Optimum Irrigation Schedules

Irrigation intervals affect leaf area index, crop growth rate, plant height, harvest index, total dry matter or seed yield of soybean (Osman et al., 2000; Yazdani et al., 2007). In Egypt, irrigation at 15-day intervals throughout the growing season has been found to be the best for the growth and yield of soybean (Osman et al., 2000). With shorter irrigation intervals the yield may increase; however, WUE and net income may decrease. The irrigation interval should therefore be such that water is used judiciously, resulting in high yields, WUE and net returns.

The crop may need a number of irrigations at different growth stages. Various parameters such as the maximum leaf area index, fraction of incoming radiation intercepted, maximum total biomass, number of pods, pod growth stage and harvest index are increased with the number of stages irrigated (de Costa and Shanmugathasan, 2002). However, for realizing high seed yields the crop should not suffer for want of moisture at the critical growth stage. When there is a limited quantity of water available, irrigation should be applied at the R5 stage only (Kim et al., 1999) as the daily mean transpiration rate, WUE and seed yield are high with irrigation at this stage. Irrigation during the reproductive phase increases seed yield by increasing seeds per plant and it can be applied either at R1, R4 or R6 (Sweeney and Granade, 2002). Irrigation frequencies have a great effect on the growth and yield of soybean. Six irrigations result in greater plant height, pods per plant, seed yield and oil content percentage than five, four, three or two irrigations (Kazi et al., 2002). In a study by Rabbani et al. (2004), the highest seed yield was with irrigation at 20, 40 and 60 days after sowing as opposed to 20, 20 and 40 or 20, 40, 60 and 80 days after sowing.

In a soybean-winter season sorghum (Sorghum bicolor) cropping system on a clay soil vertisol in Maharashtra, India, soybean showed a good response to irrigation applied at 0.60 IW:CPE (Bharambe et al., 1999). A higher soybean seed yield has been reported when the crop is irrigated at 75 mm CPE than at 100 or 125 mm CPE (Deolankar and Mogal, 1998).

In India, the seed, straw, oil and protein yields of soybean and the oil and protein contents in soybean seeds were higher under a 0.8 IW:CPE ratio than under a 0.4 or 0.6 IW:CPE ratio in Rajasthan (Kumawat et al., 2000) and Tamil Nadu (Rajendran and Lourduraj, 2000; Ramasamy and Sankaran, 2001). Irrigation at a 0.6 IW:CPE ratio results in lower canopy temperature, lower transpiration rate, higher relative leaf water content and stomatal diffusive resistance, thereby leading to higher seed yield (Elamathi and Singh, 2001).

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