Cowpea (Vigna unguiculata L.) is a versatile annual legume crop suitable for cultivation in all seasons due to its wide range of adaptability, quick growing habit, short duration, and high green fodder yield potential and better quality with palatability . Being a legume crop, it provides nutritionally superior fodder rich in crude protein, minerals, and low acid and neutral detergent  fiber,besides a minimum wastage of fodder during feeding due to high palatability and which plays a key role in livestock production [16]. It improves soil fertility through the fixation of atmospheric Nitrogen and improves soil fertility and thereby grows well in low fertile soils. The closer spacing leads to the overcrowding of plants per unit area, which compete for available limited light, moisture, and carbon dioxide. On the other hand wider spacing reduces the optimum plant population their by decreasing the fodder yield, therefore it is essential to find out the optimum special requirement for a particular variety to boost maximum yield and quality [19]. Cowpea is highly responsive to various nutrients applied through external sources, among the nutrients, Nitrogen plays an essential role cell division, cell elongation and differentiation, there by leads to better root proliferation and luxuriant growth of the crop (Shekar et al., 2019). Apart from these it is also involved in the different metabolic process of plant and helps in plant metabolism, it is a constituent of protein, chlorophyll and imparts dark green color to the plants, which helps photosynthetic activities and produce more of green biomass with better quality in a short period. Next to Nitrogen, Phosphorus is a key nutrient for increasing productivity of legumes. It is required for plant growth and root development. Phosphorus deficiency in soil is widespread and crops grown under deficient situation shows significant response to fertilizer phosphorus [14].  Phosphorus is a yield limiting nutrients and constituents of nucleic acids, proteins, amino acids and several co-enzymes, which stimulate root growth and resulted better nutrient uptake and vegetative growth of the crop [21]. Fodder cowpea Genotypes were significantly influenced by phosphorus levels with respect to green fodder, dry matter, crude protein yield, plant height and number of branches per plant [15]. Hence keeping these things in view, the present study was undertaken to standardize the optimum plant population, Nitrogen and Phosphorous requirement for higher green biomass yield and quality of fodder cowpea.

Materials and Methods

Study location: The field experiment was conducted at Zonal Agricultural Research Station, Vishweshwaraiah Canal Farm, Mandya (Karnataka) during Kharif-2015. The experimental site is situated between 12° 45’ and 13° 57’ North latitude and 76° 45’ and 78° 24’ East longitude at an altitude of 695 meters above mean sea level and comes under Southern Dry Zone (ACZ-VI) Karnataka. The normal annual rainfall is 738.5 mm and mean maximum temperature varied from 27.2 ⁰C 32.2 ⁰C and minimum air temperature ranged from 14.4 ⁰C – 20.3 ⁰C. The mean relative humidity ranged from 36 % to 58 % and daily bright sunshine hours varies between 4.9 to 9.1 hours.

Soil characteristics:  The soil of the experimental site is sandy loam in texture with neutral reaction (7.01), medium in available nitrogen (287.08 kg/ha), available phosphorus (28.10 kg/ha) and available potassium (149.81 kg/ha). Theses soils are classified as fine, kaolinitic, isohyphothermic, typic, ustipsamments as per USDA classification. These soils are yellowish red and are derived from granite gensis under subtropical semi-arid climate. The field capacity of the soils varies from 15.31 % to 22.35 % and available water ranges between 1.67 cm to 1.70 cm and bulk density of 1.36 gm/cc to 1.39 gm/cc.

Table 1. Green forage and dry matter yield of Fodder cowpea as influenced by spacing, nitrogen and phosphorus levels

       NS= Not Significant

Table 2. Proximate composition of fodder cowpea as influenced by spacing, nitrogen and phosphorus levels

NS: Not Significant, CP: Crude protein, CF: Crude fibre, EE: Ether extract, TA: Total ash, NFE: Nitrogen free extract, TC: Total carbohydrates, ADF: Acid detergent Fibre, NDF: Neutral detergent fibre

Table 3. Nutrient uptake (kg/ha) of Fodder cowpea as influenced by spacing, nitrogen and phosphorus levels

Table 4. Economics of fodder cowpea as influenced by different spacing, nitrogen and phosphorus levels

