Weed control and soybean response to preplant tillage and planting time

Clay soils in the southeastern U.S.A. Typically are tilled for seedbed preparation and weed control, prior to seeding soybean (Glycine max (L.).Merr.). However, during extended wet periods when clayey soils cannot be tilled, this practice interferes with timely planting of soybean. Consequently, a 3-year study (1985–1987) investigated the role of tillage and seeding date on irrigated soybean production on Sharkey clay (Vertic Haplaquept) near Stoneville, Mississippi. Plantings were made each year, in early May and late may or early June, in seedbeds subjected to the following treatments: (1) autumn tillage and winter fallow; (2) autumn tillage and winter wheat (Triticum aestivum L.) cover; (3) late winter or early spring tillage; (4) prepared seedbed; or (5) no tillage between harvest and planting. A disk-harrow and spring-tooth harrow were used for all tillage operations. Weeds were effectively controlled during each growing season in all treatments by selected preplant, preemergence, and postemergence herbicides, plus postemergence cultivation. Seedbed tillage had no consistent effect on soybean yield, but in two of the three years, early plantings yielded significantly more seed than late plantings. The increase from early planting was 862 kg ha⁻¹ (3598 vs. 2736 kg ha⁻¹) in 1986, and 381 kg ha⁻¹ (2899 vs. 2518 kg ha⁻¹) in 1987. These results indicate that seedbed tillage of clayey soil should be avoided if it interferes with timely planting of soybean.