Influence of Potassium Salt based Glyphosate on Weed Control Efficacy and Yield in Winter Irrigated Cotton

A field investigation was carried out at Tamil Nadu Agricultural University, during winter 2011-12 to evaluate the new formulation of potassium salt based glyphosate (Roundup Crop Shield 460 SL) on weed control in winter irrigated cotton. Experimental results revealed that, post-emergence application of glyphosate at 1800 g a.e. ha -1 at 35 and 70 days after sowing + Intercultural operation as earthing up at 45 and 55 days after sowing resulted in lower weed density, weed dry weight and higher weed control efficiency. With respect to the relative weed density, broad-leaved weeds were higher at initial stages and grassy weeds were predominant at later stages of cotton. Higher seed cotton yield was obtained with POE glyphosate at 1800 g a.e. ha -1 at 35 and 70 DAS with + Intercultural operation at 45 and 55 DAS (2930 kg ha -1 ), which was comparable with PE pendimethalin at 750 g a.i. ha -1 + Intercultural operation at 45 and 55 DAS (2710 kg ha -1 ). Compared treatments without intercultural operation, intercultural operation carried out treatments performed better and recorded more seed cotton yield.

Cotton, regarded as 'white gold' or 'money spinner' is one of the important commercial crop grown throughout the world. In textile industry, cotton plays a vital role in supplying raw materials to the tune of 85 per cent of total requirement and provides employment to about 60 million people in India (COTAAP, 2008). Cotton seed contains 18 -25 per cent oil depending up on quality of seed as well as species.
The weeds which germinate before or simultaneously with the crop are frequently capable of forming a leaf canopy over cotton. Late emerging weeds may interfere with cotton at harvest and may lower lint grade due to lint staining. Factors like inclement weather, non availability of labour for timely weeding cause delay in weeding resulting in yield losses. Weeds growing with cotton offer severe competition causing yield reduction to an extent of 74 per cent (Shelke and Bhosle, 1990). Early control of weeds could be achieved by the use of pre-emerging herbicides. Cotton with minimal weed competition during the initial phase would yield better.
Hand weeding requires more labour, consumes more time leading to higher cost of weeding. Availability of labour is also scarce. An estimate of 400-600 man hrs ha-1 (Tajuddin, 1996) is the normal labour requirement for hand weeding, which also depends upon weed infestation. Mechanical weeding was partially effective because most of the *Corresponding author email : mathumitha08@gmail.com weeds growing in intra rows escape during weeding.
Chemical weed control is easy, time saving and effective.
Pre-sowing and pre-emergence herbicides are not effective against all weeds, whereas; post-emergence herbicides can control weeds, but it needs proper time of application and skill. The combination of pre and post-emergence herbicides are required to be integrated for effective weed control and increased seed cotton yield (Shaikh et al., 2006). Post-emergence control of broadleaf weeds has typically been achieved with directed herbicide applications. For effective weed control and adequate crop safety with postemergence directed herbicides, a height differential between the crop and weeds is required (Culpepper and York, 1998). Hence, an integration of different weed management methods would be the option for the effective weed management and enhancement of productivity in cotton.
Weeds accumulated higher concentration of mineral nutrients than crops, thereby depleting soil nutrients quickly reducing the yield. Number of sympodial branches are significantly reduced due to weed infestation as reported by Balasubramanian and Sankaran (1976). Velayutham et al . (2002) reported that unweeded check reduced the boll number plant-1 and boll weight of cotton. Khan and Khan (2003) reported that grassy weeds cause 15 to 40 per cent and broad leaf weeds 15 to 30 per cent yield losses in cotton crop. Losses caused by weeds in cotton ranged from 45-75 per cent depending upon the nature and intensity of weeds (Sandhu et al., 1996). Hence, the integration of different weed management practices would be a viable option for wide spectrum weed management and enhancement of cotton productivity.

Materials and Methods
With a view to determine the influence of potassium salt based glyphosate on irrigated cotton, the present study was carried out during winter 2011-12 at Tamil Nadu Agricultural University. The soil was sandy clay loam in texture with low in available nitrogen (187.3 kg ha-1), medium in available phosphorus (12.8 kg ha-1) and high in available potassium (559.0 kg ha-1) with pH of 8.44. The experiment was laid out in randomized block design with ten treatments replicated thrice. The treatments consisted of POE glyphosate (Roundup Crop Shield 460 SL) at 1350 (T1), 1800 (T2), 2250 (T3) g a.e. ha -1 , 1800 g a.e. ha-1 + IC (T4), PE pendimethalin at 750 g a.i. ha -1 + IC (T5), HW at 35 and 70 DAS + IC (T6), HWW at 35 and 70 DAS + IC (T7), PWW at 35 and 70 DAS + IC (T8), HW at 25 and 45 DAS (T9) and Unweeded control (T10). Good viable delinted seeds of MRC 7347 BG-II were dibbled on one side of the ridges with a spacing of 90 × 60 cm. The seed rate adopted was 2.5 kg of delinted seeds ha-1. The experimental plots were applied with recommended dose of fertilizers (150:75:75 kg NPK ha-1). Nitrogen, phosphorus and potassium were applied in the form of urea (46 per cent N), single super phosphate (16 per cent P2 O5) and muriate of potash (60 per cent K2O) respectively. Full dose of P and 50 per cent of N and K were applied as basal before sowing and the balance of 50 per cent of N and K were applied as top dressing at 45 and 60 DAS in two equal splits.
The pre-emergence herbicide was applied with pendimethalin (30 EC) at 750 g a.i. ha-1 at 3 DAS followed by earthing up at 45 and 55 DAS.
Calculated quantity of herbicide with a spray fluid of 500 l ha-1 was sprayed uniformly over the plots using hand operated sprayer fitted with fan type nozzle. Calculated quantity of potassium salt based glyphosate (Roundup Crop Shield 460 SL) at three different doses viz., 1350, 1800, 2250 g a.e. ha-1 to the respective treatment plots (T1, T2 and T3) was sprayed at 35 and 70 DAS using hand operated sprayer fitted with fan type and mist type nozzle, respectively under controlled application by using hood. Treatment T4, was sprayed with potassium salt based glyphosate at 1800 g a.e. ha-1 under controlled application with hood at 35 and 70 DAS, followed by earthing up at 45 and 55 DAS. Two hand weeding were done in treatment T6 at 35 and 70 DAS followed by earthing up at 45 and 55 DAS. Hand weeder and power weeder weeding were carried out in treatment T7 and T8, respectively at 35 and 70 DAS followed by earthing up at 45 and 55 DAS. In treatment T9, hand weeding was carried out at 25 and 45 DAS. Number of bolls plant-1 was counted at 90 DAS and the seed cotton (Kapas) was harvested in three pickings commencing on 125 DAS in all the treatment plots. During the experiment, observations on weed characters like weed flora, weed density, weed dry weight were recorded on regular intervals.

