Weed Management through Imidazolinones in Pigeon Pea (Cajanus cajan L.)

To feed the continuously growing population at alarming rate, to contribute food and nutritionally secure world, one must include pulses in their diet. Pulse crops are commonly known as the poor man’s meat (Reddy, 2010). Pulses are not only the major source of dietary protein for the vegetarian population of the world but also act as a source of 20 per cent of dietary calories too. Furthermore, it can be added as healthy choice of non-vegetarians, can be added in soups, casseroles, meat sauces to add flavor. Despite being the largest producer, India lags to produce enough to meet the pace of population growth and to meet its domestic requirement India imports two million tonnes of pulses every year by spending a huge amount of foreign exchange (Chaturvedi and Ali, 2002).
       
Among pulse crops, pigeon pea [Cajanus cajan (L.) Millsp] ranks second most important pulse crops of India after chickpea. In India it occupies an area of 4.44 million hectares and production of 4.29 million tonnes with an average productivity of 967 kg ha^-1 during 2017-18 (Anonymous, 2020). Among various biotic factors limiting pigeon pea yield, weeds are of prime importance causing seed yield losses up to 80 per cent (Talnikar et al., 2008). As pigeon pea is a long-duration and rainy season crop, slow initial growth and sowing at a wider spacing of pigeon pea encourage many flushes of weeds that compete with the crop for moisture, nutrients and light. Hence, an initial period of 6-8 weeks of pigeon pea is critical in terms of crop weed competition (Channappagoudar and Biradar, 2007).
       
Imazethapyr has been reported against important weeds in soybean, groundnut and other legumes (Vencill, 2002). Various trials in different regions were conducted to study the effects of imazethapyr alone or in combination with other herbicides to control the weeds in different crops. Mishra et al., (2001) reported that Imazethapyr (75 g ha^-1) alone gave effective control of grassy as well as broad-leaf weeds in soybean. Reports suggest that application of imazethapyr + pendimethalin (RM) @ 900 g ha^-1 pre-emergence effectively controls the weeds in green gram.
       
Therefore, taking into consideration all the above facts, current experiment was carried out to evaluate better weed control measures in pigeon pea. In this study, pre and post-emergence imidazolinones [Acetolactate synthase inhibitor grouped under imidazolinones Group B classification of HRAC (Beffa et al., 2019)] alone and in combination with pendimethalin (Microtubule assembly inhibitor grouped under Dinitroanilines Group K1 classification of HRAC) was applied to find out the effect of different weed management measures on nutrient content, nutrient uptake, productivity and crop economics (BCR) of pigeon pea, worked out as basics of experimental design.

Experiment site details
 
The present field experiment was conducted at Agronomy Research Farm, CCS Haryana Agricultural University, Hisar, India during 2017-18 and 2018-19. Geographically, Hisar is situated at 29^o10'N 75^o46'E, 215.2 m AMSL. The climate of Hisar is semi-arid characterized by hot and dry summer and severe cold during the winter season. The texture of the soil of the experimental field was sandy-loam and it is slightly alkaline in reaction. The fertility status of soil was medium in organic carbon, medium in available nitrogen and phosphorus but high in available potassium. The experimental field was properly prepared in the first week of June during both years. The field was plowed twice with a tractor-drawn cultivator after harvest of the previous crop to crush clods. A uniform basal dose of 20 kg nitrogen and 40 kg phosphorus ha^-1 was applied through DAP, at the time of field preparation. On the well-prepared field, seeds of the pigeon pea crop variety ‘Paras’ @ 15 kg ha^-1 were sown by seed cum fertilizer drill. All agronomic practices as per the recommendation of CCS HAU, Hisar were performed irrespective of the treatments.
 
