Evaluation of Pre and Post Emergence Herbicides in Greengram (Vigna radiata L.) during Kharif and Rabi Seasons in the Uplands of Krishna Zone of Andhra Pradesh

Greengram is mostly grown as a kharif crop in Andhra Pradesh in the upland areas, prior to the sowing of the main crop. Because of its short duration, moderate drought resistance, it is taken up as a catch crop in both kharif and rabi seasons. Initial weed free situation is essential for this crop as the critical period of weed competition is up to 20- 30 days after sowing (DAS) and the yield losses due to weed infestation during this period ranges from 30-85 per cent (Singh et al., 2015). Due to shortage of human labour, weed control through manual means is not feasible and also uneconomical. In spite of, pre and post emergence herbicide recommendations for control of weeds in greengram there is large variation in the weed intensity and species grown in kharif and rabi greengram and that may be due to the rainfall pattern or irrigation schedules or cropping sequence. Significantly lower weed intensity and dry weight in greengram with pre emergence application of pendimethalin 30% + imazethapyr 2% @ 1.0 kg/ha and higher benefit cost ratio as compared to hand weeding treatment was reported by several workers (Tamang et al., 2015,  Kumar et al., 2019).
       
However, some of the weed species like Phyllanthus spp. during kharif, Chrozophora spp. and Phyllanthus spp. in rabi season are not controlled even after the application of the already available herbicides. This might be due to late emergence of weeds or may be due to development of certain resistance in the weeds to these herbicides. Some ready or tank mixes of compatible pre and post emergence herbicides with varying modes of action may ensure effective control of different weed species and may check the development of herbicide resistance and hence, some of the pre and post emergence herbicides mixtures available in the market need to be tested for both kharif and rabi seasons for effective control of weeds in greengram.

A field experiment was conducted during kharif and rabi seasons of 2018 at Regional Agricultural Research Station, Lam, Guntur under natural weed infestations in greengram. The soil type of the experimental field was clay loam with pH 7.8, low in available nitrogen, medium in available phosphorous and high in available potassium. The experiment consisted of nine treatments i.e. pendimethalin 30% + imazethapyr 2% as PE @ 0.75+0.05 kg a.i./ha, pendimethalin 30% as PE @  0.75 kg a.i./ha,  acifluorfen sodium 16.5% +clodinafop propargyl 8.0% as PoE @165+80 g a.i./ha, fomesafen 11.1%+fluazifop -p-butyl 11.1% as PoE @ 0.111kg a.i./ha, imazethapyr 35%+ imazamox 35% as PoE @ 35 g/ha,  propaquizafop 2.5% + imazethapyr 3.75% @ 50 + 75 g  a.i./ha as PoE at 20  DAS, imazethapyr 10% as PoE @ 50 g a.i./ha, two hand weedings at 20 and 40 DAS along with inter cultivation at 30  DAS and unweeded  check. 
       
The experiment was laid in randomized block design with three replications with gross experimental plot area of 6.0´3.6 m. The recommended basal dose of 20 kg nitrogen and 50 kg phosphorous per ha were applied at the time of sowing in the form of urea and single super phosphate, respectively. In both the seasons the crop was raised under rainfed conditions. Greengram, variety, IPM 2-14 was sown at a spacing of 30´10 cm, on 25^th June and 22^nd October, respectively during kharif and rabi seasons of 2018. An amount of 562.7 mm rainfall was received from June to October months and there was no rain after the sowing of rabi crop. All the recommended packages of practices were followed for the proper establishment of crop except weed management practices. All the PE and PoE herbicides were sprayed with knapsack sprayer fitted with flood jet nozzle as per schedule using a spray volume of 500 l ha^-1. The weed species were recorded by placing the quadrate of 50 cm×50 cm in the sampling area at 20, 45 DAS and at harvest. The weed samples for dry weight were collected from the area used for weed count.  The crop was harvested on 28.8.2018 and on 25.12.2018 during kharif and rabi seasons, respectively. The data on density of different weed species and their dry weight per square meter were recorded at 20, 45 DAS and at harvest stages of crop growth. The weed dry weight was taken after sun drying for 2-3 days followed by oven drying at 70°C for 48 hours. The weed control efficiency (WCE) was calculated at 45 DAS using the formula:
   
Where,
WCE = Weed control efficiency in percentage.
x =  Weed dry weight per unit area in weedy check.
y = Weed dry weight per unit area in treatment plot for which WCE is to be calculated.
       
The data of weed density and dry weight were subjected to square root transformation (Ö x+ 0.5) before statistical analysis to normalize their distribution (Panse and Sukhatme, 1978). Yield and yield attributes of crop were recorded at the time of harvest of crop.

Weed growth and weed control efficiency
 
The weed flora observed during kharif in the experimental plots were Phyllanthus madaraspatanense, Trianthema portulacastrum, Digera arvensis, Cynotis cucullata and Commelina benghalensis among broad leaf weeds and Echinichloa colona and Panicum ramosum were among grassy weeds. In addition to these species, during rabi, Chrozophora rottleri was the dominant dicot weed observed. The experimental plot was dominated by broad leaved weeds as compared with grasses or sedges.
       
