Preceding (Rice-Okra) Crops Herbicide Residual Effects on Weed, Growth, Yield and Economics of Succeeding Blackgram Under Different Ecosystem

Demand for pulses is increasing tremendously in the world to meet the daily protein requirement of fast growing population.  Pulses are called poor man’s meat because of these are rich in protein, carbohydrate, fat, amino acid, vitamins and minerals. Among the pulses, blackgram is one of the most important pulse crops in India. Blackgram is more suitable for various cropping system due to its short duration in nature and also enriches the soil by adding nitrogen and improves the physio-chemical and biological proprieties of soil. India is the largest producer and consumer of blackgram in the world and it is cultivated in 4.48 million ha with a production of 2.83 million tonnes and average productivity of 641 kg/ha. Tamil Nadu and Andaman Nicobar Islands produced 2,73,960 tonnes and 240 tonnes blackgram from 4,29,780 ha and 760 ha with the productivity of 609 and 322 kg/ha, respectively (India stat, 2018). Among the factors responsible for the reduction of blackgram yield, weeds are considered as prime important factor as the losses caused by weeds are exorbitant. Mishra and Bhanu (2006) reported that the yield of summer blackgram was reduced upto 63.2% when the weeds are allowed to grow throughout the crop season. Early post-emergence herbicide to control weeds with less residual toxicity would be more appropriate for better weed management. Nowadays, several new herbicide formulations are being developed and tested for weed control in leguminous crops. Low dose herbicide formulation is recommended for each crops in cropping system and sequential application of herbicides for each crop leads to residue accumulation in soil which cause adverse effect on succeeding crops reported by Sathyapriya et al., (2017). According to Naidu et al., (2012) post-emergence (POE) imazethapyr at 200 g/ha caused crop injury to blackgram to the extent of 60% under rice fallow condition. Persistence of pendimethalin in soil under diverse crops like gram, rajma, lentil, linseed, safflower, fenugreek, fennel and coriander was observed upto 75 DAS, there after there was no residue observed by Arora and Tomar (2008). Post-emergence herbicides have persisted in the soil for longer period and cause susceptible to succeeding crop (Singh et al., 2014). Persistence of herbicide in the soil is an important consideration for recommending it to the farmers as it is related to length of the time that the herbicide remains active. It also causes residual phytotoxocity which may injure the succeeding crops. With the views of above facts, the present investigation was done to find out the preceding (Rice-Okra) crops herbicide residual effects on weed, growth, yield and economics of succeeding blackgram crop.

 
 

Effect on weeds
 
Major weed species observed in the mainland experimental fields were Echinochloa colona, Leptochloa chinensis, Panicum flavidum, Cynodon dactylon among grasses, Cyperus rotundus, Fimbristylis miliaceae, Cyperus difformis in sedges, Eclipta alba, Trianthema portulacastrum, Ammannia baccifera, Convolvulus arvensis, Phyllanthus maderaspatensis, Phyllanthus niruri, Boerhavia diffusa, Cleome viscosa species in BLW. Whereas, in island ecosystem, among the grassy weeds, Echinochloa colona, Leptochloa chinensis, Acrachne racemosa, Setaria glauca, among the sedges, Cyperus haspan, Cyperus iria, Cyperus eragrostis, Fimbristylis miliaceae, among the BLW, Wedelia chinensis, Ammannia baccifera, Phyllanthus maderaspatensis, Phyllanthus niruri, Boerhavia diffusa, Cleome viscosa were the most dominant weed flora were observed in blackgram experimental field.
 
Total weed density and dry weight
 
Total weed density and weed dry weight in the blackgram were significantly influenced by the residual effect of herbicide applied to preceding crops (Table 1). Significant reduction of total weed density of 12.67 and 14.67 /m² was observed with residual effect of butachlor at 1.25 kg/ha on 3 DAT fb HW on 40 DAT in rice+ HW twice at 20 and 40 DAS in okra and blackgram (T₆) in mainland and island ecosystem, respectively which was statistically on par with residual effect of oxadiargyl loaded in biochar applied at 3 DAT + PE-metribuzin at 500 g/ha on 3 DAS fb POE quizalofop-ethyl at 50g/ha on 20 DAS(T₁). Similar trend was observed in total weed density at 40 DAS. This might be due to the residual effect of preceding crop and HW twice reduced the weed density and weed dry weight. Similar result was reported by Bommayasamy and Chinnamuthu (2019). Residual effect of butachlor at 1.25 kg/ha on 3 DAT fb HW on 40 DAT in rice+ HW twice at 20 and 40 DAS (T₆) in okra and blackgram recorded 8.75 and 7.44 times reduction in total weed dry weight under main and island ecosystem, respectively at 20 DAS. Whereas, at 40 DAS, total weed dry weight of 5.15 and 7.22 times reduction was observed in T₆ as against weedy check (T₈). In island ecosystem, similar trend was observed in weed dry weight as that of mainland ecosystem at 20 and 40 DAS.
 

 
Weed control efficiency
 
Residual effect of butachlor at 1.25 kg/ha on 3 DAT fb HW on 40 DAT in rice + HW twice at 20 and 40 DAS (T₆) in okra and blackgram recorded the highest weed control efficiency of 88.6, 86.5 and 86.2, 80.7% at 20 and 40 DAS under mainland and island ecosystem, respectively. Higher weed control efficiency may be contributed to substantial reduction in weed growth and density owing to the use of pre and post-emergence herbicides followed by hand weeding. It was in line with the results of Mansoori et al., (2015).
 
