Field experiments were conducted to study the effects of jimsonweed (Datura stramonium L.) densities and planting dates on the growth and yield of soybeans [Glycine max (L.) Merr. ‘Amsoy 71’]. Jimsonweed densities of 1.5 and 2 plants/m2 did not reduce the dry weight of soybean leaf, stem, root, or pod and seed tissues, leaf area index, plant height, pod number, or seed yield when soybeans and jimsonweed were planted at the same time. Weed densities ranging from 3 to 16 plants/m2 did reduce one or more of these soybean growth parameters. A significant density-by-year interaction was observed in which soybean growth reductions caused by high jimsonweed densities were less when competition for water was minimized by above-average precipitation in 1 of the 2 yr. No density of jimsonweed that emerged when soybeans were at the fourth trifoliolate-leaf stage reduced crop growth or yield.

Field experiments were conducted to study the effects of varying velvetleaf (Abutilon theophrasti Medic.) densities and planting dates on the growth and yield of soybeans [Glycine max (L.) Merr. ‘Amsoy 71’]. Velvetleaf densities ranging from 2.5 to 40 plants/m2 caused reductions in the dry weight of soybean leaves, stems, roots, and pods and seeds, and in the leaf area index, pod number, and seed yield when velvetleaf and soybean plants emerged at the same time. The magnitude of reduction in soybean growth and the time at which the reduction was first observed was dependent on weed density. Soybean growth reductions caused by high velvetleaf densities were less when conditions of high soil moisture content minimized the effects of competition for water. Velvetleaf emerging 21 and 23 days after soybean emergence did not reduce crop growth or yield.