Weed flora
Weed flora of the experimental field predominantly consisted of seven species of broad-leaved weeds, four species of grasses and a sedge. Dominant among the grassy weeds were Dactyloctenium aegyptium and Chloris barbata. Trianthema portulacastrum, Parthenium hysterophorus and Commelina benghalensis were the major broad-leaved weeds and Cyperus rotundus was the only sedge present in the experimental field.

Weed density
The total weed density was significantly altered by different weed control treatments. Lesser weed density was observed with HW at 25 and 45 DAS (T9) at 30 DAS followed by PE pendimethalin at 750 g a.i. ha-1+ IC at 45 and 55 DAS (T5). But at 60 DAS, POE glyphosate at 1800 g a.e. ha-1 at 35 and 70 DAS + IC at 45 and 55 DAS (T4), PE pendimethalin at 750 g a.i. ha-1+ IC at 45 and 55 DAS (T5), HW at 35 and 70 DAS + IC at 45 and 55 DAS (T6), HWW at 35 and 70 DAS + IC at 45 and 55 DAS (T7) and PWW at 35 and 70 DAS + IC at 45 and 55 DAS (T8) exhibited good control of total weeds. POE glyphosate at 1800 g a.e. ha-1 at 35 and 70 DAS + IC at 45 and 55 DAS (T4) registered good control over total weeds density at 90 DAS. Unweeded control (T10) resulted in higher weed density at all the stages of observation.

Weed dry weight
Weed management methods significantly influenced the total weed dry weight at all the growth stages. Hand weeding at 25 and 45 DAS (T9) showed lower total weed dry weight and was followed by PE pendimethalin at 750 g a.

Weed control efficiency
Higher weed control efficiency of 98.11 per cent was recorded in HW at 25 and 45 DAS (T9) at 30 DAS. At 60 DAS, higher weed control efficiency (97.38 %) was observed with PE pendimethalin at 750 g a.i. ha-1 + IC at 45 and 55 DAS (T5) and was closely followed by POE glyphosate at 1800 g a.e. ha-
Unweeded control (T10) recorded lower seed cotton yield of 911 kg ha-1. Compared to nonintercultural operation treatments, intercultural operation carried out treatments registered more seed cotton yield. POE glyphosate at 1800 g a.e. ha-1 at 35 and 70 DAS + IC at 45 and 55 DAS (T4), PE pendimethalin at 750 g a.i. ha-1 + IC at 45 and 55 DAS (T5) and HW at 35 and 70 DAS + IC at 45 and 55 DAS (T6) recorded higher seed cotton yield than non-intercultural operation treatments.

Weed index
Seed cotton yield of POE glyphosate at 1800 g a.e. ha-1 at 35 and 70 DAS + IC at 45 and 55 DAS (T4) was taken as basis to work out the weed index (WI). PE pendimethalin at 750 g a.i. ha-1 + IC at 45 and 55 DAS (T5), followed by HW at 35 and 70 DAS + IC at 45 and 55 DAS (T6) performed better and recorded lower weed index. POE glyphosate at 1350 g a.e. ha-1 at 35 and 70 DAS (T1) and HWW at 35 and 70 DAS + IC at 45 and 55 DAS (T7) were found to record higher weed index. Unweeded control (T10) resulted in higher weed index and performed poorly.

Conclusion
Experimental results revealed that, postemergence application of glyphosate at 1800 g a.e. ha-1 at 35 and 70 days after sowing + Intercultural operation as earthing up at 45 and 55 days after sowing resulted in lower weed density, weed dry weight and higher weed control efficiency. With respect to the relative weed density, broad-leaved weeds were higher at initial stages and grassy weeds were predominant at later stages of cotton. Highest seed cotton yield was obtained with postemergence application of glyphosate at 1800 g a.e. ha-1 applied at 35 and 70 DAS with intercultural operation of earthing up at 45 and 55 DAS which was comparable with pre-emergence application of pendimethalin at 750 g a.i. ha-1 + intercultural operation at 45 and 55 DAS. The treatments with intercultural operation performed better in yield attributes and higher yield compared to treatments without intercultural operation.