Description of experimental site

To evaluate the impact of mixed strip intercropping and planting geometries on the productivity of cowpea intercrops, a field experiment was conducted during summer months of 2013 and 2014 at the research area of University of Agriculture, Faisalabad (30.35-41.47°N and 72.08-73.40°E) situated at an attitude of 184 m¹⁴. The climate of the experimental area falls into Koppen’s class of semi-arid, while the soil of the experimental site is classified as Haplic Yermosols as per FAO soil classification system. The meteorological data for crop growing seasons of cowpea were obtained from the meteorological center located closer (about 1 km) to research fields (Table 1).

Experimental treatments and design

Cowpea and sorghum were sown in different strip intercropping systems and three spatial arrangements as follows: T₁A₁= 8 rows strips (cowpea-sorghum in 4-4 rows in the same strips) with 30 cm row-row spacing, T₁A₂= 8 rows strips (cowpea-sorghum in 4-4 rows in the same strip) with 45 cm row-row spacing, T₁A₃= 8 rows strips (cowpea-sorghum in 4-4 rows in the same strip) with 60 cm row-row spacing, T₂A₁= 12 rows strips (cowpea-sorghum in 6-6 rows in the same strip) with 30 cm row-row spacing, T₂A₂= 12 rows strips (cowpea-sorghum in 6-6 rows in the same strip) with 45 cm row-row spacing, T₂A₃= 12 rows strips (cowpea-sorghum in 6-6 rows in the same strip) with 60 cm row-row spacing, T₃A₁= 16 rows strips (cowpea-sorghum in 8-8 rows in the same strip) with 30 cm row-row spacing, T₃A₂= 16 rows strips (cowpea-sorghum in 8-8 rows in the same strip) with 45 cm row-row spacing, T₃A₃= 16 rows strips (cowpea-sorghum in 8-8 rows in the same strip) with 60 cm row-row spacing.

In this way, a total of 9 treatment combinations were tested in factorial arrangement of randomized complete block design (RCBD) with three replications. The strip × strip distance for all intercropping systems was kept at 70 cm. Cowpea rows were adjacent to sorghum rows in subsequent strips. There was no consideration for plant × plant distance. In total, there were 27 experimental plots which were homogeneously maintained for testing the proposed treatments.

Agronomic management plan

In order to formulate the soil fertility management plan, pre-sowing physico-chemical analysis was performed from soil samples collected from 15 and 30 cm depth (Table 2). The seedbed preparation was started with a pre-sowing irrigation of 12 cm and 3 tractor mounted cultivations each followed by planking was done. Cowpea (cv. P-51840 at kg ha^-1) and sorghum (cv. Hegari at 80 kg ha^-1) were intercropped in 30 cm spaced rows using a hand drill. Recommended dose of nitrogen (50 kg ha^-1) (urea) was applied in two splits (at the time of sowing and with first irrigation 12 d after sowing) while total phosphorous (single super phosphate) (40 kg ha^-1) was applied as basal dose. Three flood irrigations were applied at 12, 33 and 50 d after sowing. Manual hoeing was done thrice (12, 22 and 32 d after sowing) to keep weed infestation at bay. Cowpea intercrops were harvested using hand sickle at complete flowering.

Data recordings

All agronomic attributes of cowpea were recorded at the time of harvesting by following the prescribed methods. Ten plants were harvested from middle rows of each replication and then their average was taken. Plant height was recorded with the help of tailor’s measuring tape from base of the plant to the tip of the highest leaf. Stem girth was taken by using vernier caliper. Electric balance was used to take fresh weight per plant while spring balance was used to record green forage yield per plot which was then converted into tons per hectare. The agro-qualitative attributes of forage cowpea were determined by using methodologies given in Table 3.

Statistical analysis

Statistical analyses of the recorded data were done through employing analysis of variance (ANOVA) using the statistical program “Statistix 8.1”. The means were grouped for conducting orthogonal contrasts on following basis; (a) intercropping system versus year, (b) spatial arrangement versus year, (c) intercropping system versus spatial arrangement and (d) intercropping system versus spatial arrangement versus year at 5% probability level. The data were also subjected to correlation analysis in order to sort out the relationship (linear or inverse) between yield attributes and forage yield of cowpea.

