Effect of Different Weed Management Practices on Weed Dynamics, Productivity and Farm Profitability of Cluster Bean

Clusterbean (Cyamopsis tetragonoloba L.), popularly known as guar, is a drought hardy and deep rooted multipurpose legume grown as feed, fodder, green manure, vegetable and seed in dry habitat of Rajasthan. The area under this crop in Rajasthan is about 47.87 Lac ha with production of 22.23 Lac tones and average productivity of 465 kg ha^-1, (GoR, 2015-16). It is well known that weeds are ubiquitous but their presence in cropped area particularly in rainy season crops like clusterbean act as major limiting factor in achieving potential harvest. In almost all kharif crops, including clusterbean, early weed control is the basic necessity for full yield potential of the crop. Weeds should be removed from clusterbean field up to 30 DAS to minimize crop-weed competition as it is the critical period of crop weed competition. When rains occur continuously, it becomes difficult to control weeds by mechanical methods alone. In India, farmers rely predominantly on mechanical /manual methods of weed control. Being a kharif season crop, it suffers from severe infestation of weeds which reduces its seed yield by 47% (Bhadoria et al., 2000).
       
Hand weeding is a common practice of weed control but incessant rains and unavailability and high wages of labour at weeding peaks are the major constraints (Vyas and Kushwah, 2008). Further, these practices alone do not ensure weed free conditions and are expensive, cumbersome and time-consuming too; further reducing the profit margin. Most often protracted rains do not allow or delay the conventional farm operations during the critical weeding season. The pre-emergence (PE) herbicides like Pendimethalin were found effective in controlling the weeds during early stages but late flushes and escaped/regenerated weeds in later stages also hamper the crop yield to certain extent possible (Dayal, 2004). This warrants the use of post emergence (PoE) herbicide for weed control. So herbicides with no longer residual activity such as Imazathapyr which provide season long weed control is being used in many legumes. Further, diversification in herbicide use is being increasingly desired for herbicide rotations to tackle the emerging cross and multiple resistances to herbicides in weeds. This necessitates for an alternative cost-effective integrated weed-management strategy involving the pre- and post-emergence application of herbicides and intercultural operations considering the present situation of labour scarcity, quality of weed control, productivity and profitability concerns. Hence, it is worthwhile to use different herbicides at varying doses in conjunction with hand weeding to made effective weed control. So there was urgent need to test the range weed control strategies which can provide effective control of weeds in guar. Therefore, the present study was planned to test the efficacy of different weed control measures alone or in combinations with each other against weeds, crop productivity and farm profitability of guar.

A field experiment was conducted at Agricultural Research farm, Rajasthan Agricultural Research Institute, Durgapura, Jaipur, India (26^o 51´ N, 75^o 47´ E at an altitude of 390 m above mean sea level), during kharif seasons of years of 2015, 2016 and 2017 on the same site and layout. The experimental site falls in the Semi-Arid Eastern Plain Zone of Rajasthan (III-A) characterized by cold winters and hot summers. Occurrence of frost (below 0oC) during December/ January and hot desiccating winds (45^oC) in May/ June are quite common. The average annual rainfall of zone was 563 mm of which about 90 % is received during later half of June to September with erratic distribution over time and space. The soil of the experimental site was loamy sand with slightly alkaline in reaction (pH 8.1). The initial values of nutrients were 2.1 g kg^-1 organic carbon, 34 kg available P₂O₅ ha^-1, 191 kg available K₂O ha^-1, 7.8 mg kg^-1 available S.
       
Nine experimental treatments were T₁- one hand weeding at 20-25 DAS, T₂- Pendimethalin @ 0.75 kg a.i. ha^-1 as PE, T₃- Imazethapyr @ 40g a.i. ha^-1 (PoE) at 2-3 leaf stage of weeds, T₄- Imazethapyr+ Imazamox@ 40g a.i. ha^-1 (PoE) at 2-3 leaf stage of weeds, T₅- Pendimethalin @ 0.75 kg a.i.ha^-1 as PE+ Hand weeding at 20-25 DAS, T₆-  Pendimethalin @ 0.75 kg a.i ha^-1 as PE+ Imazethapyr @ 40g a.i./ha (PoE) at 2-3 leaf stage of weeds, T₇- Pendimethalin @ 0.75 kg a.i.ha^-1 as PE+ Imazethapyr+ Imazamox@ 40g a.i.ha^-1 (PoE) at 2-3 leaf stage of weeds, T₈- Weed free check and T₉- Weedy Check  tried in randomized block design (RBD) with four replications. Guar variety ‘RGC 1038’ was sown at 30 x 10 cm spacing on July 7, 2015, 03 July 2016 and 11 July 2017. The crop was fertilized with uniform dose of 20 kg N and 40 kg P₂O₅ ha^-1 through urea and SSP, respectively, at the time of sowing. Herbicides were applied with the help of knapsack sprayer fitted with flat-fan nozzle.
       
