The critical period for cowpea (Vigna unguiculata L. Walpers) leaf harvesting on grain yield of black eyed bean (BEB) cowpea type was determined by timing termination and commencement of leaf harvesting during 2005-2007 rain seasons. Two leaves were harvested from each plant per treatment up to pod formation as compared to no leaf harvesting. The times at which leaf harvesting began and ended had significant effects (p<0.05) on leaf yield, grain yield and above ground biomass. Highest leaf yield was obtained in the 2 - 8 weeks after crop emergence (WACE) leaf harvesting period for both on farm and on station in both seasons. The same period (2 – 8 WACE) produced the lowest grain yield, while highest grain yield was obtained when leaf harvesting was terminated at 2 WACE or commenced at 8 WACE. However, above ground biomass was less affected by leaf harvesting. Using Nieto curves to estimate grain and leaf yield trade offs, the critical leaf harvesting duration was 2 - 8 WACE, during which no leaf harvesting should be done to attain maximum grain yield, corresponding to start of branching to start of pod formation. It is recommended that farmers use morphological indicators to mark leaf-harvesting periods.
Key words: Black-eyed bean, cowpea, Nieto curves, critical leaf harvesting.
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