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Review of literature
In the present chapter, an attempt has been made to review the available literature on various aspects viz., associated weed flora of wheat, crop-weed competition, effect of day time of applications of post emergence herbicide against weeds on wheat crop including chemical and biological properties of soil and its persistence on wheat plant, grain, straw and soil just to know the present status of work done in India and abroad including research gap and same have been presented in this chapter under the appropriate heads.

Associated weed flora
Weeds substantially reduce the production and productivity of crops as the weeds are an integral part of the biotic community of each and every agro-ecosystem and offer severe competition with crop plants for growth resources. The associated weed flora and their correct identification provide the basis to formulate effective measures for their management in different field crops particularly irrigated crops like wheat.

Abroad
The common weeds in the experimental field at Khumaltar, Nepal during two rabi seasons 2000-01 and 2001-02 were comprised of Phalaris minor, Alopecuros sp., Chenopodium album, Stellaria media, Polygonum hydropiper, Bothiospermum, Rumex sp., Senecio vulgaris and reported that Chenopodium album as the dominant weed on all the wheat field having numbers ranged from 500 to 900 per 0.5m2 area (NARC, 2002). In another experiment during 2004-05 at Khumaltar, Lalitpur, Nepal, wheat field was infested with different types of weeds like P. minor, Alopecuros aequalis, Stellaria media, C. album, Gnaphalium affenes, Soliva anthemifolia, A. arvensis, Vicia spp. etc. Among them P. minor, A. aequalis, C. album, S. media and S. anthmifolia were common in wheat field (NARC, 2006).

Begum et al. (2003) reported that there are 73 weed species in wheat crop at Mymensingh region of Bangladesh. Among them, Gnaphalium affine (33.5%), Chenopodium album (23.3%), Polygonum plebeium (15.2%), Digitaria sanguinalis (14.8%) and Cynodon dactylon (13.6%) had higher relative abundance value as compared to other weeds. While, Mandal et al. (2014) found that wheat was infested by twenty two weed species belonging to ten families. The most important weeds were Chenopodium album, Cyperus rotundus, Eleusine indica, Cynodon dactylon, Vicia sativa, Heliotropium indicum, Raphanus raphanistrum, Brassica kaber. Among the twenty two species, fifteen were broad leaved, five were grasses and two sedges. Hossain et al. (2010) reported that dominant weed species in wheat field were Eleusine indica, Echinochloa colonam, Cynodon dactylon, Parapholis strigosa, Setaria glauca, Digitaria spp., Chenopodium album, Blumea lacera, Enydra fluctuans etc.

Hessain (2013) found that the wheat was infested with mixed population of Avena fatua, Bromus tectorum, Lolium multiflorum, Medicago sp., Brassica tournefortii, Chenopodium album, Anagallis arvensis and Convolvulus arvensis at Libya.

Amare et al. (2016) observed that predominant weed flora in wheat at Ethiopia, East Africa comprised of Avena fatua, Cynodon dactylon, Phalaris minor, Poa annua and Snowdenia polystachia among the grass weeds, and Amaranthus hybridus, Biden pilosa, Chenopodium album, Commelina benghalensis, Commelina arvensis, Datura stramonium, Galinsoga palviflora, Nicandra physelodes, Oxalis latifolia, Polygonum nepalense, and Raphanus raphanistrum were among broadleaved weeds and Cyperus esculentus was the only sedge weed.

Northern India
Bharat and Kachroo (2007) observed the weed flora associated with wheat comprised of Anagallis arvensis, Poa annua, Phalaris minor, Trachyspermum sp. and Euphorbia helioscopia under irrigated condition in Jammu region. However, Dawson et al. (2008) found the infestation of Anagallis arvensis, Chenopodium album, Parthenium hysterophorus, Vicia hirsuta and Phalaris minor in wheat at Allahabad, Uttar pradesh.
At Pantnagar, the wheat crop sown in clay loam soil was badly infested with Phalaris minor, Avena ludoviciana, Polypogon monspeliensis, Cynodon dactylon, Paspalam conjugatum, Chenopodium album, Anagallis arvensis, Fumaria parviflora, Lathyrus aphaca, Medicago denticulata, Melilotus alba, Melilotus indica, Rumex acetosella, Vicia sativa, Coronopus didymus, Eclipta alba, Ageratum conyzoides and Cyperus rotunsus (Singh et al 2004; Singh et al 2006). However, in same place Singh et al. (2010) noticed that wheat was mainly invaded by Phalaris minor, Melilotus indica, Coronopus didymus, Lathyrus aphaca and Chenopodium album which account for 26, 22, 20, 10 and 7%, respectively, at 30 days after sowing.

Yadav et al. (2009) reported major associated weeds in wheat as Phalaris minor, Coronopus didymus, Anagallis arvensis, Melilotus indica, Medicago denticulata, Rumex dentatus, Vicia sativa and Lathyrus aphaca at Karnal, Haryana. At the same place, Singh (2007) found the infestation of weeds in wheat to the tune of 29.8, 20.6, 9.3, 8.9, 4.4, 7.4, 10.5 and 9.1 % of Phalaris minor, Avena ludoviciana, Chenopodium album, Melilotus indica, Convolvulus arvensis, Anagallis arvensis, Vicia sativa and Rumex maritimus, respectively.

Kumar et al. (2011) reported that the dominant weeds in the wheat field were Phalaris minor, Avena ludoviciana, Lolium temulentum, Vicia sativa, Lathyrus aphaca, Stellaria media, Coronopus didymus, Anagallis arvensis, Spergulla arvensis and Polygonum alatum at Palampur, Himachal Pradesh. The other weed species of minor importance were Poa annua, Alopecurus myosuriodes and Plantago spp. At same place Kumar et al. (2013) found that weed flora of the experimental field was mainly composed of grassy weeds as they constituted 88.9 and 91.2% of the total weed flora at 90 DAS and at harvest, respectively. Phalaris minor (25.8 and 31%), Avena ludoviciana (31.4 and 18.6%), Lolium temulentum (14.3 and 22.1%) and Poa annua (17.4 and 19.5%) were the important grassy weeds at 90 DAS and at harvest, respectively. Vicia sativa (5.5 and 8.8% at 90 DAS and at harvest, respectively) and Coronopus didymus (5.5% at 90 DAS) were important broad-leaved weeds. Chander et al. (2013) also reported that Phalaris minor and Avena ludoviciana were the most predominant weeds constituting 66.5 and 27.65% of the total weed flora. The other weeds found growing in association with wheat crop were Lolium temulentum (2.0%), Vicia sativa (3.0%) and Coronopus didymus (0.6%) at same place.

Chhokar et al. (2011) found the infestation of P. minor, A. ludoviciana, Coronopus didymus, C. arvensis, Anagallis arvensis, R. dentatus, Melilotus indica and Medicago denticulata in wheat at Karnal, Haryana. At the same place, Malik et al. (2013) reported that the experimental field during 2006-07 was infested with Phalaris minor, Avena ludoviciana, Melilotus alba, Chenopodium album, Rumex retroflexus, Coronopus didymus, Anagallis arvensis and Convolvulus arvensis to the extent of 11.4, 57.1, 8.6, 5.7, 2.9, 5.7, 5.7 and 2.9%, respectively. Whereas, the corresponding figures during 2007-08 were 9.4, 52.3, 7.0, 9.4, 7.4, 7.0, 5.5 and 4.7%, respectively. Chhokar et al. (2015) also observed the weed flora associated with wheat was comprised of Rumex dentatus, Medicago denticulata, Coronopus didymus, Lathyrus aphaca and Malva parviflora. Among these, the most dominant weed species was R. dentatus.

Singh et al. (2012) observed that Grassy weeds viz., Phalaris minor and Avena fatua constituted 62.4% of total weed population in wheat crop, and the remaining were broad leaved species viz., Chenopodium album, Melilotus spp., Medicago denticulata, Vicia sativa, Rumex spp., Anagallis arvensis, Coronopus didymus, Lathyrus aphaca and Polygonum plebejum. Singh et al. (2015) also found that field was naturally dominated with Phalaris minor (5.74 and 40.7%) as a grassy weed and Chenopodium album (2.8 and 13.3%), Coronopus didymus (2.8 and 10.4%), Melilotus indica, (2.5 and 9.4%) Rumex spp., (2.0 and 4.8%) and Fumaria parviflora (1.8 and 3.8%), were major broad-leaved weeds infesting wheat crop during 2010 and 2011, respectively, at Pantnagar, Uttarakhand.
Katara et al. (2012) while studying the weed flora of experimental field at Palampur, Himachal Pradesh observed that wheat field was infested with both grassy and broadleaved weeds. However, the flora was dominated by grassy weeds (Phalaris minor, Avena ludoviciana, Lolium temulentum and Poa annua) constituting 70.1 and 76.1% of total weed flora during 2010-11 and 2011-12, respectively. Phalaris minor had the highest proportion (59.1 and 67.6%) of total weed flora in 2010-11 and 2011-12, respectively. Among broad-leaved weed species (Vicia sativa, Anagallis arvensis, Stellaria media, Ranunculus arvensis and Convolvulus arvensis), Anagallis arvensis was the major weed constituting 20.8 and16.9% of total weed flora during 2010-11 and 2011-12. While Katara et al. (2015) at the same place found that Phalaris minor and Anagallis arvensis were the major weeds constituting 60.8 and 21.4% of total weed population, respectively. However, Avena ludoviciana, Lolium temulentum and Poa annua constituted 4.7, 3.6 and 3.0% of total weed population, respectively. The other weeds showed their little infestation but as a whole constituted 6.5% of the total weed flora.