Plant height and stem diameter

The agronomic variables of forage cowpea were significantly improved during 2014 probably owing to higher precipitation and moderate temperatures in comparison to 2013. The interactive effect of strip intercropping systems and spatial arrangements was significant for plant height (189** and 203** during 2013 and 2014 respectively) and stem girth (88* and 98** during 2013 and 2014 respectively) of cowpea (Table 4). The tallest cowpea plants (110.3 ± 0.57 and 117.9 ± 0.83 cm during 2013 and 2014 respectively) with greatest stem girth (2.87 ± 0.67 and 2.94 ± 0.69 cm during 2013 and 2014 respectively) were recorded by cowpea sown in 12-rows strips with 60 cm spaced rows (T₂A₃), while 16-rows strips having 45 cm line-line spacing (T₃A₂) resulted in the lowest plant height (78.0 ± 0.38 and 83.1 ± 0.82 cm during 2013 and 2014 respectively) as well as stem girth (2.32 ± 0.81 and 2.53 ± 0.41 cm during 2013 and 2014 respectively) (Table 5). Correlation analysis revealed that there was linear correlation between plant height and stem girth of cowpea as depicted in Figure 1. These results are in complete confirmation with another study²³, where legumes plant height and stem diameter were influenced planting geometries of cereal-legume intercropping systems. Simultaneous cultivation of component crops in row and mixed intercropping systems intensified inter-species competition for farm applied resources which led to reduced plant height and stem girth of legumes compared to their monocultures. But when cowpea was sown in 12-rows strips (cowpea-sorghum in 6-6 rows), it might have reduced sorghum dominance in acquiring growth resources. Varied root lengths of cowpea and sorghum might be attributed as the probable reason for reducing competition for growth resources which was further supported by wider strip spacing²⁴.

Figure 1 Correlation analysis for yield attributes with green forage yield and dry matter yield of cowpea (combined analysis of pooled data of 2013 and 2014) 

Number of leaves and leaf-stem ratio

The interactive effect of intercropping systems and spatial arrangements was significant for the number of leaves (93* and 112* during 2013 and 2014 respectively) and leaf-stem ratio (83* and 96* during 2013 and 2014 respectively) (Table 4). Sorghum and cowpea 12-rows strip intercropping in 60 cm spaced lines (T₂A₃) resulted in higher number of leaves per plant (29.1 ± 0.57 and 29.9 ± 0.31 during 2013 and 2014 respectively) and leaf to stem ratio (0.59 ± 0.19 and 0.69 ± 0.21 during 2013 and 2014 respectively) (Table 5). These results corroborate with the findings of other studies^2,3,25, where it was concluded that closely spaced rows of legumes recorded minimum number of leaves and leaf-stem ratio despite exploring varied soil horizons for absorbing moisture and nutrients by sorghum and legumes, still the intra-species competition was severe enough to drastically reduce the growth of legume intercrops. Furthermore, shading effect rendered by sorghum was also found to be an important factor in reducing photosynthesis of legume plants particularly in adjacent rows with sorghum which leads to less number of leaves per plant.

