Bioefficacy, Phytotoxicity and Economics of Post-emergence Herbicides in Moth Bean (Vigna aconitifolia) in Hot Arid Regions

A field experiment was conducted at ICAR-Central Arid Zone Research Institute, Jodhpur (27°18¢N latitude and 73°01¢E longitude) from 2017 to 2019. The experimental field was well-drained with loamy sand soil low in organic carbon (0.18%) and available nitrogen (170 kg ha^-1), medium in available phosphorous (16.8 kg ha^-1) and potassium (277 kg ha^-1) with slightly alkaline in reaction (pH 8.3). The region’s average annual rainfall is about 365 mm, mainly received during the rainy season.
       
Eleven treatment combinations comprising pre-emergence and post-emergence herbicides along with hand weeding, weed-free and unweeded were applied in Kharif moth bean (Table 1). The experiment was laid out in randomized block design with three replications keeping the plot size 5 m × 3 m. The herbicides were sprayed with a knapsack sprayer fitted with a flat fan nozzle using a water volume of 500 l ha^-1.
 

       
The rainfed moth bean was sown during the first fortnight of July and harvested in the second fortnight of September. Moth bean ‘CAZRI Moth-2’ 12 kg ha^-1 was planted at 30 cm row spacing with the help of a seed drill. The recommended dose of fertilizers (10 kg N + 20 kg P) was applied as basal through urea and DAP. The crop was managed uniformly throughout the growth period, except for weed control.
       
Observations on weed infestation were recorded after harvest of crops with a quadrate of 0.5 × 0.5 m (0.25 m²) size placed at two randomly selected places in each plot, leaving an area of 30 cm from the plot borders. Weed density (no. m^-2) was recorded by categorizing the sampled weeds into grasses, broad-leaved and sedges. For recording dry matter of weeds (g m^-2), the categorized weeds were first air-dried for five days and then oven-dried at 65±5°C until constant dry weight (~48 hrs). Phytotoxicity of herbicides was recorded at 10days after application on a 0-10-point visual score scale as suggested by Rao (2000).
       
The crop dry weight was recorded from five randomly selected plants from each plot just before harvesting. Seed yield and stover yield (kg ha^-1) were recorded based on the yield obtained from the net plot (2 m × 3 m). Economics was computed based on prevailing market prices of products and inputs. Weed control efficiency (WCE) and weed index (WI) were calculated by using the standard formula suggested by Mani et al., (1973). Impact indices, viz. weed persistence index (WPI), crop resistance index (CRI) and herbicide efficiency index (HEI), were worked out as per Sen et al., (2020).
       
The collected data for weed and crop parameters were statistically analyzed separately using SPSS (version 16.0).  Before statistical analysis, the weed density and dry weed weight data were subjected to square root transformation [(x+0.5)^1/2]. Data analysis was done through analysis of variance using the F-test. Means were separated at a 5% level of significance as per Tukey’s honest significant difference test.

Weed interference
 
In terms of weed flora composition, grassy weeds were dominant, constituting about 39.6-65.3% of total weed density across the treatments, followed by broad-leaved weeds (26.3-46.6%) and sedges (6.3-28.2%). However, in terms of total weed dry matter, across the treatments dominating group was broad-leaved weeds (56.7-71.3%), followed by grassy weeds (20.7-35.4%) and sedges (4.4-13.6%) (Table 2).
 

       
Weed density and dry matter at harvest differed significantly among the weed management practices (Table 2). Among the herbicide treatments, propaquizafop + imazethapyr (PoE 125 g a.i. ha^-1) (T₅), being at par with other imazethapyr treatments (T₂, T₃ and T₄) and hand weeding (T9), recorded significantly lower total weed density (21.3 m^-2). The lowest per cent reduction in total weed dry matter compared to unweeded was recorded under clodinafop-propargyl + sodium acifluorfen @ 187.5 g a.i. ha^-1 (T₆) (41.6%). The imidazolinones herbicides (imazethapyr and imazamox) are absorbed both by the roots and the shoots (Saltoni et al., 2004); hence, imazethapyr in combination with propaquizafop provided wide spectrum weed control and resulted in lesser weed counts and produced lower weed dry matter. These findings are in line with Khairnar et al., (2014).
       
Application of clodinafop-propargyl + sodium acifluorfen both at 250 g a.i. ha^-1 (T₇) and 312.5 g a.i. ha^-1 (T₈) also led to weed dry matter suppression at par to imazethapyr treatments (T₂, T₃, T₄ and T₅) and hand weeding (T₉). Minimum weed density and weed dry matter under propaquizafop + imazethapyr (PoE 125 g a.i. ha-1) (T₅) are presumably due to effective control of grassy and broadleaf weeds, especially at the early growth stages. Further, clodinafop-propargyl + sodium acifluorfen @ 312.5 g a.i. ha^-1 (T₈) also caused effective weed control and led to 66.3% and 73.3% reductions in total weed density and total weed dry matter over unweeded (T₁₀). A narrow-spectrum activity against weeds (Table 2) rendered clodinafop-propargyl + sodium acifluorfen less effective at lower doses (T₆ and T₇). However, clodinafop-propargyl + sodium acifluorfen (@ 312.5 g a.i. ha^-1) (T₈) could cause sufficient growth suppression and found at par with imazethapyr treatments in weed dry matter reduction. Harithavardhini et al., (2016) reported higher efficacy of clodinafop-propargyl + sodium acifluorfen over imazethapyr in mung bean.
 