Singh et al. (2013) reported that grassy weeds were dominant than broad-leaf weeds in wheat field at Varanasi, Uttar Pradesh. Major associated weeds were Phalaris minor (narrow leaf-weed) and Rumex dentatus, Chenopodium album, Anagallis arvensis, and Melilotus sp. (broad-leaf weeds). In same place Singh et al. (2015) found that wheat crop was infested with weed flora of Chenopodium album, Oxalis purpurea and Anagallis arvensis, among broad-leaf weeds whereas, Phalaris minor and Cynodon dactylon among grasses. However, in case of sedges there was only one species i.e. Cyperus rotundus. While at Faizabad, Uttar Pradesh Tiwari et al. (2015) reported that the wheat crop was infested badly with Phalaris minor, Cyperus rotundus, Cynodon dactylon, Chenopodium album, Anagallis arvensis, Avena fatua, Convolvulus arvensis and Lathyrus aphaca. Similarly, Kushwaha and Singh (2000) reported that the infestation of wheat with Cinnivera dinnatifida, Phalaris minor, Cyperus rotundus, Cynodon dactylon, Anagallis arvensis, Chenopodium album, Fumaria parviflora, Portulaca oleracea, Avena fatua, Mililotus indica and Panicum sp. at Haridwar, Uttar Pradesh.

Sidhu et al. (2013) found that the main weed flora of experimental fields conducted at Ropar and Ferozpur districts of Punjab consisted of Phalaris minor, Coronopus didymus, Anagallis arvensis, Melilotus indica, Medicago denticulata, Rumex dentatus, Rumex spinosus, Trigonella polycerata, Malwa parviflora and Chenopodium album. Kaur et al. (2007) also reported wheat was infested with grass weed P. minor and broadleaf weeds like Rumex dentatus, Anagallis arvensis, Melilotus alba, Lepidium sativa, Trigonella polycerata, Malva neglecta and Medicago denticulata at Ludhiana.

Western India
Singh and Ali (2004) reported that the wheat was invaded by Chenopodium album, Chenopodium murale, Lathyrus aphaca, Angallis arvensis, Melilotus alba, Rumex dentatus, Convolvulus arvensis, Vicia hirsuta, Medicago denticulata among broadleaf weeds and Avena ludoviciana, Cynodon dactylon, Cyperus rotundus were among grasses and sedges at Udaipur, Rajasthan. However, the field was mainly dominated and colonized of broadleaf weeds (82.87%) especially Chenopodium spp. (34.5%), whereas grasses and sedges contributed 13-18%.
Sharma (2009) observed the major weeds in wheat field were Cynodon dactylon (5%), Cyperus rotundus (7%), and Asphodelus tenuifolius (13%) among monocots and Chenopodium album (15%), Chenopodium murale (14%), Melilotus alba (12%), Melilotus indica (11%), Convovulus arvensis (11%), and Rumax dentatus (12%) among the dicots at Bikaner, Rajasthan.

Major weed flora in wheat field comprised of broad-leaf weeds, viz., Chenopodium album, Melilotus indica, Anagallis arvensis, Convolvulus arvensis, Cichorium intybus, Coronopus didymus, Spergula arvensis and grassy weeds, i.e. Phalaris minor, Poa annua, Avena fatua at Rajasthan (Khokhar and Nepalia 2010; Singh 2013; Jat et al 2014) .

Eastern India
Upasani et al. (2014) reported that wheat crop was infested with grassy weeds viz., Avena fatua (1.77%), Lolium temulentum (0.71%), Polypogon monspeliensis (1.06%) and Cynodon .dactylon (5.32%) and broad-leaf weeds viz., Chenopodiun album (26.95%), Fumaria parviflora (2.13%), Coronopus dydimus (12.41%), Anagalis arvensis (5.32%) and Melilotus indica (44.33%) were dominant during the winter season at Ranchi, Jharkhand. While at Bhagalpur, Bihar Radhey Shyam et al. (2014) identified major weed flora in experimental plots was comprised of Phalaris minor among grassy weeds with Cannabis sativa, Chenopodium album and Fumaria parviflora among broad-leaved weeds. The other minor weeds were Anagallis arvensis and Solanum nigrum.

Central indiaJain et al. (2014) conducted a experiment to evaluate the effect of application of post-emergence herbicides at different levels of available soil moisture in irrigated wheat at Jabalpur, Madhya Pradesh, during winter (rabi) season of 2008–09 and 2009–10. They found that irrigated wheat ecosystem was mainly invaded with grassy weeds particularly Dinebra retroflexa (31.86 and 29.34%), Cyperus rotundus (15.03 and 18.73%) and Phalaris minor (9.72 and 7.12%); and dicot weeds Cichorium intybus (8.52 and 8.74%) Medicago hispida (7.62 and 8.86%), Alternanthera philoxeroides (7.41 and 8.61%), Melilotus indica (6.41 and 5.87%), Anagallis arvensis (5.31 and 4.62%), Chenopodium album (4.11 and 4.62%) and Trifolium fragiferum (4.01 and 3.50%) during both the years. Similarly, at the same district Yadav et al (2014) identified dominant weed species in the wheat field as Phalaris minor (66.38%), Avena ludoviciana(20.44%), Cichorium intybus (11.76%), Euphorbia geniculata (0.74%), Medicago denticulate (0.32%), Vicia sativa (0.20%), Physalis minima (0.13%), Chenopodium album (0.03%). The relative density of monocot and dicot weeds at 30 days after sowing (DAS) was 86.82 and 13.18 per cent, respectively, indicating the predominance of monocot weeds in wheat.

Tiwari et al. (2013) observed the major weed flora in the wheat field as Phalaris minor, Chenopodium album, Anagalis arvensis, Medicago denticulata, Physalis minima, Vicia sativa and Melilotus alba at Bilaspur, Chhattisgarh. While Arora et al. (2013) observed infestation of broad leaved weeds like Spergula arvensis, Anagallis arvensis, Chenopodium album, Melilotus indica and Convolvulus arvensis and grasses like Phalaris minor and Cyperus rotundus among sedges in wheat at Gwalior, Madhya Pradesh.

Losses due to crop-weed competition
Weeds compete with the crop plants for light, moisture, nutrients and space. They may cause a drastic reduction in yield to a level of one-third to almost total failure of crop when moisture and nutrients are present in limited quantity in the soil. Weeds not only deplete nutrients and moisture from the soil but also act as alternate hosts for insect-pests and diseases. It is well established that losses caused by weeds exceeds the losses from any other category of agricultural pests like insect, diseases or rodents. Unlike insect-pest and disease outbreak, losses due to weeds do not show any clear visual symptom especially at the early stages of growth. Therefore, farmers did not pay utmost attention to weed control. But, now weeds are considered as most important pests of crops as they lower yields, increase cost of production and impair the quality of produce in various ways. The magnitude of ill effect depends upon weed species as well as on their intensity and duration of infestation.

Agronomic research in major wheat growing countries of the world have indicated that the effect of weed competition on crops depends on growth habits of weeds, their relative density and periodicity of crops-weed competition. Some of the grass weeds mimic crop plants for their morphology and growth behaviour. Wild oat (Avena ludoviciana) and Phalaris minor are of such mimic weeds in wheat, in India (Singh and Ghosh, 1992).
On sandy loam soil at Hisar, Panwar et al (1998) noted 30-32% reduction in grain yield when weeds were allowed to grow throughout the season in wheat, whereas Punia et al (2003) registered 19.7% reduction in grain yield of wheat.
The study of Yadav et al. (2001) on sandy loam soil at Morena (M.P.) revealed that yield losses due to weeds were around 27.69 to 32.57%. Similarly, Vishwakarma et al. (2001) in Jabalpur recorded 34.77% yield reduction in weedy check plots in wheat. Whereas Chopra et al. (2001) found about 37.5% loss in grain yield due to weeds in wheat because of severe crop-weed competition at Meerut (U.P.).
The acute problem of both grassy weeds along with broadleaf weeds is also common in many parts of country which often results in huge yield losses and makes the weed management issue more complex (Singh and Singh, 2002).

