Drought stress can reduce the yield of soybean [Glycine max (L.) Merr.] grown in the middle and lower Mississippi River Valley. A 3-yr field study on Sharkey clay (very-fine, montmorillonitic, nonacid, thermic Vertic Haplaquept) determined the effects of four soil water levels and two row spacings on the growth and yield of cultivars Forrest (MG V), Centennial (MG VI), and Braxton (MG VII). Irrigation was applied in the four treatments when (i) soil water matrix potential (γs) at both the 15 and 30 cm depths reached −70 J kg−1 (Il), (ii) γs at all depths between 15 and 76 cm reached −70 J kg−1 (I2), (iii) the same as in I1, but only after the Rl-R2 growth stage, and (iv) there was no irrigation (NI). Soybean was planted at interrow spacings (RS) of 0.5 and 1.0 m. Irrigation significantly increased yield of each cultivar at each RS in each year, but there were no yield differences among I1, I2, and I3 regimes at either RS. Average NI yields were 2.86,3.20 and 2.97 Mg ha−1 in 1982, 1983, and 1984, respectively. Yield increase from irrigation was 10,8, and l6% in 1982, 1983, and 1984, respectively. Irrigated yields of Forrest, Centennial, and Braxton were 3.47, 3.17, and 3.40 Mg ha−1, respectively, but water use efficiency was greatest with Forrest. Irrigation prior to R2 increased lodging up to loo%, and up to 50% when irrigated after R2. Severe lodging may have reduced irrigated yields. Relative yield increase from irrigation was limited by adequate rainfall and soil water storage, which resulted in high NI yields. The most efficient water regime was one based on an early maturing cultivar and γs measured at a depth of 76 cm.
Contribution from the Louisiana State Univ. Agric. Ctr. Louisiana Agric. Exp. Stn. Approved for publication by the director of the Louisiana Agric. Exp. Stn. Approved for publication by the director of the Louisiana Agric. Exp. Stn. as manuscript no. 88-76-2083.