TIMING OF FURROW IRRIGATION TERMINATION FOR DETERMINATE SOYBEAN ON CLAY SOIL

Irrigation commencing at or near beginning bloom (R1) and continuing during reproductive development of determinate soybean [Glycine max (L.) Merr.] grown in the southern USA in dry years increases seed yield. The number of irrigations between the first and last irrigation event can influence the direct costs of irrigation; however, the optimum time to stop irrigation in relation to crop maturity has not been identified. 'Leflore' (1987-1990) and 'A5980' (1988-1990) determinate cultivars were grown on Sharkey clay (very-fine, montmorillonitic, nonacid, thermic Vertic Haplaquept) at Stoneville, MS, to determine the effect of timing of last furrow irrigation on seed yield, yield components, yield increase, and net returns. All irrigation treatments within a cultivar were started at R1 (beginning bloom) or Later. One treatment was dropped from the sequence with each successive furrow irrigation event through R6 (full seed). Irrigation applied through mid-R5 (seed-filling period) during a dry year produced the greatest seed yield increase of 1855 kg ha(-1) which was 610 kg ha(-1) greater than the yield of the treatment terminated at R4 (full pod), and equal to the yield of the treatment terminated at R6. Yield increases due to irrigation during wet years were smaller or nonexistent. In all cases, increased number of seeds was the sole contributor to increased yield, In the dry years, irrigation terminated at either mid- to late R2 or mid-RS produced the greatest yield increase per unit of irrigation water, but irrigation continued into the seedfill period and then terminated produced the highest net return. These results indicate that furrow irrigation of soybean grown on a cracking clay soil in dry years should be continued through mid-RS to achieve maximum seed yield and net returns.