Study of Azad et al. (2003) revealed that among several constraints of wheat production, weeds are one of the major constraints causing yield loss to the extent of 50%. Similarly, 54.63% reduction in grain yield of wheat was recorded due to uncontrolled weed growth at Varanasi by Singh et al. (2003). Shaikh and Lokhande (2003) found only 25% reduction in yield of wheat due to uncontrolled weed growth at Parbhani (Maharashtra).
Whereas Jat et al. (2003) observed that slow growth of wheat plant at early stage and application of more fertilizer as well as irrigation right from sowing encourages the rapid growth of weeds, making the cultivation of wheat more problematic and if weeds are not controlled in time, they cause substantial loss in yields to the tune of 15–40%. Severe infestation of weeds resulted in lower number of effective tiller and less spike weight and consequently significant reduction in seed yield. Anjuman and Bajwa (2010) reported that wheat crop incurred 60–65% biomass loss due to weed infestation. The effect was evident on tillering capacity, which was decreased by 41.6%.
Gopinath and Pandey (2004) reported reduction in grain yield due to unchecked weed growth to the extent of 25.4% in Delhi. Whereas, study of Sinha and Singh (2004) revealed that the long season weed-crop competition reduced the grain yield to the tune of 29.2-31.6% in sandy loam soil at Pusa, Samastipur, Bihar.

Weeds growing in association with irrigated and heavy fertilized crop decline its yield besides lowering down the quality of produce (Yadav et al 2006; Sharma 2014). The studies of Mishra (2006) revealed that the infestation of weeds throughout the crop growth period of wheat and reported 48.8% reduction in grain yield of wheat due to weeds at Jabalpur (M.P.). Similarly, Bharat and Kachroo (2007) under irrigated conditions of subtropical agro-ecosystem reported 38.27 to 42.29 per cent yield reduction in wheat if weeds are allowed to grow throughout the crop season. Chhokar et al. (2008) reported that 35 to 45 DAS was critical period for weed control. Weed free condition up to 45 DAS resulted in 67.9% increase in seed yield of wheat over the unweeded control plots. Malik et al. (2008) found that the weeds growth throughout the crop season reduced the grain yield of wheat to the tune of 40-46%. However higher reduction (>75%) in grain yield was noticed by Singh et al. (2002) in clay loam soil at Pantnagar (Uttaranchal).

Effective control of weeds in vitally important not only to check the losses due to weeds but also reduced the nutrient losses. Uncontrolled weed growth depleted 24.3- 28.6 % N, 13.5-16.2 % P and 22.3-25.2 % K (Pandey et al., 2001). Similarly, Kumar et al. (2005) reported the nutrient depletion by weeds to the tune of 83.4, 18.7, and 80.8 kg/ha N, P and K, respectively, which was 47.1, 11.5 and 55.21 kg/ha more than that absorbed by the wheat crop.

Thus, it may be underlined that weeds compete crop for nutrients, moisture, solar radiation and space and cause 16-75 per cent reduction in grain yield depending upon the degree of weed infestation, soil condition and agro-climatic conditions of the region.

Efficacy of herbicide
Singh and Ali (2004) reported that metsulfuron-methyl gave excellent control of broad-leaved weed species, better than 2, 4-D, farmer’s practice (one hand weeding) including weedy check. However, weed control efficiency of metsulfuron-methyl at 3, 4 and 5 g/ha was 78.2, 82.9 and 80.5%, respectively, at 90 days but either of the doses i.e. 3, 4 or 5 g/ha did not prove effective against grasses and had only little suppressing effects on sedges.
Mishra et al. (2005) found that application of isoproturon (1.0 kg ha-1) at 25 days after sowing and metribuzin (0.30 kg ha-1) at 35 days after sowing of wheat significantly reduced the population of Phalaris minor and Chenopodium album.
Kaur et al (2007) reported that the dry matter accumulation by Phalaris minor and broadleaf weed was significantly more in unsprayed control than all other herbicide treatments. However, per cent reduction in dry matter of P. minor varied from 26.19 to 30.69 in herbicide treatments over unweeded control at harvest. But increase in the dose of sulfosulfuron and mesosulfuron+iodosulfuron reduced dry weight of weeds in the decreasing trend over weedy check due to varied concentrations and longevity in the control potential.

Pandey et al. (2007) found that hand weeding significantly reduced the intensity of perennial grasses and sedges in wheat.

Barros et al. (2009) indicated that using lower dose (6+1.2 or 9+1.8 g/ ha) than the recommended dose of iodosulfuron + mesosulfuron (12+2.4 g/ ha), controlled Lolium rigidum better than broad-leaved weeds. Effective weed control was achieved mainly through the application at the first weed development stage and provided higher grain yields. The lower control efficacy of more developed Lolium rigidum and broad-leaved weeds and a longer period of competition between crop and weeds are responsible for the significantly lower crop yields for the delayed application.
Sharma (2009) adjudged the efficacy of broad spectrum post emergence herbicides viz., 2,4-D EE 500 g/ha, trisulfuron 15 and 20 g/ha, metsulfuron methyl 4 and 6 g/ha, isoproturon 750 g/ha + 2,4-D EE 250 g/ha and isoproturon 1.0 kg/ha + 2,4-D EE 250 g/ha, isoproturon 750 g/ha + trisulfuron 15 g/ha, isoproturon 750 g/ha + metsulfuron methyl 4 g/ha, isoproturon 1.5 kg/ha, farmers practice (1 hand weeding) against broad leaf weeds in wheat. He found that the maximum dry matter of weed was recorded in weedy check (372 g/m2). However, application of metsulfuron methyl at 4 and 6 g/ha reduced dry matter of weeds to maximum extent where the dry matter of weeds was in the order of 69.6 and 32.0 g/m2. Weed control efficiency of metsulfuron-methyl treated plots was also maximum at 4 g/ha (81.3%) and at 6 g/ha (91.3%). Minimum values of weed index were recorded in case of metsulfuron-methyl treatments at 4 g/ha (7.4%) and at 6 g/ha (19.5%).

Khokhar and Nepalia et al. (2010) registered that the tank mixture of isoproturon at 500 g/ha+ sulfosulfuron at 15 g/ha was most effective against grassy weeds (WCE of 93.7%) followed by sulfosulfuron at 30 g/ha (93.2%), clodinafop at 60 g/ha (92.6%), isoproturon at 500 g/ha+ clodinafop at 30 g/ha (91.8%), isoproturon at 750 g/ha (86.0%) and isoproturon at 500 g/ha+2, 4-D at 500 g/ha (84.8%), while against broadleaf weeds Isoproturon at 500 g/ha+2, 4-D at 500 g/ha was most effective (WCE 93.7%) followed by isoproturon at 500 g/ha+ sulfosulfuron at 15 g/ha (92.8%), sulfosulfuron at 30 g/ha (84%), isoproturon at 750 g/ha (83.3%), isoproturon at 500 g/ha+ clodinafop at 30 g/ha (47.9%) and clodinafop at 60 g/ha was least effective (20.2%).

Study of Tiwari et al. (2011) revealed that among the all herbicidal treatment, highest weed control efficiency and grain yield was recorded with mesosulfuron + iodosulfuron (12+2.4 g/ha) being at par with sulfosulfuron, (25 g/ha) and pinoxaden (50 g/ha).
Vazan et al. (2011) carried out an experiment to evaluate the efficiency of mesosulfuron-methyl and clodinafop-propargyl against Lolium perenne in pure stand and in mixture with wheat. They found that the mesosulfuron-methyl was more potent than clodinafop-propargyl for the control of L. perenne as the ED50 values were 0.24 (0.0070) and 0.29 (0.0091) rates for mesosulfuron-methyl and clodinafop-propargyl, respectively. However, no significant difference was found in wheat grain yields with 50% and full rate of application (24 g/ha) of mesosulfuron-methyl.

Arora et al (2013) reported that the maximum weed population and dry weight was recorded in weedy check. However, the application of isoproturon at 1.0 kg/ha caused maximum reduction in weed density being at par with isoproturon 2.0 kg/ha, two hand weeding, clodinafop 60 and 120 g/ha and found significantly superior over sulfosulfuron (25 and 50 g/ha) and fenoxaprop at (120 and 240 g/ha) including weedy check. While lowest weed dry weight was recorded in isoproturon 2.0 kg/ha followed by isoproturon 1.0 kg/ha, sulfosulfuron 50 g/ha and sulfosulfuron 25 g/ha. Dry weight of weeds under all herbicidal treatments and two hand weeding were at par to each other except fenoxaprop at both doses i.e.120 and 240 g/ha and weedy check.

Kumar et al (2012) also reported that application of clodinafop-propargyl + metsulfuron-methyl (60+4 g/ha) though statistically at par with clodinafop-propargyl + 2,4-D (60+500 g/ha), reduced the weed population significantly than weed check (97.2%), clodinafop-propargyl alone (88.0%). Maximum weed control efficiency (84.7%) was also observed with the application of clodinafoppropargyl + metsulfuron-methyl (60+4 g/ha) followed by clodinafop-propargyl + 2,4-D (81.9%). The lowest weed control efficiency (26.7%) was recorded when clodinafoppropargyl (60g/ha) was applied alone.
Effect of herbicide on growth, yield attributes and yields of wheat
Lauer and Simmons (1985) reported that the early development of new tillers is due to enhanced translocation of photosynthates from main shoot in which the higher amount of herbicide accumulated.

