Critical Growth Stage of Weed Removal in Cowpea under Water Stress

This study aimed at investigating the critical growth stage of weed removal in cowpea in order to prevent unacceptable yield loss during water deficit. The study was carried out under a screenhouse to minimize extraneous factors such as pests and rodents using a complete randomized design (CRD) with five replicates. Seeds of cowpea and early germinating seedlings of Tridax procumbens and Chromolaena odorata were used for this study and were stressed for five days. Samplings were carried out at vegetative, flowering and fruiting stage. Growth indices such as relative growth rate, net assimilation rate, leaf area ratio and tissue water of contents of cowpea were determined from plant biomass. The photosynthetic pigments were determined spectrophotometrically at different stages of growth. Weed interference index such as relative crowding coefficient and land equivalents ratio were also determined from the plant biomass. The results indicated that the growth indices of cowpea interfered by Chromolaena odorata and Tridax procumbens were greatly reduced at flowering stage compared with the growth indices at vegetative and fruiting stage. Photosynthetic pigments such as chlororphyll a, chlorophyll b, carotenoids and total chlorophyll of cowpea as interfered by Chromolaena odorata and Tridax procumbens were also lowest at flowering stage. Interference index of cowpea were lowest at flowering stage as to the vegetative and fruiting stage of cowpea. There was significant difference in the growth indices, photosynthetic pigments accumulation and interference index of at the different growth stages of cowpea at p≤0.05. From the results obtained, flowering stage is the most critical period of growth stage to remove weed in cowpea under water stress in order to prevent an unacceptable yield loss.