Experimental design and field management: The experiment was laid out in RCBD with a factorial concept and replicated thrice. The 18 treatment combinations involving three planting geometry (S₁-25cm × 10 cm), (S_{2 -30 cm × 10 cm), (S3-}45 cm × 10 cm), two nitrogen levels N₁-25 kg N/ha, N₂-37.5 kg N/ha, and three phosphorus levels (P₁-37.4 kg P₂O₅/ha), (P₂-50 kg P₂O₅/ha) and (P₃-62.5 kg P₂O₅/ha). The fodder Cowpea variety MFC-09-1 developed at mandya centre Karnataka by crossing KBC-2 X CoFC-8 and recommended for cultivation southern parts India, It is having green forage yield potential of 240-250 q/ha with protein content of 18.0% and it matures at 45 days after sowing, i.e. 50% flowering stage [11]. The crop was sown during 2^nd fortnight of July at a sowing depth of 5 cm. The Nitrogenand Phosphorus were applied in the form of Urea and superphosphate as per treatment and potassium 25 kg/ha was applied uniformly in the form of muriate of potash at the time of sowing as basal application. The cultural practices were followed as per recommended package of practices for establishment of crop. The crop was harvested for green fodder purpose at 50 per cent flowering stage. The known quantity of samples was taken and oven dried to attain constant weight in thermo statistically controlled oven at 70 ± 2 ⁰C for the estimation of dry matter content and yield as well as other quality parameters. The economics was worked out with prevailing market price of output and input cost. The statistical analysis of data was done for interpretation of the results and draw conclusion [5].

Fodder quality analysis: The determination of different quality parameters was done as per AOAC (2005). However, fiber fractions viz., neutral detergent fiber (NDF) and acid detergent fibre (ADF) were analyzed as per the method of Van Soest [20].

One gram oven dried sample ground in Willey mill using 1 mm sieve was weighed and kept in a 500 ml capacity spoutless beaker, 100 ml neutral detergent solution, 0.5 g sodium sulphite and 2 ml decahydronapthalene was added to it and the solution refluxed for one hour on the hot plate as per method of Van-soest [20]. After one hour, the contents in the beaker was filtered using vacuum filter machine and filtrate was collected in pre-weighed crucibles (W₀), washed first with hot water and then with acetone twice. Crucibles were then dried overnight in the hot air oven and weighed (W₁). The NDF content was calculated and expressed in per cent as follows:

Acid detergent fibre (ADF)

One gram oven dried sample ground in Willey mill using 1 mm sieve was weighed and kept in to 500 ml capacity spoutless beaker, 100 ml acid detergent solution, 2 ml decahydronapthalene added to it and the solution refluxed for one hour on the hot plate as per method of Goering and Van-soest [22]. After one hour, the contents in the beaker was filtered using vaccume filter machine and filtrate was collected in pre-weighed crucibles (W₀), washed first with hot water and twice with acetone and finally with hexane. Crucibles were then dried overnight in the hot air oven and weighed (W₁). The ADF content was calculated and expressed in per cent as follows:

Economics:

Results and Discussion

Green forage and dry matter yield: Research results revealed that the spacing of 30 cm × 10 cm significantly recorded higher green forage (178.94 q/ha) and dry matter yield (25.55 q/ha) over other treatments. Nitrogen had no significant influence on green forage and dry matter yield (Table 1). However, numerically higher green forage (170.82 q/ha) and dry matter yield (24.51 q/ha) were recorded with nitrogen at 37.5 kg/ha. Application of phosphorus 62.5 kg/ha recorded significantly higher green forage (179.55 q/ha) and dry matter yield (25.24 q/ha). The interaction between spacing and nitrogen levels was found significant and the rest of the treatment combinations were found non-significant. The increasing green forage yield with closer spacing is mainly due to better growth indices, which resulted higher green forage yield and contributed to higher dry matter production. Apart from this better uptake of nutrients was observed with spacing of closer spacing, which resulted in more synthesis and accumulation of photosynthates under better solar radiation.   These results confirm with the findings of [9] [10] [13]. The higher green forage & dry matter yield with a higher level of nutrients is mainly due to nitrogen Nitrogen having a positive effect on growth indices, synthesis and accumulation of photosynthates, since Nitrogen is an integral part of chlorophyll which is primary absorbance of light energy needed for photosynthesis. It also structural constituents of many amino acids, Proteins and protoplasm of the cell which are essential for the growth and development of plant cells, resulting in higher green biomass and contributed for more dry matter accumulation and production. Phosphorus is constituent of ribonucleic acid and ATP. Which regulate vital metabolic processes in the plant and helping in root growth and development, process of nitrogen fixation and yield of crop. These results are in conformity with the findings of [8] and [3].