Study of Singh and Ali (2004) showed that the highest grain yield of 4556 kg/ha was recorded under two hand weedings being at par with metsulfuron-methyl at 4 and 5 g/ha. However, the application metsulfuron-methyl at 4 g/ha was found effective and was significantly superior to 3 g/ha and at par with 5 g/ha recording grain yield of 4425 and 4470 kg/ha, respectively. While, unweeded control had recorded significantly lowest grain as well as straw yields (2655 and 3153 kg/ha, respectively).

Wang and Zhou (2006) conducted an experiment to find out the effects of herbicide chlorimuron-ethyl on physiological mechanisms in wheat and reported that plant has the capacity to counteract the oxidative stress caused by 5-150 µg/kg chlorimuron- ethyl exposure at the first stage, but the capacity is lost with exposure time. Thus, it indicated that the increase of peroxidases activity in the leaves may have been caused by H2O2 produced from sources other than SOD (Superoxide dimutases). While the 300 µg /kg chlorimuron-ethyl treatment caused significant damage to chlorophyll accumulation.

The studies of Kaur et al. (2007) revealed that all doses of sulfosulfuron (25, 37.5 and 50 q/ha) and mesosulfuron+iodosulfuron (12, 18 and 24 q/ha) were at par with each other in terms of yield of wheat as these herbicides gave an effective kill of weeds.

Study of Rahaman and Mukherjee (2009) revealed that the highest grain yield was recorded in complete weed free situation (44.5 q/ ha) which was at par with farmers practices (44.3 q/ ha) as well as pendimethalin 0.5 kg/ ha + hand hoeing (43.9 q/ ha).
Sharma (2009) found that the maximum grain and straw yield (2530 and 2900 kg/ha, respectively) were recorded with post emergence application of metsulfuron methyl at 0.004 kg/ha, due to significantly higher number of tillers/plant (5.3), length of ear (39.1 cm) and test weight (39.1 g) than weedy check. However, grain yield recorded with this treatment was at par with post emergence 2,4-D EE at 0.5 kg/ha. Similar findings were documented by Singh et al. (2002) on efficacy of metsulfuron methyl in wheat crop.

Singh et al (2010) registered that the significantly higher grain yield was with chlorsulfuron at 30 g/ha as compared to higher doses of chlorsulfuron 45 or 60 g/ha, isoproturon 1000 g/ha, and weedy plots during both the crop seasons, However it was statistically at par with chlorsulfuron 20 g/ha.
Khaliq et al. (2011) found that reduced doses of iodosulfuron + mesosulfuron (3.6-10.8 g/ha) increased wheat grain yield by 22 to 48% over the weedy control, while label dose of iodosulfuron + mesosulfuron (14.4 g/ha) improved yield by 53%. However, the wheat yields for reduced herbicide doses (50 and 75%) were statistically similar to that yield of label dose.

Singh et al. (2012) reported the highest grain yield (4.17 t ha-1) of wheat was obtained with the post-emergence application of UPH-206 (clodinafop-propargyl 15 % + metsulfuron methyl 1 %) being at par with UPH-206 at 400 g ha-1 and hand weeding at 35 and 55 days after sowing, but found significantly superior over sulfosulfuron, clodinafop and isoproturon at recommended rates. However, the application of UPH-206 at 500 g ha-1 produced the 54% higher grain yield due to more number of effective tillers and number of grains/ear in comparison to unchecked weed plots.

Kumar et al. (2012) recorded the highest grain yield of wheat (4.72 t/ha) in weed free plot due to enhanced nutrients, water, space and light supply to the wheat crop on account of zero crop-weed competition. This might have resulted in greater photosynthesis and translocation of photosynthates besides longer and stronger sink size as reflected by maximum values of yield attributes and finally the yield. However, herbicidal combinations i.e. isoproturon + 2,4-D (1000+500 g/ha), clodinafoppropargyl + 2,4-D (60+500 g/ha), fenoxaprop-p-ethyl + 2,4-D (80+500g/ha) and clodinafop-propargyl + metsulfuron-methyl (60+4 g/ha) yielded similar to that of weed free plots.
Saqib et al. (2012) reported that the statistically higher grain and straw yields (4.03 and 5.35 t ha-1, respectively) were recorded under weed free plots than rest of the treatments due to superior values of the yield attributes of wheat viz., panicle length (8.16 cm), number of spikelets per panicle (17.36), grain weight per plant (8.33 g). However, among the herbicide treatments, post-emergence application of sulfosulfuron at 25 g/ha recorded statistically higher grain and straw yields (3.71 and 4.78 t/ha, respectively) being at par with hand weeding once at 30 DAS and application of metribuzin 175 g/ha over other herbicidal treatments including weedy check.

Study of Kaur and Brar (2014) revealed that higher dose of mesosulfuron+iodosulfuron (18g/ha) had in significant less plant height than its lower dose (12 g/ha) at harvest due to toxic effect of herbicide. However, increase in grain yield was 11.2, 14.6, 11.0 and 4.45% was achieved with the application of sulfosulfuron at 25 and 37.5 g/ha and mesosulfuron + iodosulfuron at 12 and 18 g/ha over unsprayed control, respectively. Results also indicated the toxic effect of higher dose of mesosulfuron + iodosulfuron (18 g/ha) as grain yield was reduced from 15.8 to 15.6 t/ha and 16.1 to 14.4 t/ha with the increase in dose of mesosulfuron+iodosulfuron from 12 to18 g/ha during 2003-04 and 2004-05, respectively. However, the straw yield was similar during first year but there was some reduction (42.5 to 39.4 t/ha) during second year. Similar views also endorced were also obtained by Shukla et al 1998 andChandi 2004.

Nabiha et al. (2014) also reported the that total chlorophyll including chlorophyll a and chlorophyll b, decreased with increased chevalier (mesosulfuron methyl + iodosulfuron methyl-sodium) herbicide concentrations (0.6, 0.9, 1.2 and 1.5 mg/plot) in wheat leaves. The above findings are in accordance with results given by El-Rokiek et al. (2012).
Effect of Hand weeding
Surin et al (2012) reported that hand weeding twice at 25 and 50 DAS in wheat crop produced higher values of yield attributing traits i.e. 31.3% higher productive tillers/m2, 5.3% higher spike length and 8.6% higher filled grains/panicle, resulting in 35.8% higher grain and 38.6% higher straw yield compared to weedy check.
Yadav and Dixit (2014) also found that the hand weeding treatment performed better than all the herbicidal treatments in reducing the density, biomass of weeds and recorded maximum weed control efficiency (92.2%). The higher values of growth parameters viz. plant height, number of tillers/m2, leaf area index and crop biomass as well as yield attributing traits viz., effective tillers/ m2, ear head length, grains per ear head and test weight including grain and straw yields were recorded under hand weeding (30, 60 DAS) and all the herbicidal treatments.
Pandey et al. (2009) reported that the stale seedbed gave in higher wheat yield (4.45 t/ha) as compared to conventional tillage (4.22 t/ha). However, among weed control treatments, herbicides + one hand weeding (5.34 t/ha) and criss-cross sowing + herbicides + one hand weeding (5.66 t/ha) were numerically similar in term of yield but significantly superior to two hand weeding (4.78 t/ha).
Ali (2003) recorded that the higher (83.90%) weed control under hand weeding. However, chemical weed control produced highest (3974 kg/ha-1) grain yield as compared to hand weeding which produced only 3670 kg/ha-1.

Effect of day time applications
There are several environmental factors viz., temperature, relative humidity, dew and wind velocity that may cause variations in herbicide efficacy throughout the day. For example, high wind speeds during spray of post emergence herbicide may cause spray drift onto non-target crops (Duke 2005). Therefore, growers typically apply herbicides early in the morning (06:00 h), or late in the evening (21:00 to 24:00 h) when wind speeds are lowest (Waltz et al. 2004). However, efficacy of herbicides at these point of times of the day may be reduced. This reduced efficacy of herbicides when applied near or after sunset is referred to as the time-of-day (TOD) effect (Sellers et al. 2004). One theory is that dew present on leaf surfaces at these times can intercept herbicide spray droplets and increase runoff, reducing overall weed control (Fausey and Renner 2001; Kogan and Zungia 2001, Waltz et al. 2004).
Temperature is also responsible for the herbicide efficacy by indirectly affecting the absorption and translocation. Friesen and Wall et al (1991) found that when air temperature was less than 25 0C, fluazifop-P-butyl provided 50% less control of green foxtail in Manitoba. In Oklahoma, control of cheat (Bromus secalinus L.) with sulfosulfuron declined when the mean temperatures decreased from 28.3 to 16.8 0C before herbicide application (Kelley and Peeper 2003). However, an eight degree increase in air temperature (27 to 35 0C) was shown to increase the efficacy of acifluorfen on pitted morningglory, common cocklebur, and velvetleaf in Arkansas (Lee and Oliver, 1982). Doran and Andersen (1976) also observed better control of common cocklebur when herbicides were applied when the air temperature was above 25 0C. Additionally, raising the air temperature from 16 to 28 C caused the absorption of glyphosate into velvetleaf cell cultures was increased (Royneberg et al. 1992). Sharma and Singh (2001) found that glyphosate uptake and translocation increased in Florida beggar weed (Desmodium tortuosum) at 22 0C compared to 16 C.
The increased air temperatures most likely affect herbicide efficacy through the alteration of leaf cuticular wax (Hess and Falk 1990; Willingham and Graham 1988). The higher air temperatures increase cuticle and plasma membrane fluidity, resulting in increased herbicide uptake (Johnson and Young 2002) and translocation (Price 1983). On the contrary, Coetzer et al. (2001) reported that glufosinate absorption was not altered by temperature when averaged over amaranth species. However, herbicide translocation in the phloem tends to increase at higher temperatures due to increased metabolic processes and enzyme activities (Andersen 1996). But glufosinate has very little translocation following application due to rapid phytotoxicity (Coetzer et al. 2001; Devine et al 1993).
Kumaratilake and Preston (2005) reported that increased glufosinate translocation to the shoot meristem and the untreated leaves at warm temperatures, whereas at cold temperatures the majority of glufosinate was translocated to tip of the treated leaf. When plants were moved from a cold temperature to a warm temperature after glufosinate was applied, glufosinate efficacy increased compared to plants kept under cold conditions throughout the entire study (Kumaratilake and Preston 2005). Anderson et al. (1993a) found greater green foxtail injury when glufosinatee was applied in warmer temperature compared to glufosinate applications in cooler temperatures. However, cooler temperatures didn’t remove the phytotoxic injury, but merely reduced the rate of injury development (Anderson et al., 1993a).

