Productivity and Profitability of Fodder Cowpea Cultivars under Various Zinc Management Practices in IGP of India

Zinc deficiency has received great attention in India because the average level of its deficiency in soils is 49% and the value is projected to increase from 49 to 63% by 2025 (Arunachalam et al., 2013). It was concluded from a study that zinc would become so deficient by 2025 that to maintain satisfactory zinc status in crop plants, it should be applied annually to 325000 tons of zinc in India (Singh, 2008). Zinc is indispensable element for the synthesis of tryptophan that is a component of some proteins and a compound needed for the production of growth hormones (auxins) like indole acetic acid. It plays a key role in carbohydrate metabolism and imparts resistance in plants against diseases (Prasad, 2007). The metabolism of proteins, nucleic acids, carbohydrates and lipids depends on Zn to a large extent. Its deficiency causes reduced growth hormone production in plants causes shortening of internodes and smaller than normal leaves (Hafeez et al., 2013). Inactive RNAs and starch accumulate in plants as a result of Zn deficiency (Zargar et al., 2015). Henceforth, adequate supply of zinc is essential to achieve higher crop productivity.
       
Cowpea (Vigna unguiculata) is one of the crucial food legume crops originated across semiarid regions of Asia and Africa. Being a good soil renovator, the cowpea is well known as fodder because of its quick growing habits and good tonnage of fodder yield (Mallikarjun et al., 2020). Since, cowpea green manure contains 0.7% N, 0.1-0.2% P₂O₅ and 0.6% K₂O, the incorporation of its residues increases the primary macronutrient content. It is capable for accumulation of 5-6 tonnes/ha of dry biomass (Hindoriya et al., 2019) and 100 kg N/ha in 50-60 days if it is well-managed. It has good root nodules and thus, supplies atmospheric N to available form in soil and improves the soil rhizosphere conditions. Though, cowpea is sensitive to Zn content and shows Zn deficiency symptoms within 10 days after sowing (DAS). Its deficiency causes orange-yellow colour in the inter-veinal region of new leaves. At later stage, dull brown spots develop on leaf blades towards the margin of the leaf tip (Singh et al., 2011). Cowpea can grow in the potentially Zn deficient soil but the Zn concentration in shoot and biomass yield of cowpea remarkably decreased due the deficiency of Zn below the critical level. Centre for World Food Studies, Amsterdam (Netherlands) has indicated direct linkages between soil micronutrient contents and micronutrient contents of forage and fodders. The positive effect of zinc on crop yield/animal produce is possible due to zinc management (Kumar et al., 2016 and Rathore et al., 2015a and b).
 
Selection of suitable cultivars is also an important aspect of fodder production as fodder quality is a function of genetic makeup and environment. Increased efficiency of uptake and utilization of soil nutrients will aid to reduce the chemical fertilizers usage in agriculture. With respect to physiological aspect, nutritional efficiency is the ability of the genotype to absorb the nutrient from the soil, distribute it and use it internally (de Oliveira et al., 2012). The capacity of a genotype to grow in the zinc-deficient soil is expressed as zinc efficiency or the ratio of shoot dry weight under Zn deûciency soil over that soil having adequate Zn nutritional status (Gao et al., 2005). Cowpea cultivars responded differently in uptake of zinc under different level of zinc (Mfeka, 2017). Concerning all these facts, the present experiment was undertaken to study the effect of zinc management practices on cowpea cultivars in improving the yield and fodder quality along with sustaining the soil fertility.

