Experimental site description

The study was carried out at different locations of Agronomic Farm of University of Agriculture Faisalabad, Pakistan during summer months of 2013 and 2014 under same agro-climatic conditions. The geographical coordinates of the experimental site are 30.35-41.47° N latitude and 72.08-73.40 °E longitude, having an elevation of 184 m. The climate of experimental site is semi-arid according to Koppen-Geiger classification, while the soil of the experimental area belongs to Haplic Yermosols of FAO soil classification scheme⁷.

Experimental treatments and design

Two varieties of forage cowpea (P-518 and Rawan-2003) were sown at 30, 45 and 60 cm spaced rows and as broadcasted crops. In this way, there were a total of 8 treatments including V₁ (P-518 broadcasted), V₂ (P-518 sown in 30 cm spaced rows), V₃ (P-518 sown in 45 cm spaced rows), V₄ (P-518 sown in 60 cm spaced rows), V₅ (Rawan-2003 broadcasted), V₆ (Rawan-2003 sown in 30 cm spaced rows), V₇ (Rawan-2003 sown in 45 cm spaced rows), V₈ (Rawan-2003 sown in 60 cm spaced rows). The net plot size was 3.6 m × 15.0 m. Each experimental plot had 12, 8 and 6 lines for 30, 45 and 60 cm spaced rows, respectively. There were four replications for each treatment. Factorial arrangement of randomized complete block design (RCBD) was employed to carry out this field trial during both years.

Crop husbandry

For conducting pre-sowing physico-chemical analysis of the experimental units, soil samples were collected from 15 and 30 cm depth and then thoroughly homogenized, while representative samples were taken for recording the soil quality (Table 1). Meteorological data regarding temperature, rainfall and relative humidity during crop growing seasons were also collected from meteorological observation center located close to the field trials (Figure 1).

Figure 1: Meterlogical data for temperature (⁰C), rainfall (mm) and relative humidity (%) during crop growing seasons at Faisalabad (Pakistan) during 2013 and 2014 

The seed rate for both cv. P-518 and cv. Rawan-2003 was 35 kg ha^-1 and sowing was done on May 16 and May 21 during 2013 and 2014, respectively, with the help of hand-pulled single row drill. N:P at the rate of 60:22 were applied as urea and single super phosphate (SSP). Phosphorous was applied in a single dose at the time of sowing while nitrogen was applied in two equal splits (half dose at the time of sowing and remaining with 1^st irrigation at 15 d after sowing (DAS). Three irrigations were applied at 15, 30 and 50 DAS. Manual harvesting (single cut) at one inch from ground surface was done with sickle after 78 and 73 DAS in 2013 and 2014, respectively at pod filling stage.

Data collection

Dry matter yield was determined by harvesting ten plants from the middle rows of each replicate and were chopped with an electric fodder cutter. Their fresh weight was noted by using an electric balance and 500 g sample was taken from it. These samples were then placed in an oven at 70 ^⁰C until a constant weight was obtained, which was then used to calculate dry matter yield per hectare. Crude protein was determined using Macro-KJeldahl method involving acid (K₂SO₄, CuSO₄ and FeSO₄ in 10:0.5:1 ratio) digestion which gave nitrogen. The obtained nitrogen was multiplied with a constant factor (6.25) to calculate crude protein. In order to calculate crude fiber, digestion of dried samples with H₂SO₄ and NaOH was performed and then muffle furnance was used to burn non-fibrous substances. Then, crude fiber was calculated by following procedure as outlined by AOAC^10):

Soxhlet extraction apparatus was used to calculate ether extractable fat, while total ash was determined using muffle furnace technique which involved burning of dried samples to ash at 600 ⁰C (AOAC)¹⁰.

Cost of production

Cost of production for both years was calculated in order to perform economic analysis. The cost of production was computed by calculating fixed expenditures including costs of land preparation, sowing, irrigations, fertilizers, harvesting, transportation and land rent. Then the variable expenditures per treatment were also calculated. Total expenditures for each treatment were calculated by the following:

Gross income was calculated as:

Net income rendered by different treatments was calculated by deducting the total expenditure from the gross income¹¹.

Benefit-cost ratio (BCR) was determined by using the following formula:

Statistical analysis

For performing statistical analysis, the collected data were subjected to computer run statistical program “MSTAT-C”^12,13 by employing analysis of variance (ANOVA) technique. The grouping of the means was done for orthogonal contrasts; (і) variety versus spatial arrangements, (іі) variety versus year, (ііі) spatial arrangements versus year and (іν) variety × spatial arrangements × year. The level of significance was defined by P<0.05 until and unless stated otherwise.

Dry matter biomass

The individual effect of varieties and spatial arrangements was found to be significant (Table 2), while the interaction effects of cultivar × spatial arrangements as well as cultivar × spatial arrangements × year were also significant, while interaction effects of cultivar × year and spatial arrangement × year were non-significant.