Morphological and physiological factors also affect weed control (Hess and Falk, 1990) with respect to the TOD herbicides are applied. Factors such as leaf position (Andersen and Koukkari, 1978; Doran and Andersen 1976; Mohr et al. 2007; Sellers et al. 2003), exposed leaf surface area (Kraatz and Andersen 1980), thickness of epicuticular wax (Hess and Falk 1990), and plant metabolic rate (Waltz et al. 2004) may all affect post herbicide interception and subsequent absorption and translocation. These morphological and physiological characteristics are quite variable among weed species and can change diurnally, often in response to changes in environmental variables (Hess and Falk 1990). Herbicide specific factors, such as mode of action, have also been shown to play a role in TOD effects (Miller et al. 2003).

Even though there are periods throughout the day when applications of post herbicides may be more effective, weed control still remains species-specific. For example, in Arkansas, applications of acifluorfen at 21:00 h controlled planted hemp sesbania (Sesbania herbacea), pitted morningglory (Ipomoea lacunosa) and smooth pigweed (Amaranthus hybridus) more effectively than applications at 06:00 or 12:00 h, but TOD had no effect on common cocklebur (Xanthium strumarium) or prickly sida (Sida spinosa) control (Lee and Oliver, 1982).

Waltz et al. (2004) reported that the glyphosate at 840 g/ha applied before sunrise, midday, and after sunset provided 69, 100, and 37% velvetleaf control, respectively.
Study of Stopps et al. (2013) revealed that the effect of time of day varied with weed species, but weed control was generally reduced when herbicides were applied at 6:00 A.M., 9:00 P.M. and midnight. However, the herbicide activity was most frequently reduced on velvetleaf, especially for chlorimuron-ethyl, glyphosate and imazethapyr; common ragweed for glyphosate and imazethapyr, and pigweed species only showed an effect with glyphosate, suggesting that post herbicides are most effective when applied midday, rather than in the early morning or late evening.

Stewart et al. (2009) investigated how weed control in corn was affected by the time of day when herbicides were applied. Weed control following the application of six POST herbicides (atrazine, bromoxynil, dicamba/diflufenzopyr, glyphosate, glufosinate, and nicosulfuron) at 06:00, 09:00, 12:00, 15:00, 18:00, 21:00, and 24:00 h was assessed. For many weed species, herbicide efficacy was reduced when applications were made at 06:00, 21:00 and 24:00 hrs. Velvetleaf was the most sensitive to the time of day effect, followed by common ragweed, common lambsquarters, and redroot pigweed. Annual grasses were not as sensitive to application timing; however, control of barnyardgrass and green foxtail was reduced in some environments at 06:00 hr and after 21:00 hr. Only in the most severe cases, the grain yield of corn was reduced due to decreased weed control.
However, previous research has demonstrated that the efficacy of POST herbicides can vary depending on day time applications. The effect of day time applications on weed control has been previously reported for acifluorfen (Lee and Oliver 1982), bentazon (Andersen and Koukkari 1978; Doran and Andersen 1976), chlorimuron ethyl (Miller et al. 2003), fluazifop-Pbutyl (Friesen and Wall 1991), flumiclorac, fluthiacet-methyl (Fausey and Renner 2001), fomesafen (Miller et al. 2003), linuron (Kraatz and Andersen 1980), glufosinate (Martinson et al. 2002; Miller et al. 2003), and glyphosate (Martinson et al. 2002; Miller et al. 2003; Mohr et al. 2007; Norsworthy et al. 1999; Sellers et al. 2003; Waltz et al. 2004), atrazine, bromoxynil, dicamba/diflufenzopyr, and nicosulfuron on weed control, in a corn cropping system (Stewart et al. 2009). No studies however, have examined possible effects of day time applications of mesosulfuron-methyl on weed control conducted in wheat.
Nutrient uptake
Jain et al. (2007) observed the increased uptake of N by 44.29-51.24 kg/ha, 12.00- 12.87 kg/ha of P and 8.37- 9.87 kg/ha of K, respectively, by wheat due to application of clodinofop followed by 2,4-D, whereas isoproturon+2,4-D increased the uptake of N by 10.85-15.50 kg/ha, 3.12-4.63 kg P /ha and 2.31- 2.89 kgK/ ha compared to weedy check plots on clayey soils of Jabalpur (M.P.). Similar finding were also endorsed by Brar and Walia (2007).

Study of Singh et al. (2009) revealed that hand weeding twice at 30 and 45 days after sowing recorded maximum and significantly higher N,P,K uptake by wheat than herbicidal treatments. The increase in the N,P,K uptake was 50.5, 56.8 and 52 %, respectively, during 2003-04, and 55.0, 61.9 and 56.9 %, respectively, in 2004-05.

Khokhar and Nepalia (2010) reported that nutrient uptake was maximum (150.20 kg N, 41.00 kg P and 194.14 kg K/ha) with isoproturon 500 g/ha+ sulfosulfuron 15 g/ha being at par to isoproturon 500 g/ha+2, 4-D 500 g/ha but proved significantly superior to isoproturon, clodinafop and sulfosulfuron alone at 750, 60 and 30 g/ha, respectively, and tank mixture of, isoproturon 500 g/ha + clodinafop 30 g/ha. While the minimum nutrient uptake (87.87 kg N /ha, 23.82 kg P/ha) by crop was registered under weedy check plots.

Kumar et al. (2012) found 8.0, 11.6, 3.7, 8.2, 10.8, 4.6, 10.2 and 13.5 per cent significantly higher total uptake of NPK by wheat under weed free conditions compared to pendimethalin, isoproturon, isoproturon + 2,4-D, isoproturon + metsulfuron-methyl, fenoxaprop-p-ethyl, fenoxaprop-pethyl + 2,4-D, fenoxaprop-p-ethyl + metsulfuron-methyl and clodinafop-propargyl + metsulfuron-methyl, respectively. However, all the herbicidal treatments recorded significantly higher total nutrient uptakes than the weedy check. The lowest nutrient uptake by wheat grains and straw in weedy check seems to be due to the increase in weed dry matter accumulation. The similar results have been reported by Pandey et al. (2001).

Economics
Study of Mani et al. (1998) revealed that the chemical weed control in wheat was found more remunerative than hand weeding. Rastogi et al. (1994) and Thakur and Singh (1998) also found that the maximum benefit-cost ratio was registered under herbicidal treatments as herbicides not only pushed-up grain yield, but also fetched higher monetary returns. Similarly, study of Radhey Shyam et al. (2014) also revealed that maximum net returns and B:C ratio (31,475/ha and 1.80) were obtained with sulfosulfuron 33.3 g/ha being at par with weed free and one hand weeding at 30 DAS.

Ali (2003) also reported that chemical weed control had higher cost-benefit ratio (1:2.88) and revenue per crop per day (Rs. 77) as compared to hand weeding.

Khaliq et al. (2011) found that reduced doses of iodosulfuron + mesosulfuron were quite effective in suppressing total weed density (72-95%) and biomass (83-94%). The maximum marginal rate of return was recorded for 50% of the label herbicide dose (7.2 g/ha of iodosulfuron + mesosulfuron), followed by 25% of the label dose (3.6g/ha).

