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Dr. Bill Simmons
Department of Natural
Resources and Environmental
How And When Weeds Steal Resources
Weed management strategies are very important for maintaining the maximum genetic yield in corn and soybean. Weed species, duration and timing of weed growth, and seasonal climatic conditions all affect the competition for light, water, space, and nutrients. Introduction of herbicide resistant crops and the accompanying herbicides have allowed producers to remove weeds in a wider timing window. The optimum time of glyphosate application is before the weeds begin to compete with the crop and late enough that the crop canopy can interrupt the development of a second flush of weeds by shading the soil. Most research indicates that when weeds are allowed to emerge with the crop and controlled at 4 weeks after soybean emergence, no yield reduction occurs Research included single weed species, grasses, broadleaf weeds, and mixed stands. Other research indicates this duration interval may be smaller or larger in certain cases. In a study consisting of a natural weed infestation, weeds needed to be controlled at 4 weeks after emergence to preserve yield under ample soil moisture. However, a yield reduction occurred earlier than this under drought conditions or extremely high weed pressure. Some research has indicated percent soybean yield loss due to weed competition increased as planting date was delayed. This may be especially true where fast growing competitive weeds are present or where soils have limited water-holding capacity. Later planted soybeans have less chance of closing canopy when planted in wide rows and thus the critical importance of good weed control. Canopy structure may be affected by delaying weed control as the soybeans grow vertically without much branching in an attempt to intercept sunlight
Corn is more sensitive to weed competition than soybean. Measurements 70 days after planting at Urbana showed that allowing weeds to grow to 9 inches or more reduced corn height by 18% or more compared to 5% or less reduction when weeds were controlled with a soil applied herbicide or at 4 inches or less with glyphosate. One exception was the single glyphosate application at 2-inch weed height where a secondary flush of weeds was uncontrolled. The yield penalty for this regrowth was at least 40 bushels when compared to control with a single application at 4 inches or excellent weed control involving two applications.