Interference between soybeans, common cocklebur and jimsonweed under Indiana field conditions

Specific understanding of the interaction between crop plants and weedy plant species under field conditions is essential in making accurate weed control decisions. Common cocklebur (Xanthium strumarium L.) and jimsonweed (Datura stramonium L.) are two commonly encountered broad leaf weeds in soybeans (Glycine max (L.) Merr.). Little research has been done to examine the interaction between soybeans and these weeds on an individual plant basis. A three year study examined the interactions between these plant species to determine the ability of the two weeds to interfere with the growth and seed production of the soybeans and to determine the effect of the soybeans on the growth and reproduction of the two weedy plants. Effects of this interference were monitored over time during the growing season and distance within the row. Common cocklebur was capable of significant interference in the growth and reproduction of soybeans, causing increasing reduction in soybean growth throughout the growing season. At harvest, seed yield of soybeans in the 1.5 m of soybean row adjacent to each common cocklebur plant was reduced 16.0% compared to weed-free soybeans. Interference effects of common cocklebur were linear over distance within the row, and this linearity increased as the growing season progressed. When areas of interference from adjacent common cocklebur plants overlapped, the resulting influence on the soybeans between the weeds was additive. The effect of jimsonweed interference on vegetative growth of soybeans was variable and small, but seed yield of soybeans was reduced 12.5% within the 1.2 m of soybean row adjacent to individual jimsonweed plants at harvest. Interference effects of jimsonweed over distance within the row were variable. Some additivity in influence from adjacent jimsonweed plants was detected at harvest. When common cocklebur and jimsonweed were grown together with soybeans, the larger influence of common cocklebur masked the influence of jimsonweed. Soybean interference reduced growth of common cocklebur by 50 to 77% and jimsonweed growth by 75 to 93% compared to weed plants grown free from soybean interference. The economic threshold population for common cocklebur would be 6800 plants/ha, assuming a $61.00/ha cost of control, 18.3$ cents$/kg selling price for soybeans and a 2695 kg/ha soybean seed yield. Under the same economic assumptions, the economic threshold population for jimsonweed would be 11,000 plants/ha