Ammonium sulfate enhanced the initial control of johnsongrass by glyphosate and SC-0224 applied in the fall or summer. Glyphosate at 0.42 kg ha-1in combination with ammonium sulfate at 3.33 kg ha-1provided levels of fall johnsongrass control and spring regrowth control comparable to glyphosate at 0.84 kg ha-1. Spring applications to mixed populations of seedling and rhizome johnsongrass showed that ammonium sulfate added to glyphosate or SC-0224 afforded little enhancement of herbicide efficacy. Glyphosate or SC-0024 at 0.42 kg ha-1applied in combination with logarithmically decreasing rates of ammonium sulfate demonstrated reduced weed control with ammonium sulfate rates above 9.7 kg ha-1.

Jointed goatgrass, downy brome, and horseweed are increasingly troublesome winter annual weeds during fallow periods in conservation-tillage systems in the southern Great Plains. These experiments determined the optimum weed size, vigor, and minimum herbicide rate required for 95% or better control of these weeds on fallow land. Jointed goatgrass and downy brome were controlled best when plants were 10 cm or less tall and growing vigorously at time of treatment. Horseweed was controlled best when plants were 30 cm tall and growing vigorously. Based on local retail and application costs and assuming optimum conditions for control, the two most economical herbicide treatments that controlled each weed 95% or better were: jointed goatgrass, clethodim at 250 g ai/ha and glyphosate + 2,4-D at 249 + 479 g ae/ha; downy brome, quizalofop at 18 g ai/ha and glyphosate + 2,4-D at 582 + 950 g ae/ha; and horseweed, 2,4-D at 560 g ae/ha and metsulfuron at 5 g ai/ha.

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R^sumi; ZusammenfassungSoybean yields, pod and branch numbers were inversely related to the height of velvetleaf {Abutilon theophrasti). This effect was attributed to increasing interception of light with increasing height of A. theophrasti. Reductions in soybean yield and yield components were greater if A. theophrasti was maintained at various heights for only 3 weeks rather than 6 weeks following soybean emergence. This effect was attributed to a longer duration of light competition by the weed. Even when the A. theophrasti canopy was kept 25% below that of soybean for 4 weeks, soybean yields, pod and branch numbers decreased when the weed was allowed to regrow during the remainder of the season, but not if the weed was removed after the initial 4-week chpping period. No reduction in seed yields, pod and branch numbers was