Response of cowpea to variable rates and methods of zinc application under rainfed and supplementary irrigation conditions

Abstract

Agronomic field trial was planted at two sites, Ukulima and Syferkuil, in Limpopo Province, to determine the response of cowpea to variable rates and methods of zinc application under supplementary irrigation and rainfed conditions, respectively. The experiment was laid out in a split plot arrangement and fitted into a randomized complete block design (RCBD). Treatments consisted of two factors namely zinc rates (0, 5, 10, 15, 20 and 25 kg ha-1) and methods of application (soil and foliar) with white cowpea variety (IT00K-1217) seeds sown. The zinc fertilizer was applied as ZnSO4 (38.5% Zn) while basal phosphorus (30 kg P ha-1) was applied in the form of single super phosphate (10.5% P) so as to eliminate P constraints. Soil application of the zinc fertilizer was band placed near the row of cowpea plant while foliar application was done 3-5 weeks after plant emergence. Growth parameters measured included plant height, number of primary branches, canopy cover, number of trifoliate leaves, days to 50% flowering and pod formation, and fresh biomass as well as yield component attributes. Twenty young cowpea leaves and immature green pods were each harvested at approximately 75% physiological maturity. Crude protein, total nitrogen, potassium, phosphorus, zinc, and iron content and uptake were assessed in the different plant parts. Growth, yield and nutrients data collected were subjected to analysis of variance. Treatments means were separated using Tukey’s test at probability level of 5%, while the response variables were modelled using quadratic polynomial equation. Results obtained revealed that total above ground biomass yield of 6219 kg ha-1 and fresh pod weight of 142.3 g were obtained when zinc was soil applied under rainfed condition compared to 6019 kg ha-1 and 138.0 g for total above ground biomass yield and fresh pod weight, respectively when foliar applied. Soil application gave a total above ground biomass yield of 6298 kg ha-1 and pod fresh weight of 150.9 g while foliar application gave total above ground biomass yield and pod fresh weight of 4791 kg ha-1 and 124.0 g, respectively at Ukulima. Soil application also gave a significantly higher grain yield (2251 kg ha-1) than the foliar application (1503 kg ha-1) at Ukulima. A higher but inconsequential effect on grain yield was obtained with soil over foliar application at Syferkuil. Application of zinc fertilizer at a rate beyond 5 kg ha-1 resulted in a grain yield reduction of up to 22.2% and 6.6% respectively at vi Ukulima and Syferkuil. Thus, this rate appeared optimum for both grain and fodder production when soil applied at both sites. Based on the quadratic model, total biomass (4897 kg ha-1) and grain (1602 kg ha-1) yields were optimized at an estimated zinc rate of 54.7 and 33.4 kg ha-1, respectively under supplementary irrigation, while total biomass (5913 kg ha-1) and grain (2696 kg ha-1) yields were obtained at an estimated optimum zinc rate of 20.1 and 26.8 kg ha-1, respectively under rainfed condition. A scorching effect of cowpea leaves was observed following foliar application of zinc fertiliser at 25 kg ha-1 that resulted in a decreased growth and yield at Ukulima. Foliar application resulted in improve cowpea leaf zinc concentration (43.9 mg kg-1) compared to soil application (23.2 mg kg-1) at Syferkuil. It also resulted in increased crude protein, total N, P, Fe and Zn uptake compared to soil application at Ukulima. Both soil and foliar zinc application gave 28.5% zinc concentration in the fresh immature pods samples collected. Findings from this study reveal that foliar zinc sulphate application resulted in higher grain and biomass yields as well as zinc concentration in the various cowpea plant parts. Based on the quadratic model used in this study, the optimum zinc rate required to guarantee high cowpea yield and zinc-rich grain and leaf content at both trial sites is highly variable; and thus requires further study for validation. Keywords: Cowpea production, grain yield, dryland farming, leafy vegetables, foliar fertilizer, improved nutrition, zinc deficiency.