Observations on weed count and weed dry weight were recorded just before spray and 30 days after spray were recorded by using a quadrate measuring 50 × 50 cm at two randomly selected spots in each plot and converted into one square meter area and these data were subjected to square root transformation √(x+0.5) before analysis. The weed control efficiency (WCE) and weed index were calculated by using the following formulae:
                                                             
Where,
X = Weed dry matter in weedy check (g).
Y = Weed dry matter in respective treatment (g).
 
Where,
X = Yield in weed free check (kg).
Y = Yield in respective treatment (kg).
       
The crop was harvested at physiological maturity when plants turned golden yellow. After threshing, winnowing and cleaning was done and seeds were weighed separately to record seed yield and all the yield attributing parameters. The net returns (NR) of each treatment were calculated by deducting the total cost (TC) of cultivation from gross returns (GR) of respective treatments and the benefit: cost ratio was calculated by dividing the net returns with total cost of cultivation.  All the data were subjected to statistical analysis by adopting appropriate method of analysis of variance assuming homogeneity; pooled analysis of the data was also carried out to establish the trend of treatments applied as per Gomez and Gomez (1984). Wherever, the F values were found significant at 5 % level of probability, the critical difference (CD) values were computed for making comparison among the treatment means.

Effect on weed dynamics and control efficiency
 
The regular survey of experimental field revealed that the field was infested by Cynodon dactylon, Cyperus rotundus, Echinochloa colona, Echinochloa crus-galli among monocot and Amaranthus viridis, Amaranthus spinosus, Commelina benghalensis, Parthenium hysterophorus and Trianthema portulacastrum among dicot weeds. Overall the experimental field was dominated by population of dicot weeds over monocots. Result of three years pooled data (Table 1) shows that all the weed control treatments significantly reduced density and dry matter of weeds over weedy check. The significantly minimum weed density at 25 DAS was observed with application of Pendimethalin @ 0.75 kg a.i. ha^-1 as PE fb one hand weeding (3.58 m^-2) and Pendimethalin 0.75 kg/ha at PE fb Imazathapyr+ Imazamox @ 40 g a.i. ha^-1 PoE (4.64 m^-2) compared to weedy check (6.82 m^-2). These two treatments proved their potential in terms of broad spacterm weed control as they shows comparable results to weed free plot. Among the weed control practices highest weed control efficiency was recorded under weed free (100%) and followed by Pendimethalin @ 0.75 kg a.i. ha^-1 as PE fb one hand weeding at 25 DAS (83.7%) and Pendimethalin @ 0.75 kg a.i. ha^-1 as PE+ Imazathapyr+ Imazamox @ 40 g a.i. ha^-1 PoE (74.0%), while minimum was under weedy check (0.00%) followed by Pendimethalin @ 0.75 kg a.i. ha^-1 as PE treatment (49.8%). The minimum weed index was observed with weed free (0.00%) followed by application of Pendimethalin @ 0.75 kg a.i. ha^-1 as PE fb one hand weeding at 25 DAS (10.41) and Pendimethalin 0.75kg/ha at PE fb Imazathapyr + Imazamox @ 40 g a.i. ha^-1 at 2-3 leaf stage of weeds (20.43) compared to weedy check (58.36). Lower weed density and higher weed control efficiency in Pendimethalin @ 0.75 kg a.i/ha as PE fb one hand weeding at 25 DAS and sequential application of Pendimethalin @ 0.75 kg a.i./ha as PE + Imazathapyr + Imazamox@ 40 g a.i./ha PoE might be due to the fact removal of weeds manually the field efficiently control weeds after pre-emergence application of herbicide which emerged during early as well as later stages of crop growth resulted in excellent performance compared to herbicides specially applied alone and sequential application of Pendimethalin controlled early flush of weeds while post emergence application  of ready mix (Imazathapyr+ Imazamox)  destroyed late flush of weeds most efficiently during entire crop season compared to weedy check and herbicide applied alone. These results are in close conformity with the findings of Sharma and Sagarka (2015) and Kumar et al., (2015).