The experiment was conducted at Research Farm of Agronomy Section, ICAR-National Dairy Research Institute, Karnal, India during Kharif, 2019. Soil of the experimental field was clay loam having pH 7.48 and electrical conductivity 0.309 dS/m. Available N, P and K content of soil was 188.9, 30.5 and 192.4 kg/ha, respectively. The experiment was laid out in factorial randomized block design with three cultivars, i.e., V₁: C-152, V₂: MFC-08-14 and V₃: MFC-09-1 and five zinc management practices, i.e., Zn₁: Control, Zn₂: 10 kg ZnSO₄/ha, Zn₃: 20 kg ZnSO₄/ha, Zn₄: 0.5% ZnSO₄ foliar spray at 20 DAS, Zn₅: 0.5% ZnSO₄ foliar spray at 20 and 40 DAS and replicated thrice. Nitrogen and phosphorus were applied to each plot uniformly through DAP as basal dose (RDF: N₂₀P₆₀). Zinc was applied in the form of zinc sulphate heptahydrate. Seeds of cowpea varieties were treated with liquid rhizobium culture @ 50 ml/ seeds required for one acre. Then, seeds were mixed well with culture using one litre water and kept in shade on a clean Pakka floor for drying. After half an hour of drying, seeds were used for sowing. The crop was sown on August 06, 2019. A pre-emergence application of Pendimethalin @ 1 liter a.i./ha was used to check weeds at initial crop growth stage. Two irrigations were given at 25 and 45 DAS. Cowpea was harvested for fodder purpose on October 02, 2019.
       
The standard agronomic methods were used to measure growth attributes of fodder cowpea. The fodder samples taken at harvest stage were dried in hot air oven at 60-70°C for 72 hrs till it attains a constant weight and grounded (Wiley mill) to pass through one mm screen for the estimation of nutrient content. Total nitrogen, phosphorus and potassium content were estimated using modified Kjeldhal method (Jackson, 1967), ammonium vanadomolybdo phosphoric acid yellow colour method (Richards, 1968) and Flame Photometer method (Jackson, 1973), respectively. While, the zinc content in fodder samples was analysed using di-acid (HClO₄ + HNO₃ in 3:10 ratio) digestion followed by reading at Atomic Absorption Spectrophotometer (Jackson, 1967). The uptake of N, P, K and Zn were calculated by multiplying their concentrations with respective yield of cowpea fodder. Soil samples taken at 0-15 cm depth after crop harvest were used for estimation of chemical properties. Wet oxidation method (Walkley and Black, 1934), alkaline permanganate method (Subbiah and Asija, 1956), 0.5 M sodium bicarbonate method (Olsen et al., 1954) and ammonium acetate extraction method (Jackson, 1967) were used for estimation of soil organic carbon, available N, P and K, respectively. DTPA-CaCl₂-TEA method (Lindsay and Norvell, 1978) was used for micronutrients estimation.
 
Experimental data were analyzed using analysis of variance (ANOVA) as per factorial randomized block design (Gomez and Gomez, 1984). Significance of the treatment means were tested using F test with 5% level of significance (P<0.05) and means were compared using the least significant difference (LSD) test at a = 0.05.

Growth attributes
 
Results (Table 1) indicated that the cultivar C-152 showed significantly best plant growth in terms of plant height, number of leaves, leaf length, number of branches and total root nodules per plant at both 30 DAS and harvest stage amongst cultivars. The cultivar C-152 was found at par with MFC-09-1 for number of branches and total root nodules per plant at harvest stage, whereas, the lowest growth performance was noted with the cultivar MFC-08-14. Though, the leaf width was remained statistically unaffected among the cultivars. The differences in growth attributes could be associated with genetic potential of cultivars and also the response of cultivars to local climatic conditions. Win and Oo (2015) also concluded in their studies that cowpea genotypes exhibited variable shoot length. Varying degree of genetic variability for number of branches per plant (Hedge and Mishra, 2009) and nodulation (Ayisi et al., 2000) were recorded in cowpea genotypes.


       
The effect to Zn management on all the growth attributes was found significant, except leaf width at 30 DAS (Table 1). At initial crop growth stage (30 DAS), the application of 20 kg ZnSO₄ (Zn₃) significantly recorded the highest plant height, number of leaves, leaf length, number of branches and total root nodules per plant amongst zinc management practices. Though, it remained statistically at par with 10 kg ZnSO₄ (Zn₂) for the leaf length, number of branches and total root nodules. At the harvesting stage, the application of 20 kg ZnSO₄ (Zn₃) exhibited significantly superior growth which remained at par with 10 kg ZnSO₄ (Zn₂). The higher plant growth with zinc application might be due to the fact that zinc acts as cofactor of several enzymes and helps in cell division and elongation, metabolism of auxin and nucleic acid as well as carbohydrate and protein synthesis by activating particular enzymes in plants. Our results are in conformity with the findings of Pandya and Bhatt (2007) and Kumar et al., (2016).
 