Treatment details
 
The experiment was laid out in randomized block design using eighteen treatment combinations viz., T₁- imazethapyr @ 75 g ha^-1 as PRE, T₂- imazethapyr@ 100 g ha^-1 as PRE, T₃-imazethapyr@ 75 g ha^-1 at 45 DAS, T₄- imazethapyr@ 100 g ha^-1 at 45 DAS, T₅- imazethapyr + imazamox @ 70 g ha^-1 as PRE, T₆- imazethapyr + imazamox @ 100 g ha^-1 as PRE, T₇- imazethapyr + imazamox @ 70 g ha^-1 at 45 DAS, T₈- imazethapyr + imazamox @ 100 g ha^-1 at 45 DAS, T₉- pendimethalin + imazethapyr @ 750 + 50 g ha^-1 as PRE, T₁₀- pendimethalin + imazethapyr @ 1000 + 50 g ha^-1 as PRE, T₁₁- pendimethalin + imazethapyr@ 750 + 65 g ha^-1 as PRE, T₁₂- pendimethalin + imazethapyr @ 1000 + 65 g ha^-1 as PRE, T₁₃- pendimethalin + imazethapyr @ 750 + 75 g ha^-1 as PRE, T₁₄- pendimethalin + imazethapyr @ 1000 + 75 g ha^-1 as PRE, T₁₅- pendimethalin @ 1000 g ha^-1 as PRE, T₁₆- weed free, T₁₇- weedy check and T₁₈- two hoeing at 40 and 70 DAS, replicated thrice. 
 
Formulation of herbicides
 
Pendimethalin 30% EC of Rallis India Limited a Tata Enterprise with trade name of TATA PANIDA 30 E, imazethapyr 10% SL of BASF with trade name of Pursuit 70WG and imazethapyr 35% + imazamox 35% WG (RM) of BASF with trade name of Odyssey 70WG were used in the experiment. The required amount of herbicides for the experimentation was calculated by using the following formula:
   
Where,
F = Formulated product required in kg or lha^-1.
R = Dose in active ingredient in kg ha^-1 to be sprayed (recommended rate).
A = Area to be sprayed (m²).
Application of herbicides
 
In pigeon pea crop, imazethapyr, imazethapyr + imazamox (RM), pendimethalin + imazethapyr (TM) and pendimethalin, were applied as PRE spray to the soil surface as per treatment on the day of sowing and, imazethapyr, imazethapyr + imazamox (RM) were applied as POE at 45 DAS. Herbicides were applied with the aid of manually operated knapsack sprayer fitted with flat fan nozzle. Hand hoeing was done manually with the help of hand hoe to keep the field free from weeds in the weed-free treatment plots at 20, 40, 60 and 80 DAS.
 
Nutrient analysis
 
The samples of seeds and stalk of pigeon pea (at harvest) were collected and sundried. Then oven-dried at 65±5^oC till constant weight achieved and ground to pass in a 40 mesh sieve and analyzed for total nutrient content. Nitrogen, phosphorus and potassium content in seeds and stalk of pigeon pea were analyzed by micro Kjeldahl (Bremner and Mulvaney, 1955), Ammonium-vanadomolybdo phosphoric acid yellow color (Koenig and Johnson, 1942) and Flame emission spectrometric method (Zagatto et al., 1979), respectively. Nutrient uptake by seeds and stalk (kg ha^-1) for each nutrient was worked out by multiplying the seed and stalk yield (kg ha^-1), respectively with their respective nitrogen, phosphorus and potassium content divided by 100.
 
Seed and stalk yield
 
Harvested plants of one square meter were sundried for at least two week and threshed manually by beating. Then, threshed seeds obtained from each plot after proper cleaning were weighed using electronic weighing machine and converted to expressed as kg ha^-1. Stalk yield was calculated by subtracting the seed yield from the biological weight of the respective plot and expressed in kg ha^-1.
 
Economics
 
The economics of different treatments was computed in terms of, gross returns, net returns and BCR, based on prevailing market prices for different inputs and outputs as per information collected from the Department of Agricultural Economics, CCS HAU, Hisar.
       
