Performance of Early Post-Emergence Herbicide in Irrigated Soybean ( Glycine max (L.) Merill)

A study was carried out to evaluate the effect of early post emergence herbicide, imazethapyr against weeds in irrigated soybean ( Glycine max (L.) Merill) at Agricultural Research Station, Bhavanisagar. Weeds, viz ., Dactyloctenium aegypticum , Acrach ne racemosa, Cyperus rotundus , Boerhaavia diffusa, Digera arvensis, Parthenium hysterophorous were the dominant weeds in the experimental field. Imazethapyr at 100 and 200 g ha -1 applied on 15 DAS provided 87 to 91 % weed control efficiency compared to unweeded control. However, imazethapyr at 200 g ha -1 had slight phytotoxicity on soybean in the initial stages. Higher grain yield of 1645 kg ha -1 was obtained with application of imazethapyr 100 g ha -1 .

Soybean (Glycine max (L.) Merill) is a dual purpose, important crop to meet protein and oil requirements.It has a great nutritional significance, with over 40 % protein and 20 % oil and has been recognized as a potential supplementary source of edible oil.The combined effect of weeds, insects and diseases sets serious limitations to increasing agricultural production.The reduction in yield of soybean ranged from 10 to 73 per cent due to weed competition and the yield can be enhanced by almost 50 per cent by adopting timely chemical weeding (Tiwari and Trivedi, 1985).Critically viewing, the manual and mechanical methods of weed control, besides being less effective, are costly and time demanding.Mechanical method was partially effective because most of the weeds growing in intra-rows escaped in weeding.Thus, chemical weed control became a promising means to control weeds at initial stages of crop growth.Many pre -emergence herbicides control weeds only for a limited period and hence late emerging weeds escape killing.So, there is ample scope for controlling weeds by application of early post emergence herbicides.Hence, the present experiment was carried out to evaluate the efficiency of Imazethapyr for control of weeds in soybean and its influence on productivity.
The post emergence herbicide imazethapyr at 50, 75, 100, 200 g ai ha -1 was sprayed 15 days after sowing.Data on total weed density and weed dry weight were recorded 30 days after sowing using 0.25 m 2 quadrate at 4 places in randam and analysed after subjecting the original data to log transformation.The total annual rainfall received during the crop season was 610 mm distributed in 28 rainy days.

Weed density
Effect of imazethapyr was found significant in reducing the broadleaved weeds and grasses.At 30 days after sowing, hand weeding on 25 DAS showed low broadleaved density followed by early post emergence application of imazethapyr 200 g ha -1 and imazethapyr 100 g ha -1 .Whereas the remaining treatments recorded higher broadleaved weed density.It was more effective against broadleaved weeds and it controlled all major broadleaved weeds like Boerhaavia diffusa, Digera arvensis, Parthenium hysterophorous and Trichodesma indicum.Hand weeding on 25 days after sowing recorded lower grass weed density followed by early post emergence application of imazethapyr at 100 and 200 g ha -1 (Table 1.) Weed dry weight showed a decreasing trend with increasing rate of imazethapyr.Among the weed control treatments, early post emergence application of imazethapyr at 100 and 200 g ha -1 significantly reduced dry matter of weeds.The weed control efficiency was tremendously enhanced (77-91%) due to higher dose of imazethapyr at 30 days.However, imazethapyr at 200 g ha -1 had phytotoxic effect on soybean in the initial stages causing stunted growth and chlorosis but the crop recovered after one or two irrigations (Table 2).

Grain yield
Early post emergence application of Imazethapyr at 100 g ha -1 significantly increased the grain yield compared to other treatments.Whereas 200 g ha -1 had little phytotoxic effect causing chlorosis and stunted growth during initial stages and it got recovered after one or two irrigation, thus caused reduction in yield.Chandel and Saxena (2001) found that post emergence application of imazethapyr at 100 g ha -1 was effective in controlling weeds and increasing seed yield and weed control efficiency.Tewari et al. (2004) also found that imazethapyr at 100 g ha -1 as pre emergence excelled all the herbicide treatments with respect to effective weed  Thus, imazethapyr at 15 days after sowing is suitable for the field infested with more of broadleaved weeds and grasses than sedges.

Conclusion
From the results of the present study, it can be concluded that early post emergence application of imazethapyr at 100 g ha -1 15 DAS with hand weeding on 45 DAS provided better weed control and resulted in increased yield compared to other weed control methods.
original values, Data subjected to log transformation (log(x+2)), NA-Not analysed control (64.4 per cent WCE) and increased grain yield at par to manual weeding twice in green gram.

Table 1 . Effect of weed control treatments on density of weeds in soybean
EPOE-Early post emergence, E.up-Earthing up, DAS-Days after sowing, PW-Power weeder, HW-Hand weeding, Figures in parenthesis areoriginal values, Data subjected to log transformation (log(x+2))

Table 2 . Effect of weed-control treatments on dry matter of weeds at 30 DAS and yield of soybean
EPOE-Early post emergence, E.up-Earthing up, DAS-Days after sowing, PW-Power weeder, HW-Hand weeding, Figures in parenthesis are