Response of six vegetable crops to irrigation schedules

Field studies were carried out for 2 years (1995 and 1996) on sandy soil in the north western region of Botswana to examine the effect of irrigation schedules (18 mm of water in each irrigation at 11, 22, 33, 44 and 55 mm of cumulative pan evaporation, CPE) on marketable yield, irrigation production efficiency and economic return of cabbage, spinach, rape, carrot, tomato and onion under sprinkler irrigation. The higher mean marketable yield of cabbage (71.65 t/ha), spinach (33.53 t/ha), rape (73.22 t/ha), carrot (56.76 and 38.39 t/ha), tomato (46.81 t/ha) and onion (56.05 t/ha) were recorded for irrigation scheduled at CPE of 22, 11, 22, 22, 22 and 11 mm, respectively. The irrigation at CPE of 22 mm resulted in higher irrigation production efficiency of cabbage (11.32 kg/m3), spinach (3.35 kg/m3), carrot (9.83 and 6.66 kg/m3), tomato (5.90 kg/m3) and onion (6.26 kg/m3), but rape (12.03 kg/m3) gave higher irrigation production efficiency at CPE of 33–55 mm. Irrigation scheduled at 22 mm CPE resulted in a higher net return of 47 131, 104 398, 76 691 and 93 192 P/ha for cabbage, rape, carrot and tomato, respectively, but spinach and onion gave a higher net return of 27 086 and 83 934 P/ha at 11 mm CPE(1 US$ = 4.55 P). Irrigation scheduled at 22 mm CPE gave a higher B/C ratio of 2.92, 1.94, 5.40, 4.98, 4.91 and 4.82 for cabbage, spinach, rape, carrot, tomato and onion, respectively. Seasonal water applied and marketable yield of vegetable crops exhibited quadratic relationships (R2 = 0.85–0.99). The fitted regression models attained the maximum yield, net return and B/C ratio at CPE of 16–18 mm. The results revealed that rape is the most remunerative crop followed by tomato, onion, carrot, cabbage and spinach.