EFFECT ON CROP
 
Plant growth attributes
 
Herbicide residual effect of preceding crops weed control treatment did not show any significant difference on blackgram germination. Significant differences in plant height were noticed with herbicide residual effect and their weed control treatment of preceding crops (Table 2). In mainland ecosystem, higher plant height recorded in weed free check (T₇) which was comparable with residual effect of butachlor at 1.25 kg/ha on 3 DAT fb HW on 40 DAT in rice + HW twice at 20 and 40 DAS in okra and blackgram (T₆). Whereas, in island ecosystem, these two treatments were on par with oxadiargyl loaded in biochar applied at 3 DAT in rice+ PE-metribuzin at 500 g/ha on 3 DAS fb POE quizalofop-ethyl at 50 g/ha on 20 DAS in okra (T₁).
 

        
Herbicide residual effect of preceding crops showed significant influence on leaf area index in both the ecosystems. In mainland ecosystem, weed free check (T₇) was recorded significantly higher LAI of 5.00 which was comparable with residual effect of butachlor at 1.25 kg/ha on 3 DAT fb HW on 40 DAT in rice+ HW twice at 20 and 40 DAS in okra and blackgram (4.92) (T₆). Whereas, a similar trend was followed in island ecosystem. This might be due to higher leaf area at flowering stage enabled the increased source-sink relationship through higher photosynthetic area of crop. This is in conformity with the findings of Rajput et al., (2017).
        
Root characters like root length and root dry weight showed significant influence in herbicide residual effect of preceding crops at 50 DAS. Weed free check (T₇) and residual effect of butachlor at 1.25 kg/ha on 3 DAT fb (HW) on 40 DAT in rice + HW twice at 20 and 40 DAS in both okra and blackgram was recorded 39.6, 46.9% and 51.4, 43.8 kg/ha of higher root length and root dry weight in main and island ecosystem, respectively compared to weedy check (T₈) at 50 DAS. Rooting depth positively correlated with plant height, root dry weight and root thickness. In both the ecosystem, the shorter root length and lower root dry weight were recorded in weedy check (T₈). The results were in conformity with Bommayasamy et al., (2018).
 
Crop yield attributes and yield
 
Number of pods/plant and seed yields were significantly influenced by the herbicide residue and their weed control treatments imposed to previous rice and okra crops in both the ecosystems (Table 3). Among the weed control treatments, residual effect of butachlor at 1.25 kg/ha on 3 DAT fb HW on 40 DAT in rice + HW twice at 20 and 40 DAS in both okra and blackgram (T₆) recorded 2.15 and 1.83 times higher number of pods/plant than weedy check (T₈) under mainland and island ecosystem, respectively. There was no significant difference observed in seed test weight in all the treatments in both the ecosystems. Seed test weight is a genetic character of plant which doesn’t changes with management practices whereas it differs numerically. 
 


Among the weed control treatments, residual effect of butachlor at 1.25 kg/ha on 3 DAT fb HW on 40 DAT in rice+ HW twice at 20 and 40 DAS (T₆) in okra and blackgram recorded 64.8% higher seed yield under mainland ecosystem. The next order best treatments were combined residual effect of oxadiargyl loaded with biochar on 3 DAT in rice + PE-metribuzin at 500 g/ha on 3 DAS fb POE-quizalofop-ethyl at 50 g/ha on 20 DAS in okra (T₁), combined residual effect of oxadiargyl encapsulated with starch on 3 DAT + PE-metribuzin at 500 g/ha on 3 DAS fb twin-wheel hoe weeder at 40 DAS (T₃) and oxadiargyl loaded with zeolite on 3 DAT in rice + PE-metribuzin at 500 g/ha on 3 DAS fb HW on 40 DAS (T₂) in ok. These three treatments were on par with one another and registered 34.5, 33.4 and 18.4% higher grain yield compared to weedy check (T₈). Similar trend was observed in island ecosystem also. The higher seed yield in these treatments was due to effective control of weeds in preceding crops which reduced weed density and dry weight. The similar findings reported by Kumar and Singh (2018). Significantly the lowest seed yield was recorded under weedy check (T₈) in both the ecosystem.
 
Economics
 
The data on the economic analysis of blackgram are presented in Table 3. Weed free check recorded higher net return of Rs. 17175/- and Rs. 9457/- with the cost benefit ratio of 1.49 and 1.24 under main and island ecosystem, respectively. This might be due to continuous weeding in preceding crops reduced build up of soil weed seed bank and minimize production cost in succeeding crops. Among weed control treatments, residual effect of oxadiargyl loaded in biochar applied at 3 DAT in rice+ PE-metribuzin at 500 g/ha on 3 DAS fb POE quizalofop-ethyl at 50g/ha on 20 DAS in okra recorded Rs. 9,420 higher net return than weedy check (T₈). It may be due to severe reduction in weed density and dry weight which enhances seed yield of blackgram. It was in conformity with the observation of Raman et al., (2005).

It can be concluded that combined residual effect of oxadiargyl loaded with biochar on 3 DAT in rice + PE- metribuzin at 500 g/ha on 3 DAS fb quizalofop-ethyl at 50 g/ha on 20 DAS in okra for realizing the higher weed control, productivity and profitability under the constrained situation of manual weeding.