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Herbicide-resistant Corn for Reducing Use of Residual Herbicides and for Wirestem Muhly Control 1999

Project Leader: Russell R. Hahn, Department of Crop and Soil Sciences


The introduction of Liberty Link (glufosinate resistant) and Roundup Ready (glyphosate resistant) corn hybrids has the potential to reduce the use of residual herbicides and to provide new control options for hard-to-control perennial weeds such as wirestem muhly. For these weed control programs to be successful, the potential yield losses associated with total postemergence weed control programs must be understood. In addition, maximum environmental benefit of these programs will only be recognized if it is known how much residual herbicide, if any, is required to make these programs dependable in field corn. Experiments were conducted at several locations across NY state in 1999 to collect additional information concerning these two issues.

In most of the experiments, early postemergence control programs resulted in better weed control and higher corn yields than the standard preemergence herbicide programs. This was attributed to the lack of adequate rainfall for activation of the preemergence herbicides. Silage corn yields decreased by 20 and 32% respectively when Liberty ATZ was applied mid- and late postemergence compared with the early postemergence application. Application timing affected grain corn yields in only one of four Roundup Ultra experiments. In that experiment, at Aurora, the mid- and late postemergence applications resulted in yield losses of about 10 and 60% respectively compared with yield from the early postemergence treatment. It is difficult to explain why application timing had no effect in the other three Roundup Ultra experiments. In two of these experiments, Aurora and Valatie, overall yields were depressed due to drought stress and soil variability within the plot areas. At Mt. Morris, the deep silt loam soil was very forgiving when rains came later in the season. Potential yield losses associated with postemergence weed control programs are influenced by several factors, including weed pressure, amount and timing of rainfall events, and the water-holding capacity of the soil, as well as time of application. Clearly, it would be a mistake to ignore the potential importance of timing since it is a factor that can be controlled.

After three years of field research in NY, there is still no evidence that residual soil applied herbicides are needed with Roundup Ultra treatments. In addition, preliminary data suggests that reduced rates of Roundup Ultra in combination with reduced rates of other postemergence herbicides can provide excellent weed control and would guard against the development of resistant weed biotypes. Finally, when using either Liberty or Roundup Ultra for control of perennial weeds such as wirestem muhly, it is recommended that a one-half rate of an appropriate residual herbicide program be applied at planting to suppress annual weeds and then the postemergence application be timed for maximum effect on the target perennial species. resistant corn hybrids could result in control recommendations for this perennial grass that would involve the use of environmentally and toxicologically friendly herbicides like Roundup Ultra and Liberty which have sites-of-action that are different from the sulfonylurea herbicides.