Abstract
Field studies were carried out for two years (1997 and 1998) on a sandy loam soil in the north-western region of Botswana to determine the effect of five irrigation schedules (0.4, 0.6, 0.8, 1.0, and 1.2) measured as the ratio IW:(Ep - R) where IW is the amount of irrigation water, EP is the sum of the daily evaporation, and R is the sum of the corresponding daily rainfall on marketable yield, irrigation production efficiency, and economic return of winter cabbage. The yield and yield components increased significantly with increase in IW:(Ep - R) ratio up to 1.0. The mean marketable yield, marketable heads per square metre, head weight, and harvest index for the two years were 94.72 and 96.95 t ha-1, 3.81 and 3.87 m-2, 2.45 and 2.54 kg, and 0.610 and 0.637, respectively, with irrigation at IW:(Ep - R) ratio of 1.0. The head weight was more affected by irrigation schedules than the marketable heads per square metre. Further increase in irrigation levels from IW:(Ep - R) ratio of 1.2 led to an adverse effect on marketable yield and irrigation production efficiency. The irrigation production efficiency (21.90 and 22.03 kg m-3) was maximum when irrigation during the crop growth periods was applied at IW:(Ep - R) ratio of 0.8. The seasonal water applied and marketable yield showed a quadratic relationship (R2 = 0.85 and 0.93). The quadratic crop-water production functions attained a maximum yield of 95.7 and 96.5 t ha-1 at 525 mm and 550 mm of seasonal water application. Irrigation scheduled at IW:(Ep - R) ratio of 1.0 resulted in the highest gross return (U.S. $20 623.08 ha-1), net return (U.S. $13 978.24 ha-1), and benefit:cost (B:C) ratio (2.97).