Effect of herbicide on biological properties of soil
The microbial flora in soil is an important constituent for enhancing soil fertility as these are involved in nutrient retention and recycling and ultimately affect agriculture productivity. However, the microbial biomass and their activities in soil may fluctuate due to different soil management practices. Herbicides may have either positive or negative effect on microbial growth. The available research work on the effect of herbicide on soil micro flora have been reviewed critically and being presented as below:
Nada and Mitar (2002) reported that the Azotobacter is most sensitive to herbicide application. It is, therefore, a reliable indicator of the biological value of soil. The numbers of this group of nitrogen-fixing bacteria decrease considerably in the period of 7—14 days after herbicide application. Simultaneously, numbers of Actinomycetes and so of fungi increased, indicating that these microorganisms use herbicides as sources of biogenous elements. Rate of herbicidal decomposition depends on the properties of the preparation applied, herbicide dose as well as on the physical and chemical soil properties, soil moisture and temperature, ground cover, agro technical measures applied and the resident microbial population.
Bensulfuron-methyl, nicosulfuron and rimsulfuron have been found to decrease significantly the abundance of bacteria in top soil (Djuric and Jarak 2006). Study of He et al. (2006) revealed that metsulfuron-methyl distinctly inhibited the common aerobic heterotriphic bacteria, but the effect on common fungi and common actinomycete were not evident. In the meantime, the number of tolerant fungi increased greatly in the rhizosphere after the application of metsulfuron-methyl in contrast to the significant decrease of the amount of tolerant actinomycete. The population of aromatic compounds-decomposing bacteria, aerobic Azotobactor and nitrite bacteria all increased in the earlier period, but the aerobic Azotobactor decreased rapidly in number 30 days later, and the amount of nitrite bacteria also showed a temporary decrease with time i.e. 15 days later. This decline in Azotobactor population at later period was in line with the result of Lone et al. (2014) as they also noticed an initial enhancement in population of Azotobactor followed by depression after apllication of two sulfonylurea herbicides viz., Atlantis ( MesosulfuronMethyl 3% + Idosulfuron Methyl Sodium 0.6% WG) @ 400 g/ha and Sulfosulfuron 75% WG @ 33.33 g/ha. Contrary to this, influence of isoproturon was neutral throughout the period. Similarly, the study of Lenart (2012) also found that the growth of Azotobacter chrococcum was not inhibited after linuron application.
Sharma (2007) reported that the total microbial population in soil was significantly declined with post emergence application of isoproturon at 1.5, 2.0 and 2.5 kg/ha over control. However, the inhibitory effect of isoproturon was seen upto 10 days only. But at 60 days after herbicide application and at harvest of the crop, the differences among different treatments was not significant and thereby indicated that bacterial, fungal and actinomycetes population in isoproturon treated soil recovered from the initial setback under microbial population
Kucharski and Wyszkowska (2008) reported that Apyros 75 WG disturbs soil’s homeostasis, as it disrupts multiplication of some microbial groups, inhibits the activity of soil enzymes and depresses the yield of spring wheat, even if applied in a recommended dose. Among the soil enzymes, dehydrogenases and urease were the least tolerant to the effect of the herbicide. The vulnerability of microorganisms to soil pollution with the herbicide can be arranged in the following decreasing order i.e. ammonifying bacteria ; Pseudomonas ; copiotrophic bacteria ; oligotrophic bacteria ; nitrogen binding bacteria ; spore-forming oligotrophic bacteria ; Arthrobacter ; cellulolytic bacteria ; Actinomyces ; fungi.
Tamilarasi et al. (2008) found that the bacteria population in wheat field increased correspondingly with time being maximum at harvest, suggesting that wheat rhizosphere is beneficial for the growth and reproduction of bacteria.
Lovieno and Baath (2008) assessed the effect of various level of soil moisture on bacterial respiration and growth rate and found that the bacterial respiration and growth rate were low in air-dried soil, but increasing rapidly to high stable values in moist soils.

. Zabaloy et al. (2008) reviewed the intensive use of herbicides in agricultural soils of the Pampas region (Argentina), which was a matter of environmental concern and studied the impacts of three widely used post emergence herbicides, glyphosate, 2,4-dichlorophenoxyacetic acid (2,4-D) and metsulfuron-methyl, on soil microbial communities by an integrated approach using short-term soil incubations and noticed that the addition of these herbicides at a dose 10 times higher than the normal field application rates, caused minor changes to soil microbial activity, bacterial density and functional richness. While Ratcliff et al. (2006) did not observe any change in microbial abundance (bacteria and fungi) in soil treated with glyphosate dose of 50 mg kg-1 but bacterial counts increased in response to 100-fold increase in herbicide dose.
Study of Sebiomo et al (2011) indicated that bacterial, fungal and actinomycetes populations decreased upon treatment with four herbicides viz., atrazine, primeextra, paraquat and glyphosate compared to control. However, there was significant response of soil microbial activity to herbicides and increased adaptation of the microbial community to the stress caused by increase in concentration of the herbicides over weeks of treatment. But actinomycetes population was ever the highest in control soil samples compared to herbicidal treated soil samples.
Microorganisms consume the herbicide molecules and utilize them as a source of energy, carbon and nutrients for growth and reproduction (Sondhia et al. 2013; Xu et al. 2009). Bacteria in general have a higher capability of decomposing or digesting the herbicide and use them as a source of biogenous element as compared to actinomycetes and fungi. Soulas (1982) indicated that there was antagonistic inhibition between the herbicide degradating bacteria and those which could not utilize herbicide as carbon or nitrogen source for their growth.
Studies of Sondhia et al. (2013) also indicated that on 0 day (2 hours after application), the concentration of pyrazosulfuron-ethyl was high, so it was not utilized by fungi immediately after application but fungal growth was increased with time and utilized pyrazosulfuron-ethyl successively. The growth of the fungi was more in 2mg/kg treatment than 4 mg/kg treatment because it contain double dose of pyrazosulfuron-ethyl that affect the growth of the fungi. A stimulatory effect of increased dose of herbicides on fungal counts was observed by several workers i.e. Sondhia et al. (2016) on penoxsulam at a dose of 25 g ha-1, Crouzet et al. (2010) on mesotrione applied to soil at 0.45 to 45 mg kg-1, and Zabaloy et al. (2010) on 2,4-dichlorophenoxyacetate applied in soil at 1 to 10 mg kg-1. Sondhia et al. (2013) and Zain et al. (2013) underlined that the herbicides affect microbial growth, either positively or negatively; depending on chemicals (class and concentration), microbial species and environmental factors such as temperature, moisture and pH etc.

According to Ali et al (2014) use of herbicides influenced the biological balance of soil microflora, which has an important role in soil fertility and microbial ecosystem. They found that application of MCPB, Bentazon, MCPB + Fluozifop-p-butyl., Bentazon+Fluozifop-p-butyl, Metribuzin, Flouzifop-pbutyl+ Metribuzin, Cycloxydin, and Sethoxydin increased the population of soil fungi 4 to 10 times compared with the control. However, the above said herbicides had no significant effects on nitrogen fixing bacteria.
According to Radivojevic et al. (2014) influence of metsulfuron-methyl on the soil microbial activity in soil depend on the rate of application and duration of activity. They found that the application of metsulfuron-methyl at 1 and 5 mg/kg soil did not have any effect on microbial parameters but the effects were only detected at higher doses (25 and 50 mg/kg) and they were slight and transitory. However, higher doses (25 and 50 mg/kg) induced increasing activity of soil microorganism from 5th to 40th day.

Sharma et al. (2014) reported that the total bacterial count in two hand weeding and weedy check treatments were at par to each other (42.3 x 106 and 39.6×106 CFU respectively) but it was almost double than other two treatments (metsulfuron-methyl 4 g/ha and metsulfuron- methyl 4 g/ha fb manual weeding 50 DAS). However, fungal population at harvesting was at par in the plots receiving two manual weedings and metsulfuronmethyl 4 g/ha + manual weeding 50DAS (40.2 x104 and 37.3×104 CFU respectively) showing no considerable effect of herbicide on fungal population.

Chandran and Atmakuru (2015) found that the application of metsulfuron-methyl (0.02, 0.20 ?L/kg soil) caused a marked increase in the microbial biomass for C: N ratio over the untreated control. However, the soil microbial effect was significant upto 42 days of incubation for nitrogen and 28 days for carbon. But there was no long term influence on soil microbial properties.

Studies of Bacmaga et al (2015) revealed that a mixture of diflufenican + mesosulfuron-methyl + iodosulfuron-methyl-sodium increased the counts of total oligotrophic bacteria and spore-forming oligotrophic bacteria, organotrophic bacteria and actinomycetes, while decreased the counts of Azotobacter and fungi, and ultimately modified the structure of soil microbial communities. The herbicide applied in the recommended dose stimulated the activity of catalase, urease and acid phosphatase. But the highest dose of the analyzed substances (36.480 mg kg?1) significantly inhibited the activity of dehydrogenases, acid phosphatase, alkaline phosphatase and arylsulfatase. The foresaid herbicide had an adverse influence on spring wheat yield, and higher doses (18.240 and 36.480 mg kg?1) led to eventual death of plants.