The data obtained on various observations were tabulated and subjected to statistical analysis by using statistical tool ‘The SAS System 9.3’. 

The major weeds appeared in experimental field comprised of Echinochloa colona, Cynodon dactylon, Dactyloctenium aegyptium and Digitaria sanguinalis among grassy weeds; Trianthema portulacastrum and Digera arvensis among broad leaved weeds and Cyperus rotundus among sedges. During initial period of crop growth, Digitaria sanguinalis and Dactyloctenium aegyptium were the dominated weeds with relative density of 23.7% and 17.4% during 2017 and 24.7% and 21.0% during 2018, respectively at 30 DAS. Whereas Digitaria sanguinalis and Trianthema portulacastrum were the dominated weeds with relative density of 23.2% and 17.8% during 2017 and 22.8% and 17.2% during 2018, respectively at 60 DAS.
       
Among different weed species, Cynodon dactylon was not controlled by any of the herbicide treatment and PRE alone application of pendimethalin @ 1000 g ha^-1 did not control Digera arvensis and Cyperus rotundus throughout the season.
 
Nutrient content in pigeon pea as affected by weed control measures
 
Crop-weed competition affects the nutrient content of crop plants. Obviously significantly highest nutrient content in seed and stalk of pigeon pea were recorded in the weed-free plot (T₁₆) over the weedy check (T₁₇) which was statistically at par with treatment where two hoeing at 40 and 70 DAS (T₁₈) were employed. Among herbicidal treatments, highest nutrient content in seed and stalk of pigeon pea were recorded in PRE application of pendimethalin + imazethapyr @ 1000 + 75 g ha^-1 (T₁₄). It may be due to the absence of crop-weed competition in these treatments.
       
The perusal of two years of pooled data of nutrient content presented in Table 1 indicates that nitrogen content in the seed is not affected by any of the herbicides treatment applied either as PRE or POE. Among herbicidal treatments, the highest nitrogen content (3.294%) in seed of pigeon pea was observed treatment T₁₄. Alone application of imidazolinones did not affects the phosphorus content in seed significantly either applied as PRE or POE and significantly higher phosphorus content (0.552%) in seed was observed in treatment T₁₄ which was statistically at par with T₁₁, T₁₂ and T₁₃. Whereas all of the herbicide treatments significantly enhanced the potassium content in seed over the weedy check and the highest potassium content (0.814%) in seed was observed in treatment T₁₄ over rest of the other herbicidal treatments except treatment T₁₃ to which it was statistically at par.
 

       
All of the herbicidal treatments significantly positively correlated with enhanced nitrogen, phosphorus and potassium content in the stalk of pigeon pea over the weedy check plot. Among herbicidal treatments, significantly higher nitrogen (1.245%), phosphorus (0.185%) and potassium (1.590%) content in the stalk of pigeon pea was observed in treatment T₁₄ over alone application of imidazolinones either applied PRE or POE. These results are in contrary with the findings of Dhaker et al., (2015) who reported that weed management did not affects the nutrient content of seed and haulm of soybean.
 
Seed and stalk yield of pigeon pea as affected by weed control measures
 
Obviously weed free treatment (T₁₆) gave significantly higher seed yield (1914 kg ha^-1) of pigeon pea among different treatments, but statistically at par with T₁₈ and T₁₄. Similarly Significantly higher stalk yield (6259 kg ha^-1) of pigeon pea was obtained in treatment T₁₆ over other treatments except T₁₈, T₁₄ and T₁₃ to them it was statistically at par. Whereas among herbicidal treatments, combined PRE application of pendimethalin + imazethapyr @ 1000 + 75 g ha^-1 (T₁₄) gave significantly higher seed (1873 kg ha^-1) and stalk yield (6170 kg ha^-1) of pigeon pea over alone application of imidazolinones either applied as PRE or POE and combined PRE application of imazethapyr at lowest dose of 50 g ha^-1 with pendimethalin at either dose (750 or 1000 g ha^-1). It may be due to the absence of crop-weed competition for moisture, nutrient, space and sunlight because unchecked growth of weeds in crop suppresses the growth of crop plant and effectively competes with the crop throughout the growing season thus adversely affects the yield. The findings are in collaboration with results reported by Kumar et al., (2015).
 
