Effect of Cowpea Density and Nitrogen Fertilizer on a Sorghum-Cowpea Intercropping System in Kobo, Northern Ethiopia

Sorghum productivity is limited in Kobo area mainly due to deterioration of soil fertility and continuous cropping. Hence, an experiment on sorghum/cowpea intercropping was conducted in 2014/15 cropping season to assess the effect of density of the intercropped cowpea and nitrogen fertilizer rates on yield and yield related traits of sorghum and cowpea as well as to determine the appropriate plant density of cowpea and nitrogen fertilizer rate that maximize the productivity of the intercrop system. Treatments consisted of factorial combinations of three cowpea densities (50, 75 and 100%) and four levels of nitrogen rates (0, 20.5, 41 and 61.5kg N ha -1 ) accompanied with sole sorghum and sole cowpea, carried out in Randomized Complete Block Design (RCBD) with three replications. The analyzed result showed that plant height and dry biomass yield of sorghum as well as leaf area and leaf area index of both crops showed an increased trend as N level increased linearly. Leaf area index of cowpea was also affected by cowpea density in which the largest data was recorded at high population density. Similarly, grain yields of component crops were significantly influenced (P<0.01) due to the interaction effect. Thus, the highest grain yield of sorghum (2370.4 kg ha -1 ) and cowpea (821.3 kg ha -1 ) were obtained from the combination of 41 kg N ha -1 + 75% sole cowpea density, and 20.5 kg N ha -1 + 100% of cowpea density, respectively. Similarly, 20.5 kg N ha -1 + 100% planting density produced the highest dry biomass (2375.0 kg ha -1 ) of cowpea. The land equivalent ratio (LER) and gross monetary value (GMV) showed that sorghum-cowpea intercropping was highly superior to and more advantageous over sole cropping. The highest values of LER and GMV were obtained from combination of 41 kg N ha -1 and 75% sole cowpea density. This is, therefore, sorghum-cowpea intercropping was proved to be more productive and efficient system in utilizing land compared to sole cropping with carefully managed N fertilizer and cowpea plant density. Hence, combination of 41 kg N ha -1 and 75% sole cowpea density can be recommended for the farmers in the study area to improve sorghum productivity and the cropping system at large.