Bacmaga et al. (2016) conducted a experiment to evaluate the sensitivity of soil microorganisms viz., Azotobacter spp., Arthrobacter spp., Bradyrhizobium spp. (lupini), Rhizobium leguminosarum bv. viciae, Streptomyces intermedius, Streptomyces viridis, Streptomyces longisporoflavus, Streptomyces odorifer, Fusarium spp., Aspergillus spp., Penicillum spp., Rhizopus spp. to metazachlor, a mixture of diflufenican + mesosulfuron-methyl + iodosulfuron-methyl-sodium, and a mixture of terbuthylazine + mesotrione + s-metolachlor. They found that fungi were more sensitive to herbicides than bacteria and actinomycetes. The tested microbes were mostly resistant to increased doses of the mixture of diflufenican + mesosulfuron-methyl + iodosulfuron- methyl-sodium. While, the increased doses of metazachlor had highest predicted environmental concentrations (PEC) and posed the greatest threat for soil-dwelling microorganisms which points to a high risk of soil contamination with this weed control agent.
Dissipation and persistence of herbicide in soil and plant
Sulfonylureas, a class of very effective herbicides introduced in the early 1980?s, are used at very low rates. Chemical hydrolysis and microbial breakdown are the most important pathways of sulfonylurea degradation in soil, whereas photolysis and volatilization are relatively minor processes. The degradation of sulfonylurea herbicides is affected by several soil and environmental factors like pH, temperature, organic matter, soil moisture, microbial activity etc. The degradation rates of sulfonylureas are negatively correlated with soil pH. Soil pH may directly or indirectly influence the activity and detoxification of herbicide by affecting the ionic or molecular character of the chemical, ionic character of the soil colloid, the cation exchange capacity and capacity of microbial population to attack on herbicide.

Some of the factors that influence the persistence of pesticides are common to both plants and soils. These are, firstly, the characteristics of the pesticide, including its over-all stability either as parent compound or metabolites, its volatility, solubility, formulation, and the method and site of application. A second group includes the environmental factors, particularly temperature, precipitation (and humidity) and air movement (wind). The other factors depend on the properties of the plant or soil. Characteristics influencing the persistence of pesticides in plants include the plant species involved, the nature of the harvested crop, the structure of the cuticle, the stage and rate of growth and the general condition of the plant (Edwards, 1975).
Increasing temperature is known to accelerate numerous processes involved in pesticide dissipation (Stenersen, 2004), for example, increasing a substance’s solubility and increasing enzymically mediated metabolic activity (Willis and McDowell, 1987). However, the effects of temperature on the half life of herbicide in plant under field conditions was assessed by Koeppe et al (2000) and observed that temperature had a marked effect on the rate of rimsulfuron metabolism in maize, particularly in roots. Metabolic half life of rimsulfuron were 2.7 hours and more than 48 hours at in shoot and root at 10 oC, which were decreased to 1.2 and 25 hours at 25 oC and further 10 oC increase in temperature it dissipated with in 1.5 and 8.7 hours, respectively.

In addition to environmental factor (temperature), the crop growth rate is a plant characteristic plays a major role to cause the faster degradation of pesticides. Growth dilution refers to the effect that with increasing weight of sampled plant material, the proportion of residual pesticide concentration (weight-based) decreases, which is also known as “apparent elimination” (Fenoll et al 2008). Growth dilution depends on substance stability, that is, the more stable a pesticide, the more growth dilution will usually contribute to its dissipation (Zongmao and Haibin, 1988). Many studies explicitly avoided the effect of growth dilution by considering equal periods between planting and harvest, harvesting samples of equal size, or by expressing residues on a harvest area basis rather than on a weight basis. Some studies reported contributions of growth dilution to pesticide dissipation range from around 10% for thifensulfuron-methyl in whole soybean plants (Brown et al, 1993) to 82% for dimethoate in artichoke heads (Cabras et al, 1996).

Some of the studies considering pesticide dilution due to plant growth corrected dissipation half-lives by incorporating a growth dilution factor (Cabras et al. 1996; Hill et al 1982; Hong et al. 2011). This factor expresses the relationship between the pesticide residue on the day of application multiplied by the plant component weight on the day of application divided by the plant component weight on the day of sampling (Hong et al. 2011).
Paterio-Moure (2008) reported that the persistence of pesticide in soil related to climate, soil properties and the physical and chemical properties of the pesticide. Some studies reported that the degradation rate for sulfonylurea herbicides is dependent on several factors such as temperature, pH, moisture content, and the biological activity of the soils (Beyer et al 1987, Beyer et al 1988, Blair and Martin, 1988). However, the study of Joshi et al. (1985) revealed that the degradation rates of sulfonyurea herbicide are negatively correlated with soil pH and positively correlated with temperature, soil moisture content, organic matter content and microbial biomass in soil. Sulfonylurea herbicides are generally weakly adsorbed by soil. Moreover solubility of mesosulfuron-methyl is high, hence able to leach from the top-soil to deeper surface soil (0-20 cm). The sulfonylurea group herbicides have low octanal/water partitioning coefficients (Kow value) and relatively higher water solubilities, resulting in a high potential mobility in soils (Anderson and Dulka 1985; Brown 1990).
Pesticide washoff from plant surfaces by rainfall can be an important dissipation process, especially shortly after substance application, and is an indirect route by which many pesticides reach the soil (Fisher et al., 2002 and Katagi 2004). However, the ability of rain to either help the pesticide penetrate into the plant interior or to wash it off depends on rain quantity, time between pesticide application and rainfall, pesticide solubility, the structure of the plant surface, and formulation (Willis and McDowell, 1987 and Cabras et al 2001).

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However, there are various factors have been reported to influence the degradation of sulfonylurea herbicides in the field, which may result in shorter half-lives than those found under controlled environment conditions. Sarmah and Babadie (2002) reported that the chemical degradation tends to be temperature-dependent for SUs with half-life (DT50) decreasing with increasing temperature. James et al (1999) observed that the respective half-lives ranged from 22 to 38 days for chlorsulfuron and from 31 to 44 days for triasulfuron under five controlled temperature/ soil moisture regimes, ranging from 10 to 30 0C and between 40-80% maximum water holding capacity. Half in the field were considerably shorter (13 days for chlorsulfuron and from 12-13 days for triasulfuron). Hence they suggesting that the degradation rates of the herbicides were influenced more by soil temperature than by soil moisture content. While in a New Zealand acidic soil (pH 5.7) both herbicides disappeared within 11-15 weeks during the summer. Similarly, according to EFSA (2008), with the decrease in temperature from 200C to 100C, half life of mesosulfuron-methyl increased upto 154 from 49.1 days under controlled condition. Dermiyati and Yamamoto (1997) was also reported that decreasing the temperature from 30 to 4 °C reduced the degradation rate of halosulfuron-methyl in two different soils at 50% water-holding capacity; average half-lives at the two temperatures were 13 and 98 days, respectively, in both of the soils.
European Food Safety Authority (2008 and 2016) reported that temperature and microbial activity are major factors affecting the mesosulfuron-methyl degradation. However, the microbial activity is not started immediately after application of herbicide, then temperature becomes a major factor for its degradation. Saha and Kulshrestha (2002) also reported that the temperature plays a role in environmental degradation during the early hours of application of the sulfonylurea compound under field condition. In Addition to temperature, presence of heavy dew on the leaves of plant especially during winter season may cause to herbicide runoff and indirectly affect the amount of herbicide going in the soil and affect the initial concentration of herbicide in soil. However, mesosulfuron-methyl is not photodegraded to significant extent at wavelengths >290 nm on soil surfaces. Soil photolysis will therefore not contribute notably to elimination from the terrestrial environment, and will not lead to the generation of relevant degradates EFSA (2008).

Walker et al. (1989) studied the degradation of herbicide chlorsulfuron and metsulfuron methyl in soils from different soil depths and observed that the degradation rate decreased with increasing depth in the soil. Half-lives of both herbicides in different soils were found to be considerably varying from 20 to more than 150 days and this may be due to variation in soil characteristics. Highly significant negative correlations between degradation rates and soil pH, and highly significant positive correlations with microbial biomass were observed.
Kotoula-syka et al. (1993) conducted a study to determine phytotoxicity and persistence of chlorsulfuron, metsulfuron methyl, triasulfuron and tribenuron methyl in three soils i.e. sandy loam, sandy clay loam and silty clay loam. All the bioassays indicated that phytotoxicity of all herbicides was not affected by soil texture but increased with increasing herbicide concentration and soil pH. In field studies, persistence and leaching of all above herbicides in all soils increased with increasing rate of application.
James et al. (1995) reported that the degradation of metsulfuron methyl did not follow pure first order kinetics as during first 2 weeks the initial concentration decreased more quickly than expected. However, thereafter degradation appeared to follow first order kinetics. The rate of degradation of metsulfuron methyl was sensitive to temperature and half-life of metsulfuron methyl ranged from 8 to 36 days.
Cranmer et al. (1999) carried out sorption, dissipation and leaching studies of metsulfuron methyl in six Colorado soils and found that dissipation followed first order kinetics and half-life of metsulfuron methyl in these six Colorado soils ranged from 11.8 to 27.7 days.