 
Effect of different IWM practices on crop productivity
 
All the weed control treatments recorded significantly higher seed, stover and biological yield as compared to weedy check. Among different weed control treatments, two hand weeding (weed free) recorded maximum seed (1268 kg ha^-1), haulm (3590 kg ha^-1) and biological yield (4859 kg ha^-1). Among herbicidal treatments application of Pendimethalin @ 0.75 kg a.i/ha as PE fb one hand weeding at 25 DAS and Pendimethalin 0.75 kg ha^-1 at PE fb Imazathapyr + Imazamox @ 40 g a.i. ha^-1 at 2-3 leaf stage of weeds recorded significantly maximum seed (1136 kg ha^-1 and 1009 kg ha^-1), haulm (3304 kg ha^-1 and 2943 kg ha^-1) and biological yield (4440 kg ha^-1 and 3953 kg ha^-1), respectively (Table 2). Whereas the lowest seed (528 kg ha^-1), haulm (1504 kg ha^-1) and biological yield (2032 kg ha^-1) were recorded under weedy check. Two hand weeding treatment provided the long time weed control and hence resulted in appreciably higher yields over weedy check and other treatments. Application of Pendimethalin @ 0.75 kg a.i/ha as PE fb one hand weeding at 25 DAS and Pendimethalin 0.75 kg ha^-1 at PE fb Imazathapyr + Imazamox @ 40 g a.i. ha^-1 at 2-3 leaf stage of weeds were the next superior as these treatments kept the crop almost weed free during 25-45 DAS that markedly reduced the competition for nutrients and other growth resources by weeds as a consequence of which reduction in dry matter and nutrient depletion by weeds occurred. Reduced weed-crop competition under these treatments, saved a considerable amount of nutrients for crop growth that led to enhanced crop growth by utilizing greater moisture and nutrients from deeper soil layers. These favorable effects in rhizosphere were more conspicuous in HW twice and one HW treatments as this improved soil tilth by making it vulnerable for the plants to utilize water and air. The higher yield under these treatments were attributed to lower weed density, weed dry weight and better weed control efficiency. Increase in crop productivity might be due to the direct influence of various weed management treatments on the suppression of weeds. The results corroborate with the findings of Yadav and Sheoran (1991), Donald and Humblin (1976), Tiwari et al., 2014 and Singh et al., (2006).
 

 
Effect of different IWM practices on farm profitability
 
Economic evaluation of different weed management treatments (Table 3) indicated that among herbicidal treatments application of Pendimethalin @ 0.75 kg a.i/ha as PE fb one hand weeding at 25 and which was followed by with Pendimethalin @ 0.75 kg a.i.ha^-1 as PE+ Imazathapyr+ Imazamox@ 40 g a.i.ha^-1 at 2-3 leaf stage of weeds recorded maximum cost of cultivation (INR 16500 ha^-1 and INR 14900 ha^-1), gross return (INR 55352 ha^-1 and INR 49189 ha^-1)  net returns (INR 38852 ha^-1 and INR 34289 ha^-1) and B:C ratio (2.35 and 2.20). However, weed free plot involved higher cost of cultivation (INR 19500 ha^-1) and gross returns (INR 61490 ha^-1) and net returns INR 41990 ha^-1) but lower B:C ratio (2.15) as compared to application of Pendimethalin @ 0.75kg a.i/ha as PE fb one hand weeding and Pendimethalin @ 0.75 kg a.i.ha^-1 as PE+ Imazathapyr+ Imazamox@ 40 g a.i.ha^-1 at 2-3 leaf stage of weeds. Whereas, minimum cost of cultivation (INR 10000 ha^-1) and gross returns (INR 25632 ha^-1), net returns INR 15632 ha^-1) and B:C ratio (1.56) were recorded under weed check. The higher net reruns and B: C ratio recorded under these superior treatments can be explained easily with the corresponding higher grain yield under these treatments. The higher crop productivity of crop might be the principal reasons for higher net returns under weed free condition by two hand weeding and integrated weed management using Pendimethalin @ 0.75 kg a.i ha^-1 as PE fb one hand weeding at 25 DAS. The lowest grain yield due to unrestricted weed growth in weedy check treatment was eventually reflected in lowest net returns. Positive effect of Pendimethalin alone and in combination with one hand weeding on yield and yield economics was observed by kumar and Sharma (1996), Kumar and Sharma (1997), Kumar and Chauhan (2015).
 

The result of three years investigation reveals that weed management by two hand weeding 20 and 40 DAS resulted in highest weed control efficiency and crop productivity however pre-emergence application of Pendimethalin 0.75 kg ha^-1 fb one hand weeding gave comparable results with it. So looking to the laborious pressure of hand weeding, cost effectiveness ease of application of chemical herbicides, application of Pendimethalin 0.75 kg ha^-1 fb one hand weeding can be an effective weed control option in cluster bean crop with higher seed and haulm yield under rainfed Agroecosystem in Semi-arid and Arid regions of Rajasthan.