Replacement studies were conducted under greenhouse conditions to determine if available soil water influences the competitive interaction between soybean and Florida beggarweed. Stomatal conductance and leaf water potential were determined for both species under different levels of available soil water to identify possible mechanisms involved in changes in the relative competitiveness induced by the water deficits. Soybean leaf area and aboveground biomass were greater than for Florida beggarweed under optimum water, but equal to or less than Florida beggarweed with water stress. Soybean was more competitive than Florida beggarweed when there was adequate soil water but less competitive than Florida beggarweed under water stress. Stomatal conductance was higher for soybean with optimum soil water (at high leaf water potentials) but equal to Florida beggarweed as soil water became limiting (low leaf water potentials). These data indicated that water stress differentially affected soybean and Florida beggarweed.

Field experiments were conducted in 1986 and 1987 to evaluate the effect of herbicide incorporation to various depths on common cocklebur control with imazaquin, a tank mix of imazaquin and metribuzin, and a preformulated mixture of chlorimuron and metribuzin in soybeans. Herbicide rates included were two-thirds and the full registered rate. All treatments, except chlorimuron plus metribuzin (50 plus 320 g ai/ha, respectively), controlled more common cocklebur when incorporated to depths of 2.5 to 7.6 cm than when left unincorporated. Also, imazaquin at 90 g ai/ha and the tank mix of imazaquin plus metribuzin at 90 plus 280 g ai/ha, when incorporated, controlled more common cocklebur than higher rates of the same combination when not incorporated. Common cocklebur control was similar with 2.5-, 5.0-, and 7.6-cm soil incorporation depths.