Green fodder yield
 
Results (Table 1) showed that cowpea cv. C-152 recorded significantly highest green fodder yield (30.23 t/ha) as compared to MFC-08-14 and MFC-09-1. Further results revealed that MFC-09-1 (26.95 t/ha) cultivar showed significantly higher green fodder yield than MFC-08-14 (25.32 t/ha). The significant variations in cultivars might be due to the differences in growth pattern resulted from their genetic makeup. Kumar (2017) also reported the significant variations in cowpea varieties due to their genetic potential.
       
Among zinc management practices (Table 1), the application of 20 kg ZnSO₄ was found at par with foliar spray of 0.5% ZnSO₄ at 20 and 40 DAS and both produced significantly maximum green fodder yield (29.75 and 29.39 t/ha, respectively) as compared to remaining practices. The biomass yield is a function of other growth parameters like plant height, number of leaves etc. The differentiation in cell growth and development of plant was evidenced under adequate Zn supply because zinc protect cells from oxidative degradation of growth hormone indole-3-acetic acid (IAA). The present results are in line with earlier findings of Kumar et al., (2016).
 
Nutrient content and uptake
 
The primary nutrient contents were significantly differed among cowpea cultivars (Fig 1 and 3). Significantly the highest N, P and K contents (2.77, 0.310 and 1.95%, respectively) were noted with C-152 as compared to MFC-08-14 and MFC-09-1. Though, zinc content was found statistically similar with all the three cultivars (Fig 5). The nitrogen fixation depends upon nodule count of the plant which reflects the N content in the plant shoot (Rhoden and Allen, 1982). Genotypic characters were the prime cause of differences in nutrient content. Safaya and Singh (1977) also reported the varying P accumulation capacity of cowpea genotypes. In case of uptake (Fig 2, 4 and 6), significantly the highest N, P, K and Zn uptake were noted with C-152 as compared to other cultivars. The higher nutrient uptake could be attributed to their higher nutrient content and dry fodder yield.


       
Zinc management practices caused significant variations on the N, P, K and Zn content (Fig 1, 3 and 5). Significantly the higher N and Zn contents were noted with 0.5% ZnSO₄ at 20 and 40 DAS, which was at par with Zn₂ and Zn₃ for N and Zn₃ for Zn. Likewise, application of 0.5% ZnSO₄ at 20 and 40 DAS (Zn₅) and 0.5% ZnSO₄ at 20 DAS (Zn₄) recorded the highest P content, while all the zinc applied plots showed statistically similar and higher K content over control. For nutrient uptake (Fig 2, 4 and 6), the application of 20 kg ZnSO₄(Zn₃) and foliar spray of 0.5% ZnSO₄ at 20 and 40 DAS (Zn₅) recorded considerably maximum uptake of N, K and Zn as compared to the remaining practices. Though, significantly the highest P uptake was noted with 0.5% ZnSO₄ at 20 and 40 DAS (Zn₅) which was at par with 20 kg ZnSO4 (Zn₃) and 0.5% ZnSO4 at 20 DAS (Zn₄). Addition of Zn to soil enhanced the content and uptake of N, K and Zn resulting from synergistic effect of Zn with these nutrients. Similar findings were also observed by Keram et al., (2012) in wheat. The higher nutrient content due to foliar application of Zn under present investigation was also supported by Yilmaz et al., (1997) who reported significantly higher Zn content in plants due to foliar application than basal. It might be due to easily penetration of Zn through leaves via. stomatal pathway or transportation and homogenization of spray cause higher diffusion of Zn in leaves. Higher absorption of Zn is due to combined application of zinc through soil and foliar application which leads to higher absorption of zinc (Oseni, 2009). Our results of zinc uptake are in consonance with finding of Debnath et al., (2018).
 
