Row Spacing Determines Critical Period of Weed Control in Crop: Cowpea (Vigna unguiculata) as a Case Study

To further understand the importance of row spacing as an agronomic practice, a study was conducted to evaluate how levels of row spacing determines the critical period of weed control (CPWC) in cowpea (Vigna unguiculata). The experiment was laid out as a split-plot design with eight periods of weed interference as the main plots and three row spacing (60, 75 90 cm) as the subplots with three replications of each treatment combination. Period of weed interference consisted of weed removal for 14, 35, 54, and 77 days after emergence (DAE), and weed infestation for 14, 35, 54, and 77 DAE. Results indicated that the mean yield of cowpea was higher at narrow spacing (60 cm) than at wide spacing (90 cm) under season-long weed infestation plots but no difference in yield was found among the row spacing in the season-long weed removal plots. There was no difference in the beginning of the CPWC among the row spacing. However, 90 cm row spacing requires longer period of weed removal to avoid unacceptable yield loss when compared to 60 cm row spacing. The end of the CPWC coincides with the period of canopy closure by the crop. This finding suggests that it took the crop longer time to close canopy at wide row spacing (90 cm) compare to reduced row spacing. The differences in the duration of weed control intervention in crop row spacing suggest the importance of integrating decisions regarding row spacing and period of weed control in weed management strategies.