Plants fresh and dry weights, green forage yield and dry matter biomass

The interactive effect of intercropping system and spatial arrangements was also significant for fresh weight (274** and 297** during 2013 and 2014 respectively) and dry weight (187** and 257** during 2013 and 2014 respectively) per plant of cowpea along with green forage yield (266** and 287** during 2013 and 2014 respectively) as well and dry matter yield (134** and 120** during 2013 and 2014 respectively). The highest fresh weight (188.6 ± 0.67 and 190.5 ± 0.61 g during 2013 and 2014 respectively) and dry weight (59.1 ± 0.67 and 66.3 ± 1.19 g during 2013 and 2014 respectively) per plant (Table 5) were rendered by 12-rows strip having 60 cm apart rows (T₂A₃). Correlation analysis depicted a linear relationship for fresh and dry weights per plant with green forage and dry matter yields (Figure 1). The same intercropping system (T₂A₃) was instrumental in yielding the maximum green forage yield (22.2 ± 0.28 and 23.7 ± 0.34 t ha^-1 during 2013 and 2014 respectively) and dry matter biomass (6.63 ± 0.26 and 6.94 ± 0.19 t ha^-1 during 2013 and 2014 respectively) of forage cowpea (Table 6), while it was followed by 12-rows strips sown in 45 cm spaced rows (T₂A₁). In contrast, sorghum-cowpea 8-rows strips having 30 cm row-row spacing (T₂A₁) remained superior as far as green forage biomass and dry matter yield of sorghum were concerned. It was followed by the same intercropping system having row-row spacing of 45 cm, while sorghum-cowpea intercropping systems comprising of 16-rows strips with 60 cm row-row spacing (T₃A₃) remained inferior to rest of intercropping systems and spatial arrangements (Table 6). The T₂A₃ intercropping system resulted in superior agronomic attributes including plant height, stem girth, fresh and dry weights per plant which ultimately enhanced green forage biomass as well as dry matter yield. These findings are in line with others^22,26, who inferred that productivity of cowpea in narrowly spaced (30 and 45 cm) intercropping systems remained below-par to solo cowpea despite well-developed nodulation and fully functional biological nitrogen fixation (BNF). Similar findings were also reported by other researchers^10,27, where cowpea remained recessive in acquiring nutrients and moisture compared to cereals. In addition, legume intercrops suffered losses in productivity owing to their dependence on soil solution for nitrogen before the initiation of BNF after 27-35 d of sowing. Moreover, in strip intercropping systems, cowpea rows adjacent to sorghum confronted lesser competition for growth resources by exploiting different soil horizons but had to face shading effect rendered by taller sorghum plants. Similarly, inner rows of cowpea faced lesser shading effect but competition for growth resources intensified owing to having same root length which led to reduced herbage yield²⁸.

Crude protein and crude fiber contents

All quality traits were significantly influenced by intercropping systems and spatial arrangements including crude protein (120** and 135** during 2013 and 2014 respectively), crude fiber (142* and 169** during 2013 and 2014 respectively), ether extractable fat (200** and 225*during 2013 and 2014 respectively) and total ash (101* and 109* during 2013 and 2014 respectively) (Table 4).

Protein content occupies vital position in determining the nutritional quality of forage while agronomists as well as animal nutritionist recommend protein-rich forages for boosting the performance of dairy animals. Cowpea-sorghum intercropping in 12-rows strips having 60 cm row-row spacing (T₂A₃) effectively improved crude protein (19.9 ± 0.21 and 19.6 ± 0.37 during 2013 and 2014 respectively) of cowpea forage with the minimum crude fiber (26.1 ± 0.51 and 26.0 ± 0.90 during 2013 and 2014 respectively) contents (Table 6). This was followed by 12-rows strips having 45 cm spacing (T₂A₁), while strips having 16-rows performed below par under all spatial arrangements. Earlier research works^1,14 are in conformity with these findings, as it was reported that substantial enhancement in crude protein of mixed forage could be achieved by intercropping cowpea with cereal forages under optimized spatial arrangements. It was suggested that type of intercropping could influence nitrogen fixed by cowpea which might be attributed for improved crude protein content and reduced fiber as the absorbed nitrogen and protein content were linearly correlated. Type of intercropping and planting geometries as in our research have also been reported to improve the efficacy of applied nutrients which imparted a significant influence on protein and crude fiber contents of forages^14,29.

Ether extractable fat and total ash contents

Fats are pivotal quality attribute of forages as these secrete higher amounts of energy during metabolism than proteins. Similarly, mineral constituents of forages required to perform various metabolic processes are measured as ash. The intercropping system (T₂A₃) resulted in the maximum fat (1.91 ± 0.17 and 1.95 ± 0.29 % during 2013 and 2014 respectively) and total ash (11.78 ± 0.16 and 11.7 ± 0.21 % during 2013 and 2014 respectively) (Table 6), 16-rows strips registered the minimum fat and ash contents without any regard to spatial arrangements. Strips having 8-rows under all planting geometries performed better in terms of forage quality than 16-rows strips but it remained below par to cowpea sown with forage sorghum in 12-rows strips. These findings also match with a previously conducted study³⁰, which revealed that considerably higher herbage yield with improved quality attributes could be obtained by optimizing intercropping type and spatial arrangement of component crops.