Statistical analysis revealed that both varieties of cowpea differed significantly (P≤0.01) in their potential for dry matter biomass production (Table 2). It was also observed that spatial arrangements were effective (P≤0.05) in influencing the productivity of forage cowpea in terms of dry matter biomass. It was found that Rawan-2003 was more productive than P-518 (P≤0.05) especially when it was sown at 45 cm spaced rows (V₇) (8.26 and 9.03 t ha^-1 in 2013 and 2014, respectively). The same cowpea variety sown in 30 cm spaced rows (V₆) followed it, while the lowest dry matter yield (6.27 and 6.39 t ha^-1 in 2013 and 2014, respectively) was recorded by P-518 which was broadcasted (V₆). Overall, Rawan-2003 performed better at 45 cm spaced rows, while P-518 had better results at 30 cm spaced rows.

One of the apparent reasons for this performance might be the spreading nature of Rawan-2003 which required more space to grow in comparison with P-518. Another reason could be higher genetic potential of Rawan-2003 than P-518 as far as dry matter biomass production was concerned. Similar findings were reported by other researchers¹⁴, who suggested that very few varieties of cowpea have good genetic potential to yield reasonably higher quantities of green forage owing to higher photosynthesis rate and producing more number of leaves and branches along with utilizing plant nutrients more efficiently. Furthermore, it was recorded that an appropriate agronomic management including an optimum spatial arrangement was vital to achieve higher forage yield as well as genetic potential of the variety. It was also observed that cowpea performed better when inter-row spacing was maintained at 45 cm in comparison with 60 and 75 cm spaced rows. Cowpea forage yield was increased with decreasing row spacing, thus narrow row spacing could bring positive results for erect type varieties of cowpea¹⁵. Contrarily, another study¹⁶reported that cowpea intercropped with pearl millet performed better in 30 cm spaced rows in comparison with wider row spacing.

Quality of forage

The productivity and performance of dairy cows is directly influenced by quality attributes of feedstuffs, especially higher crude protein has been reported to be effective in increasing milk production. Similarly, fiber is considered to be an anti-nutritional factor in forages and its low concentration reduced the bulkiness of feed which caused a significant increase in feed intake¹⁵.

Rawan-2003 cultivar was significantly (P≤0.05) superior in term of crude protein contents (18.93 and 18.97 % in 2013 and 2014, respectively) and the lowest crude fiber (25.64 and 25.63 % in 2013 and 2014, respectively) especially when it was sown in 45 cm spaced rows (V₇). Protein of Rawan-2003 cultivar decreased with increase in inter-row spacing. P-518 recorded comparatively lower crude protein and significantly (P≤0.05) higher crude fiber contents than Rawan-2003, especially when it was broadcasted (V₇) (Table 3). These results corroborate with the conclusions made by another field research¹⁷, where it was reported that crude protein and crude fiber contents of forage cowpea varieties could be due to genetic potential, however closer line sowing (30 cm) was also found to be effective in increasing crude protein and reducing fiber contents of cowpea forage. Furthermore, spreading type of cultivars recorded higher protein while erect verities of cowpea yielded more fiber contents.

Fats and ash are also important quality parameters of animal feed owing to their vital role in a variety of metabolic processes. Rawan-2003 recorded significantly (P≤0.05) higher ether extractable fat (1.91 and 1.9 4% in 2013 and 2014, respectively) and total ash (11.90 and 11.92 % in 2013 and 2014, respectively) contents particularly it remained unmatched when it was sown in 45 spaced rows (V₇). On the other hand, P-518 sown as broadcasted crop (V₁) recorded the lowest ether extractable fat and total ash contents. These results are in complete confirmation with those of another study¹⁸ which reported that spatial arrangement was found to be an important factor in influencing fat and ash contents. However, these finding contradict with the conclusions reported by another field investigation¹⁹ which suggested that spatial arrangements did not affect fat and ash contents of cowpea.

Economic analysis

Profit has occupied central place in recent commercial and profit-oriented farming as reduction in economic returns result in shifting to other crops. Rawan-2003 sown in 45 cm spaced rows gave the highest net income (US$. 869.50 and 923.50 in 2013 and 2014, respectively) (V₇) and it was followed by same cowpea variety sown at 30 cm spaced rows (V₆) (Tables 4 and 5). The lowest net income was generated by P-518 when it was broadcasted (V₁). However, P-518 recorded the highest net income when it was sown at 30 cm spaced rows. Broadcast method of sowing resulted in the lowest net income for both varieties of forage cowpea.

Following the trend, Rawan-2003 sown at 45 cm spaced rows (V₇) remained unmatched in terms of benefit-cost ratio (BCR) (4.66 and 4.85 in 2013 and 2014, respectively) and it was followed by same cowpea variety sown at 30 cm spaced rows with BCR of 3.94 and 4.32 in 2013 and 2014, respectively, while wider row spacing did not work at par with those of 30 and 45 cm spaced rows. Both varieties sown with broadcast method resulted in significantly lower BCR particularly P-518 witnessed the lowest BCR and this trend was evident during both years. The results of this study corroborate the findings of other investigations^20,21, where comparatively closer spatial arrangement for different cowpea cultivars was instrumental in generating the highest net income owing to higher production per unit of land basis, while wider intra-row spacing caused a considerable reduction in economic yield which was bound to bring down the net income. Similarly in complete agreement of this research’s findings, a number of researches^22,23,24) also reported that closer row spacing of different legumes including cowpea were effective in increasing net income as well as benefit-cost ratio. They concluded that by optimizing spatial arrangements, there was no additional cost involved but it significantly increased forage yield of soybean which increased net income as well as benefit-cost ratio.