Evaluation of Pendimethalin 38.7 EC on Weed Management in Winter Irrigated Cotton

Field experiments were conducted during winter 2008-09 and 2009-10 at Eastern Block, Tamil Nadu Agricultural


Materials and Methods
Field investigations were carried out at Tamil Nadu Agricultural University, Coimbatore during winter, 2008-09 and 2009-10.Field trials were laid out in randomized block design with treatments replicated thrice.The weed management practices evaluated in the present study consisted of chemical weed control (application of pre-emergence and early postemergence herbicides), cultural practices (mulching with straw) and manual weeding (hand weeding once and twice) and weed free situation (hand weeding 10 times) and unweeded control.

Weed flora
Weed flora of the experimental field was taken at 30 DAS in the control plot.Seventeen weeds were observed in the experimental fields and out of the 17 weeds observed, Cynodon dactylon and Dactyloctenium aegypticum were the dominant grasses, Cyperus rotundus was the only sedge and Trianthema portulacastrum, Digera arvensis and Parthenium hysterophorus were the predominant broad leaved weeds.

Weeds
The density of grasses, sedges and broad leaved weeds increased upto 45 DAS, then there was a decline in weed density.Similar results were observed by Kumar, (2004), who found that hand weeding and earthing up of cotton had smothering effect on weeds at later stages.The weed control methods reduced the density of all the weeds at different stages of crop growth as compared with unweeded control (Table 1).Pendimethalin (38.7%) at 2.0 to 4.0 kg ha -1 followed by one hand weeding at 45 DAS resulted in effective control of grasses, broad leaved weeds and to some extent sedges due to its broad spectrum action.It enters grasses through the coleoptile and shoot of the seedling below the ground.Thus, grasses were effectively controlled with this herbicide.The left over weeds were controlled by manual weeding at 45 DAS.Application of pendimethalin at higher dose of 1.5 kg ha -1 recorded lesser population of weeds when compared to lower doses as was reported by Chander et al. (1997).
Weed dry weight depicted a similar response in line with the weed density in various treatments.The reduced weed density under pendimethalin (38.7%) at 2.0 to 4.0 kg ha -1 resulted in reduced weed dry weight at all the stages of crop growth (Table 2).This might be attributed to rapid depletion of carbohydrate reserve of the weeds through rapid respiration as pointed out by Prakash et al. (1999).The dry weight of grasses, sedges and broad leaved weeds were lesser due to different weed management practices.The dry weight of weeds showed an increasing trend from germination to 90 DAS.It might be due to early germination, establishment and quick growth of weeds than the crop.

Weed control efficiency (WCE)
Weed control efficiency showed the maximum value under pre-emergence application of pendimethalin (38.7%) at 4.0 kg ha -1 at 25 and 45 DAS.Pre-emergence application of pendimethalin registered higher WCE ranging between 93.45 and 65.8 per cent (Table 3).The results of the present study indicated that application of pendimethalin (38.7%) at different doses followed by hand weeding produced higher WCE throughout the crop period which was comparable with the conventional weeding at 45 DAS.The integrated weed management practice gave the broad spectrum weed control as a result of longer persistency in the soil profile.Manual weeding is difficult especially during the monsoon seasons due to intermittent rains and consequently the moisture content of the soil would be too high for mechanical manipulation.Hence, application of pendimethalin (38.7%) at 2.0 to 4.0 kg ha -1 followed by hand weeding is a quite suitable option to overcome the weed problem in cotton.Similar finding was reported by Balasubramanian (1992) who found that the weed control efficiency was comparatively higher with the application of pendimethalin at 1.0 kg ha -1 as compared with 0.5 and 0.75 kg ha -1 .

Seed cotton yield
Pendimethalin (38.7%) at 2.0 kg ha -1 + hand weeding recorded higher seed cotton (Table 4) yield of 58 and 32 per cent during winter 2008-09 and 2009-10 seasons, respectively over unweeded control.The next best treatment was the pendimethalin (38.7%) at 2.5 kg ha -1 + hand weeding.Gnanavel and Babu, (2008) also reported maximum seed cotton yield with pendimethalin and fluchloralin combination coupled with hand weeding as compared with control.Hand weeding twice recorded lower seed cotton yield during winter season due to poor control of grasses and broad leaved weeds.Cotton being a wide spaced and slow  Data not statistically analysed yield.Weeds compete with crop for light, nutrients and water.Hence, the crop under unweeded control may not be able to obtain the above growth factors in optimum quantity resulting in reduced leaf area, dry matter production and number of leaves.This would have reflected in poor yield under unweeded control.Presence of weeds throughout the growing season caused poor crop growth and caused yield reduction in unweeded check (Bhoi et al., 2007).
From the above study, it could be concluded that pre-emergence application of pendimethalin (38.7%EC) at 2.0 kg ha -1 on 30 DAS followed by hand weeding and earthing up on 45 DAS can keep the weed density and dry weight reasonably at a lower level and increase the yield in cotton under irrigated condition.