Nutrient uptake by pigeon pea as affected by weed control measures
 
The data presented in Table 2 showed that there was a significant difference in uptake of major nutrients N, P and K. The highest nutrient uptake in weed-free treatment (T₁₆) was observed due to the absence of weed infestation, the competition of weed and more production of dry matter and higher seed yield due to optimized utilization of available resources as compared to other treatments. Similarly Beura and Reddy (2003) have reported better control of weeds and favorable conditions for crop growth resulted in higher nutrient uptake by the crop. However, in weedy check treatment (T₁₇), the rate of uptake of nutrient by plants was very low. This is due to weed suppress the vegetative growth of pigeon pea by competition to light, moisture and nutrient. Similar results were also reported by Sable et al., (2013).
 

       
Whereas among herbicidal treatments, the highest nitrogen (61.7 and 76.8 kg ha^-1), phosphorus (10.3 and 11.4 kg ha^-1) and potassium (15.2 and 98.1 kg ha^-1) uptake in seed and stalk, respectively was reported in treatment T₁₄ (combined PRE application of pendimethalin + imazethapyr @ 1000 + 75 g ha^-1) over alone application of imidazolinones either applied as PRE or POE (T₁ to T₈) and combined PRE application of imazethapyr at lowest dose of 50 g ha^-1 with pendimethalin at either dose  of 750 or 1000 g ha^-1 (T₉ and T¹⁰). The results conform to the findings of Sanjay et al., (2006).
 
Economics of pigeon pea as affected by weed control measures
 
Among the weed-control measures, the lowest net return and benefit cost ratio (BCR) were recorded under weedy check plot. These increased remarkably under weed-control treatments. Involvement of labor in conventional hoeing marginalized the net return and BCR from this treatment.
       
The perusal data of economics presented in Table 3 revealed that the highest cost of cultivation (Rs. 59,925 ha^-1) with lower BCR (1.97 in 2017 and 1.99 in 2018) in comparison to herbicidal treatments was occurred in weed-free treatment (T₁₆) that may be due to highest treatment cost. The lowest net returns (Rs. 1,081 ha^-1 in 2017 and Rs. 3,151 ha^-1 in 2018) with lowest BCR (1.03 in 2017 and 1.08 in 2018) were obtained in weedy check treatment (T₁₇). It may be due to lowest productivity of pigeon pea in this treatment.
 

       
While comparing the herbicides treatments, the highest net returns (Rs. 71,703 ha^-1 in 2017 and Rs. 73,245 ha^-1 in 2018) with BCR (2.64 in 2017 and 2.68 in 2018) was obtained in treatment T₁₄ (PRE application of pendimethalin imazethapyr @ 1000 + 75 g ha^-1) which was closely followed by treatment T₁₃ with the same BCR (2.64 in 2017 and 2.68 in 2018), thus proved to be the most economical weed control treatment. Similar results were reported by Gupta et al., (2017) and Kaur et al., (2015).

Keeping the field weed free via hoeing is laborious, time consuming and costlier weed management practice. In pigeon pea which is a rainy season crop in Haryana is not possible to employee hoeing in rainy condition. Moreover, combined application of imidazolinones significantly improves the nutrient content, nutrient uptake and productivity of pigeon pea over alone application of imidazolinones. Therefore, PRE combined application of pendimethalin + imazethapyr @ 1000 + 75 g ha^-1 proved to be an effective and a profitable alternative to the existing recommendation (two hoeing at 25 and 45 DAS) of weed control in pigeon pea in Haryana locality.

We would like to thank the field staff of the Department of Agronomy, CCS Haryana Agricultural University, Hisar, India for caring for this research work.