Post-harvest fertility status of soil
 
Data presented in Table 2 indicated that the pH, EC and soil organic carbon and available micronutrient content in soil (Mn, Cu and Fe) were not affected significantly. Though, available N, P and K and DTPA extractable Zn were differed significantly due to cultivars. Significantly highest levels of available N and K status after cowpea harvest were noted with C-152 as compared to rest of both cultivars. However, MFC-08-14 showed the maximum levels of available P content in soil. Likewise, significantly the highest DTPA-extractable Zn content in the soil was noted under MFC-09-1 which was at par with MFC-08-14. The variations in soil nutrient content may be due to differences in root volume, release of chemical exudates and nutrient uptake of cultivars. Adjei-Nsiah et al., (2008) observed a wide range of nitrogen recycling by the roots assuming that 30% of nitrogen in plant contained in roots.


       
Zinc application did not significantly affect the soil pH and EC. However, all others studied soil fertility parameters were significantly differed due to zinc application (Table 2). All the zinc applied plots (Zn₂ to Zn₅) showed significantly highest available N and K content and DTPA-extractable Zn status in soil. Amongst all Zn management practices, the highest values of these nutrients (N, K and Zn) were found with basal application of zinc @ 20 kg/ha (Zn₃). Available P status was significantly higher with foliar spray of 0.5% ZnSO₄ at 20 DAS as compared to soil application, though, significantly lower than control. Similarly, DTPA-extractable Mn, Cu and Fe content also indicated a decreasing trend from control to zinc application. Significantly the lowest DTPA-extractable Mn, Cu and Fe content were noticed under Zn₃ and Zn₅ which might be due to negative interaction with Zn. The increased N and K content due to Zn application in present experiment are also supported by findings of Kumar et al., (2017). Decreasing trend of P with increase in Zn concentration of soil might be due to increased negative surface charges on P which ultimately increased the sorption of Zn in soil lead to reduced DTPA-extractable Zn (Prasad et al., 2016). Keram et al., (2012) reported that application of 20 kg ZnSO₄/ha recorded significantly higher DTPA-extractable Zn under wheat crop. As far as method of application is concerned, the basal application enhanced the Zn content in soil as it directly contributes to nutrient pool and increase availability through complexion. The results are in line with the findings of Das et al., (2005). The lower DTPA-extractable Cu could be due to the fact that both Cu and Zn cations compete with each other for adsorption site on soil clay minerals and interact negatively (Prasad et al., 2016).
 
Economics
 
Growing of cowpea cv. C-152 incurred the lowest cost of cultivation (INR 22495/ha), but highest gross returns, net returns and benefit cost ratio among all the tested cultivars (Table 3). The variations in cost of cultivation and gross returns were due to differences in their seed price and green fodder yield, respectively. Velayudham et al., (2015) also observed significant variation in economics due to differential yield of cowpea varieties.


       
In case of zinc management practices (Table 3), the maximum cost of cultivation was noted with application of 20 kg ZnSO₄ as basal (Zn₃). Similarly, significantly the highest gross (INR 53542/ ha) as well as net returns (INR 30456/ha) of cowpea were obtained with application of 20 kg ZnSO₄ (Zn₃) which remained at par with foliar spray of 0.5% ZnSO₄ at 20 and 40 DAS (INR 52894 and 29973/ ha, respectively). Our findings are analogues with the findings of Soni and Kushwaha (2020) who reported that foliar spraying of 0.5% ZnSO₄ at different growth stages exhibited the maximum net return in mungbean. Similar results have also been observed by Rathore et al., (2015c) who reported highest BCR under 20 kg ZnSO4 as soil application.
 
Correlation studies
 
Correlation studies (Table 4) indicated that the growth attributes of cowpea, i.e., plant height (r=0.879), number of leaves (r=0.872), leaf length (r=0.849), leaf width (r=0.806), number of branches (r=0.861), number of root nodules (r=0.831), N content (r=0.860) and K content (r=0.890) were strongly and positively correlated with green fodder yield. However, moderate positive correlation was noted between the green fodder yield vs. P (r=0.628) and Zn content (r=0.650). The non-significant correlation was noted between Zn vs. P (r=0.276) and K content (r=0.466). Likewise, the correlation between the number of branches (r=0.462) and the number of root nodules (r=0.360) with P content were found non-significant.

Results from the present study revealed that fodder cowpea cv. C-152 along with application of either 20 kg ZnSO₄ as basal or 0.5% ZnSO₄ as foliar spray at 20 and 40 DAS was found to be most productive as well as profitable approach to attain quality fodder with sustaining the soil fertility.