Fodder quality parameters: The wider spacing of 45 cm × 10 cm recorded significantly higher content of crude protein (16.40 %), crude fibre (22.60%), ether extract (2.86 %) total ash (11.10%) and acid detergent fibre (31.28 %) and neutral detergent fibre (34.94%) (Table 2). This is mainly due to vigorous growth of plant and better growth indices, which helped plant to uptake of more of nutrients by plants which resulted in higher proximate composition. Whereas Nitrogen free extract and total carbohydrate content was higher with a closer spacing of 25 cm × 10 cm (51.06 % and 72.90 % respectively). This might be due to lower crude protein, crude fibre, ether extract and total ash. The similar results were reported by [10] [1] [12]. Application of Nitrogen at 37.5 kg/ha recorded significantly higher content of crude protein (16.60 %), ether extract (2.87 %) and total ash (10.47 %). This is mainly due better uptake of Nitrogen, which resulted increases protein content of herbage. These findings are in conformity with the findings of [6]. Application of phosphorus 62.5 kg/ha recorded significantly higher content of crude protein (16.15 %), crude fibre (23.30 %), total ash (10.32 %), acid detergent fibre (29.77 %) and neutral detergent fibre (34.07 %). This may be ascribed to phosphorus as constituents of ribonucleic acid and ATP, which regulated metabolic processes in plants and helping root formation, better growth induces and nitrogen fixation which resulted in improved proximate content of plant. This is in line with findings of [2]. The Nitrogen free extract and total carbohydrates content were not significantly influenced by Nitrogen and phosphorus levels at different spacing’s, whereas, crude protein and crude fibre content were not influenced by spacing’s at different levels of Nitrogen and phosphorus.

Nutrient uptake: The uptake of nitrogen phosphorus and potassium was not significantly influenced by spacing (Table 3). However higher uptake was observed with the spacing of 30 cm × 10 cm (62.76 kg, 4.18 kg and 23.70 NPK kg/ha, respectively). Application of nitrogen 37.5 kg/ha recorded significantly higher uptake of NPK (65.50 kg, 4.19 kg, and 23.38 NPK kg/ha). Among phosphorus levels, significantly higher uptake was observed with application of 62.5 P₂o₅kg/ha (65.05 kg, 4.44 kg, and 24.05 NPK kg/ha). The higher uptake with higher Nitrogen and phosphorus due to better root growth and growth indices which resulted more uptake of nutrients. This is in accordance with the findings of [8] [7] [3] and [4].

Economics: The spacing of 30 cm × 10 cm recorded higher gross returns (26,842 Rs./ha), net returns (17754 Rs./ha) and benefit-cost ratio (2.95) (Table 4). The higher level of nitrogen 37.5 kg/ha recorded higher gross returns, net returns and benefit cost ratio (25,622 Rs. ha^-1, 16,371 Rs. ha^-1 and 2.77 respectively). Application of phosphorus 62.5 kg/ha significantly recorded higher gross returns, net returns and benefit cost ratio (26,933 Rs. ha^-1, 17,415 Rs. ha^-1, and 2.83 respectively). The higher net returns and benefit-cost ratio is mainly due to optimum plants population under planting geometry of 30 cm × 10 cm and luxuriant vegetative growth of the crop with higher level of nutrients, resulted more green biomass. These results are in line with the findings of [17] [18].

Conclusion

To overcome the deficit of fodder, dairy farmers resort, to the increased use of costly concentrate feeds, which increases the cost of production of milk. This has to be reduced to the extent possible by nutritive green fodder. Although, source of cereals fodder are widely used in livestock nutrition for their high dry matter production and low cost, but they have low nutritive value due to their poor crude protein content. On the other hand leguminous fodder crops are known for their quality, providing supplementary proteins, Phosphorous, Iron and certain water soluble vitamins. They can be used in small quantities to compensate poor quality bulk forages or substitutes for concentrates. Among leguminous crop, the fodder cowpea variety MFC-09-1 with row spacing of 30 cm and 10 cm between the plants within a row with application of nitrogen 37.5 kg/ha and phosphorus 62.5 Kg/ha as basal dose at the time of sowing found optimum and economical, which recorded higher green forage, dry matter yield and better quality content and yield, hence it can be recommended to the farming community for adoption of cultivation in southern parts of Karnataka under rainfed ecosystem.

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