Crop phytotoxicity
 
The phytotoxicity rating of applied herbicides on crop growth revealed that PoE application of imazethapyr-containing herbicides (T₂, T₂, T₄ and T₅) caused a phytotoxic effect (score 1-2) on moth bean. The phytotoxic effect of herbicides caused stunting and discolouration of leaves which persisted up to flowering and reflected on the final yield of the crop as well (Table 3). Similar results of imazethapyr phytotoxicity were reported earlier by Punia et al., (2015) on mung bean.
 

 
Crop growth and yield
 
The favourable growth environment in weed-free (T₁₁) treatment resulted in the significantly highest crop dry weight (613 g m^-2), seed yield (6.92 q ha^-1) and stover yield (23.9 q ha^-1) over other treatments (Table 3). On the other hand, the reduction in seed yield in unweeded compared to weed-free and hand weeding was 50.8% and 42.1%, respectively. These results are in agreement with as reported by Upadhayay et al. (2013). Moreover, clodinafop-propargyl + sodium acifluorfen (PoE 312.5 g a.i. ha^-1) (T₈) being at par with propaquizafop + imazethapyr (PoE 125 g a.i. ha^-1) (T₅) and hand weeding (T₉) recorded significantly higher crop dry weight (513 g m^-2). Thus, the variation in crop dry weight under different herbicides was ascribed both due to effective weed control and the phytotoxic effect of imazethapyr on moth bean. The significant difference in crop growth under the different magnitude of crop-weed interference resulted in significant differences in the yields of moth bean (Table 3). On the other hand, the favourable environment provided due to the higher weed control efficacy of imazethapyr-containing treatments (T₂, T₃, T₄ and T₅) was suppressed by the phytotoxic effect on moth bean and it was manifested in the growth and yield of the crop.
       
Because of its at par weed control efficacy and no phytotoxic effect on moth bean, clodinafop-propargyl + sodium acifluorfen (PoE 312.5 g a.i. ha^-1) (T₈) being at per with hand weeding (T₉), recorded significantly higher seed yield (5.53 q ha^-1) over imazethapyr containing treatments (T₂, T₃, T₄ and T₅) and pendimethalin (T₁). However, in terms of stover yield, clodinafop-propargyl + sodium acifluorfen (PoE 312.5 g a.i. ha-1) (T₈) was found at par with propaquizafop + imazethapyr (PoE 125 g a.i. ha^-1) (T₅) and hand weeding (T9). The higher yield under these treatments might be due to effective control of the weeds as indicated through higher WCE. Clodinafop-propargyl + sodium acifluorfen (PoE 312.5 g a.i. ha^-1) (T₈) produced 162.4% and 180.0% higher seed yield and stover yield over unweeded, respectively. In this study, weed competition and crop phytotoxicity have manifested in crop growth and yields.
 
Economics
 
The higher seed and stover yield recorded under weed-free (T₁₁) treatment resulted in the highest gross returns and net benefits of moth bean, which was closely followed by hand weeding (T₉)(Table 4). These results corroborated with the finding of Ram Pratap et al., (2018). The weed management in moth bean, either through hand weeding or effective PoE herbicides (T₈), accounts for around 24-25% of the cost of cultivation. Although imazethapyr-containing herbicides (T₂, T₃, T₄ and T₅) have been more effective on weeds, as evidenced through WCE (Table 5), their crop phytotoxic effect resulted in low yields, therefore, low gross returns and net benefit. Thus, the highest gross returns (Rs. 26421 ha^-1) and net benefits (Rs. 23535 ha^-1) of moth bean were recorded under clodinafop-propargyl + sodium acifluorfen (PoE 312.5 g a.i. ha^-1) (T₈).
 

 
@table5
 
Impact assessment indices
 
The significant difference in weed dry matter in different weed management practices was manifested in WCE and WI (Table 5). The better control of both grassy and broad-leaved weed under propaquizafop + imazethapyr (PoE 125 g a.i. ha^-1) (T₅) results in the highest WCE (73.8%). However, due to the crop phytotoxic effect of imazethapyr, the minimum value of WI (20) was recorded in clodinafop-propargyl + sodium acifluorfen (PoE 312.5 g a.i. ha^-1) (T₈). It reflects the effectiveness of applied herbicide in securing yield loss against weed competition and a lower value of WI means high herbicide efficiency.
       
As for efficient weed management, lower WPI and higher HEI values are desirable. Clodinafop-propargyl + sodium acifluorfen (PoE 312.5 g a.i. ha^-1) (T₈) recorded the highest HEI (3.9), indicating the higher yield advantage (Table 5). The lowest WPI was recorded in propaquizafop + imazethapyr (PoE 125 g a.i. ha^-1) (T¹⁰) (1.03). The highest CRI (4.70) was recorded in clodinafop-propargyl + sodium acifluorfen (PoE 312.5 g a.i. ha^-1) (T₈). This might have been due to satisfactory weed control under this treatment and no phytotoxic effect on the crop. These results corroborate the findings of Kumar et al., (2018).