Pre emergence application of pendimethalin 30%+ imazethapyr 2% at 0.75+0.05 kg ha^-1 and pendimethalin 30% at 0.75 kg ha^-1 significantly reduced both weed density and dry matter at 20 DAS during kharif season (Table 1). Similar reduction due to PE application of pre-mix pendimethalin + imazethapyr at 1.0 and 0.75 kg/ha was reported by Singh et al., 2017. At 45 DAS, weed density and dry matter of all the PE and PoE herbicide applications was significantly lower than that of unweeded treatment. Among the PoE herbicides imazethapyr 10% at 50 g ha^-1 and propaquizafop 2.5% + imazethapyr 3.75% at 50 + 75 g ha^-1 were effective in controlling both density and dry matter of weeds and on par with that of hand weeding treatment. In greengram weed density and dry weight was also observed to be low with PoE application of imazethapyr by Kumar et al., 2016. The WCE of all  the PE and PoE treatments except acifluorfen sodium 16.5% + clodinafop propargyl 8.0% at 165+80 g ha^-1 (ranging between 74.4 to 98.5%) was on par with hand weeding treatment at 45 days stage indicating that any one of the PE or PoE treatments can effectively control the weeds in kharif season. The weed control efficiency was highest in hand weeding treatment with intercultivation (99.8%). Aktar et al., 2015 reported similar results in greengram. Acifluorfen sodium 16.5% +clodinafop propargyl 8.0% was not effective in kharif in controlling the weed population in greengram.
 

       
Post emergence application of imazethapyr 35%+ imazamox 35% at 35 g ha^-1, fomesafen 11.1%+fluazifop-p-butyl 11.1% at 0.111 kg ha^-1 and propaquizafop 2.5% + imazethapyr 3.75% at 50 + 75 g ha^-1 were able to check the density of broad leaved weeds of greengram up to 45 DAS and fomesafen 11.1%+fluazifop-p-butyl 11.1% controlled the BLW population even up to harvest stage during kharif (Table 2) which may be due to the two herbicide combination of the product. However, the grass weed density was not significantly influenced by both pre and post emergence treatments at all stages of crop growth.
 

       
During kharif, among the dicot weeds, the density of Phyllanthus (Table 3) was significantly low with pre emergence treatments at 20 DAS. But later at 45 and at harvest stages, the post emergence herbicide imazethapyr 35%+ imazamox 35% recorded lesser density of Phyllanthus and was on par with hand weeding. Pre emergence pendimethalin 30% + imazethapyr 2% recorded a Phyllanthus population which was significantly lower than unweeded treatment at 45 DAS indicating that pendimethalin 30% is not effective for the control of Phyllanthus weed which may be due to the late emergence of the weeds in the season.
 

       
During rabi, the PE herbicides significantly reduced the dry matter of weeds but not the weed density, at 20 DAS as compared to unsprayed treatment (Table 4). At 45 DAS, weed density and dry weight of PE application of pendimethalin 30% + imazethapyr 2% was on par with that of hand weeded treatment indicating the superiority of this treatment as compared with spray of pendimethalin 30%  in controlling the weed density and dry weight. All the PoE herbicides are comparable to hand weeding in both density and dry matter of weeds with no significant difference in the WCE recorded at 45 days after sowing.
 

       
Among the PE herbicides, pendimethalin 30% + imazethapyr 2% reduced the density and dry matter of Phyllanthus as compared to the pendimethalin 30% and both the PE herbicides were not effective for the control of Chrozophora during rabi season (Table 5). All the PoE herbicides tested were able to reduce the Phyllanthus density at 45 DAS on par with that of hand weeded treatment indicating that PoE herbicide application is essential for control of Phyllanthus weed in greengram. However, the Chrozophora, the problem weed in rabi greengram could not be controlled by any of the post emergence  herbicides tested during rabi.
 
  
 
During kharif, the seed yield of greengram recorded with application of all the PE and PoE herbicides (1098 to 1326 kg ha^-1) was significantly superior over that of unweeded control (730 kg ha^-1) and on par with two times hand weeding control with inter cultivation (1407 kg ha^-1) (Table 6). The yield increase was mainly due to increased number of pods per plant which was lowest in unweeded treatment (17.9) and ranged from 27.5 to 32.9 in other treatments. Kumar et al., 2019 reported higher yield of greengram due to pre emergence herbicide pendimethalin 30 EC + imazethapyr 2 EC at 1.0 kg ha^-1 followed by one hand weeding.
 

       
Similarly, during rabi also increase in yield in all the herbicide treatments was recorded except imazethapyr 35%+imazamox 35% (Table 7) and the yield increase was mainly due to higher pod number. The yield reduction in imazethapyr 35%+imazamox 35% application treatment might be due to reduced plant height and no. of pods per plant indicating certain toxicity on plant growth during rabi season. Sprays of some herbicides despite of selectivity mechanisms in crops may induce oxidative stress effecting leaf expansion and biomass accumulation (Cobb and Reade, 2010).
 

Among the two PE herbicides tested on greengram during kharif and rabi seasons, pendimethalin 30%+imazethapyr 2% at 0.75 + 0.05 kg ha^-1 was found to be more effective in reducing the weed density and dry matter and recorded higher yield as compared with pendimethalin 30%. The PoE herbicides mixtures tested viz., fomesafen 11.1% + fluazifop-p-butyl 11.1% at 0.111 kg ha^-1, propaquizafop 2.5%+ imazethapyr 3.75% at 50+75 g ha^-1 were effective in reducing weed density and weed dry matter and recorded seed yield on par with imazethapyr 10% at 50 g  ha^-1 in both the seasons in greengram in the uplands of Krishna zone of Andhra Pradesh.