James et al. (1999) found that chlorsulfuron and triasulfuron degraded rapidly in the acidic soil (pH 5.7) with high organic matter levels (7.3 per cent) and followed first order rate kinetics. The degradation rates were influenced more by soil temperature than by soil moisture content. The results also showed that the effects of chlorsulfuron disappeared within 8 weeks. Residues of metsulfuron methyl in soil applied at 15 and 30 g a.i.ha-1 rates disappeared within 9 and 14 weeks respectively. Similarly, Rouchaud et al. (1999) reported metsulfuron methyl residues in soil were non detectable at harvest when applied at 6 g ha-1 in winter wheat in Belgium.
Fan et al. (2004) carried out dissipation studies of monosulfuron in soil and wheat. Soil degradation studies showed that monosulfuron degraded faster in acidic soil and strong alkaline soil than neutral or weak soil. No residues of monosulfuron were detected in wheat samples collected at 75 days after herbicide application.
Bedmar (2006) conducted a study to determine phytotoxicity and persistence of chlorimuron (0, 3.6, 7.2, 14.4 and 21.6 ng g-1) and metsulfuron (0, 1.15, 2.3, 4.6 and 6.9 ng g-1) in two soils i.e. Balcarce loam soil (7.5 per cent organic matter content, pH 5.9) and San Cayetano loam soil (4.0 per cent organic matter, pH 6.8) of Argentina by bioassay technique. They observed that persistence and phytotoxicity of both herbicides was dose dependent and increased with increasing the doses of herbicides application. Half-lives depending on rate of application for chlorimuron ranged from 30 to 43 days in Balcarce soil and from 50 to 69 days in San Cayetano soil. Whereas, in case of metsulfuron methyl values ranged between 38 to 51 days in Balcarce soil and 54 to 84 days in San Cayetano soil, respectively.

Sanyal et al. (2006) evaluated metsulfuron methyl (Ally 20 WP) and chlorimuron ethyl (Classic 25 WP) for their dissipation behaviour in three different soils and reported that herbicides persisted for 45 to 60 days irrespective of the treatment combinations. Half-lives of metsulfuron methyl and chlorimuron ethyl ranged from 10.75 to 13.94 days irrespective of soils and doses applied. No residues were detected in the harvested crop samples of rice, wheat and soybean, therefore these two herbicides metsulfuron methyl and chlorimuron ethyl could safely be recommended.
Singh and Kulshrestha (2006) reported that triasulfuron residues in wheat field soil at two levels of application i.e. 15 and 20 g ha-1 dissipated from soil within 30 days with a half-life of 5.8 and 5.9 days respectively. No residues were detected in soil at harvest time (112 days). The dissipation behaviour of triasulfuron in soil followed a first order rate kinetics.
Han et al (2007) found that carfentrazone-ethyl dissipated rapidly in soil after application. Its half-lives in soil were 5.8 and 3.8 h in Beijing and Jilin, respectively. The terminal residues of carfentrazone-ethyl in soil samples were very low (around 0.003–0.005 mg/kg), and the residues in wheat grain were not detectable. The use of carfentrazone-ethyl in wheat could be considered to be safe.

Sondhia et al (2007) carried out residue studies of sulfosulfuron under field conditions in soil at 1, 7, 15, 30 and 60 days after herbicide application and wheat grains and straw at harvest by HPLC. She found that arround 90.9 per cent of applied sulfosulfuron degraded in the soil within 30 days indicating rapid degradation of sulfosulfuron residues. At the time of harvesting, residues of sulfosulfuron were not detected in wheat grains and wheat straw.
Sondhia (2007) found the 0.002, 0.006, 0.0075 and 0.010 ?g/g residue level of imazosulfuron in soil after 60 days at 30, 40, 50 and 60 g/ha application rates, respectively. However, no residues were found after 90 and 120 DAS. At harvest imazosulfuron residues were not detected in rice grains and straw.

Paul and Singh (2008) investigated phototransformation of metsulfuron methyl and identified major photo products viz., 2-amino-6-methoxy-4-methyltriazine, methyl-2-sulfonyl-amino-benzoate and saccharin (o-sulfobenzoimide).

Sondhia (2008) reported that metsulfuron methyl residues in soil decreased with the passage of time and by 60 days residues were found below detectable limits (<0.001 ?g g-1). At harvest metsulfuron methyl residues were not detected in wheat grain and soil samples.
Sondhia (2008) reported that the residue level of metsulfuron-methyl in soil at harvest was found below the detection limit (<0.001 ?g g-1) at 3, 4 and 5 g a.i./ha application rates and 0.002 ?g/g at 8 g a.i./ha metsulfuron-methyl, respectively. No residues of metsulfuron-methyl were detected in wheat grains at 3 and 4 g a.i./ha rates. However, residues were detected in wheat straw at 5 and 8 g a.i./ha application rates.
Sondhia and Singhai (2008) conducted field experiment at Jabalpur during 2005-06 in rabi season to evaluate persistence of sulfosulfuron residues in soil applied at 25, 50 and 100 g a.i. ha-1 in wheat crop. The rate of dissipation of sulfosulfuron was faster in top soil (0-15 cm) as compared to subsurface soil (15-30 cm). Residues of sulfosulfuron at 100 g ha-1 dose were not present in surface and sub-surface soil after 200 days.
Paul et al. (2009) carried out residue studies of metsulfuron methyl in wheat at 28 days after sowing and found that metsulfuron residues were non – detectable at 15 days, 20 days and 30 days of herbicide application when applied at 4, 8 and 12 g ha-1 respectively. The half-life of metsulfuron methyl ranged from 6.3 to 7.8 days.
Sondhia (2009) conducted a field experiment in Jabalpur to study metsulfuron methyl residues in soil, rice grain and rice straw. The residues of metsulfuron methyl after 30 days of herbicide application in soil were 0.008, 0.010, 0.011 and 0.016 ?g g-1 at 2, 4, 5 and 8 g ha-1 application rate respectively. Soil, rice grains and straw at harvest contained non-measurable residues of metsulfuron methyl (below 0.001?gg-1).
Zhang et al (2009) was also conducted an experiment to study the dynamics and final state of residues of mesosulfuron-methyl soil in Hebei and Hubei of China and found that the dissipation per cent were 61.5 and 53.3; 72.3 and 66.7; 76.9 and 75.6 per cent in soil and 83.7 and 68.2; 87.0 and 80.8; 90.2 and 90.4 per cent in wheat plant in 7, 14 and 21 days after herbicide application, respectively. However, half-life of mesosulfuron-methyl was 10.7 and 9.47 days in soil and 6.42 and 8.32 days in wheat plant in Hebei and Hubei, respectively.

Saini et al. (2010) observed that the sulfosulfuron exist in the 0-15 cm soil depth upto five days after the spray. At subsequent samplings (30 and 60 days after spray and at harvest), the residue of sulfosulfuron, mesosulfuron + iodosulfuron and pinoxaden at the soil depths (0-15 and 15-30 cm) were found to be below 0.02, 0.001 and 0.05 ppm, respectively, which were the minimum detectable limit.

Wang et al. (2010) applied two herbicides chlorsulfuron and imazosulfuron to Podu and Wolylin soils in Taiwan at two levels of concentrations i.e. 10 mg kg-1 and 50 mgkg-1 and noticed that both herbicides (chlorsulfuron and imazosulfuron) dissipated faster in acidic soil. The half-lives of chlorsulfuron and imazosulfuron ranged from 6.8 to 28.4 days and 6.4 to 14.6 respectively.
Singh et al. (2012) studied the persistence of pyrazosulfuron methyl in rice field water and soil at IARI, New Delhi and noticed that dissipation of herbicide was faster in water than soil and followed first order kinetics with half-life of 5.4 and 0.9 days in soil and water respectively. Degradation was faster in non-sterile soil than in sterile soil suggesting the significant role of soil microbes in dissipation of this herbicide.

Arora et al (2013) found that residue of 0.006 and 0.021 ?g/g isoproturon (2.0 kg/ha) and clodinofop (120 g/ha), respectively were detected in post harvest soil whereas no residues of fenoxaprop (240 g/ha) and sulfosulfuron (50 g/ha) were detected in soil. Similarly in wheat grain, isoproturon and clodinofop residues were present at the level of 0.041 and 0.096 ?g/g, which were below the MRL (0.05 mg/kg for isoproturon and 0.1 mg/kg for clodinofop). In wheat grain sulfosulfuron residues were below detectable limit. In straw only isoproturon residue could be detected to the level of 0.022 ?g/g.

Ghosh et al. (2014) conducted a study for two years in B.C.K.V. experimental farm , Mohanpur, West Bengal to investigate the persistence of mixed herbicide formulation (pyroxsulam 4.5 per cent OD+ sulfosulfuron 75 per cent WDG) at 18 g ha-1 and 36 g a.i. ha-1 in wheat plant and field soil. They reported that irrespective of dose more than 47 per cent of initial residues in field soil and 58 per cent of the initial residues in wheat plant were degraded within 3 days. No residues of pyroxsulam and sulfosulfuron were detected in harvest samples of wheat straw, grain and field soil.
Lee et al. (2015) studied orthosulfamuron residues in rice by Quechers method. The limit of quantitation of method used was 0.03 mg kg-1. No residues were detectable in rice grain and rice straw at 116 days after orthosulfamuron spray (harvest).

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