Double-crop corn (zea mays) weed control in Virginia
King, Steve Russell
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Double-crop production of corn (Zea mays L.) for grain following the harvest of small grain is not currently practiced in Virginia. Historical precipitation and evapotransportation data indicate that delayed corn planting could result in a higher probability of moisture during critical periods of crop development. Double-crop corn may also reduce economic risk as two crops would be harvested in the same year. Field experiments were conducted in three Virginia locations in 1998 and 1999 to determine the herbicide inputs required for double-crop corn production relative to those required in full-season no-till corn production. Experiments were conducted in a split-plot, randomized complete block design with cropping system as the main plot and herbicide treatment as the subplot. Herbicide treatments included combinations of nonselective herbicides for no-till establishment and/or preemergence residual herbicides and/or selective postemergence herbicides in both production systems. Glyphosate-tolerant corn was planted in all experiments and postemergence glyphosate treatments were also evaluated. In each experiment, dependent variables included weed control by species evaluated throughout the season, as well as weed biomass and corn yield evaluated at the end of the growing season. Generally, nonselective herbicides were not required in the double-crop system where atrazine was applied as a preemergence treatment, or where selective postemergence treatments were applied. Where a significant proportion of the infestation was comprised of perennial species, however, atrazine treatments were not sufficient in the double-crop system. Postemergence glyphosate treatments provided excellent broad-spectrum weed control in this situation. In heavy annual grass infestations, postemergence glyphosate treatments provided superior weed control to preemergence treatments alone, and equivalent weed control to treatments in which both preemergence and postemergence herbicides were applied. Corn yield response to weed control and cropping system variables varied significantly between the 1998 and 1999 growing seasons. Where adequate late-season rainfall was received, economic return from small grain and corn crops in the double-crop system was higher than the return in the full-season system, particularly in infestations where the double-crop system allowed significant reduction in herbicide input.
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