Field experiments were conducted to evaluate soybean interference on common cocklebur and entireleaf morningglory under irrigated and nonirrigated conditions. Interference from the mixture of soybean and entireleaf morningglory, and soybean alone reduced the total leaf area index (LAI), LAI within the common cocklebur canopy, and growth rate of common cocklebur more than did entireleaf morningglory; more reduction occurred without irrigation than with irrigation. Irrigated common cocklebur produced 687 burs per plant, while nonirrigated produced 359. Irrigated and nonirrigated soybeans and the mixture of soybeans and entireleaf morningglory reduced common cocklebur bur production 43 to 47%. Irrigated entireleaf morningglory reduced common cocklebur bur production 42%, while nonirrigated reduced bur production 28%. Interference from the mixture of irrigated soybeans and common cocklebur reduced the total leaf area index (LAI), LAI within the entireleaf morningglory canopy, and growth rate of entireleaf morningglory more than did soybean or common cocklebur alone. Irrigated entireleaf morningglory produced 542 and 1107 seeds per plant in 1982 and 1983, respectively. The mixture of irrigated soybeans and common cocklebur reduced entireleaf morningglory seed production 84 to 90%. Nonirrigated entireleaf morningglory was not competitive in any treatment combination either year due to lack of sufficient LAI.

Field experiments were conducted to evaluate common cocklebur and entireleaf morningglory interference on soybean under irrigated and nonirrigated conditions. Total leaf area index (LAI), LAI within the soybean canopy, crop growth rate, and seed yield of soybean were decreased more by common cocklebur than by entireleaf morningglory. Interference from entireleaf morningglory, common cocklebur, or both species reduced soybean yields 21, 57, and 64%, respectively, with irrigation and 12, 60, and 76%, respectively, without irrigation. Soybean yield reduction from interference with entireleaf morningglory, common cocklebur, or both species was not influenced by soybean date of planting. Soil water was extracted from greater soil depths by soybean growing with the weeds than by soybean alone. High WUE without irrigation suggests that soybean uses water more efficiently when soil moisture is limiting than when soil moisture is available under irrigated conditions.