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Herbicide-Resistant Corn for Reducing Use of Residual Herbicides and for Wirestem Muhly Control 2000

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


The introduction of Roundup Ready (glyphosate-resistant) and Liberty Link (glufosinate-resistant) corn hybrids has the potential to reduce the use of residual herbicides that may contaminate surface and groundwater and to provide control options for perennial weeds such as wirestem muhly in corn. The maximum environmental benefit of these weed control programs will only be recognized if it is known how much residual herbicide, if any, is required to make these programs dependable. In addition, the potential yield losses associated with total postemergence weed control programs must be understood if these programs are to be successful. Four experiments were conducted at several locations across NY state in 2000 to collect additional data concerning these issues.

In experiments conducted in Cayuga and Livingston Counties, the addition of residual herbicides to early and mid-postemergence (EPO and MPO) Roundup Ultra applications did not improve weed control ratings or result in significantly different grain corn yields than when Roundup Ultra was sprayed alone. Time-of-application of the Roundup Ultra treatments had no impact on grain corn yields with moderate weed pressure and a highly productive soil in Livingston County. In Cayuga County, with heavy common ragweed and green foxtail pressure, late postemergence (LPO) Roundup Ultra application did result in a significant yield reduction (134 bu/A) compared with an average yield of 148 bu/A for the EPO and MPO Roundup Ultra treatments. When giant foxtail was suppressed with a preemergence (PRE) application of Dual II Magnum in Columbia County, the timing of sequential Roundup Ultra applications had little influence on grain corn yields. With Roundup Ultra only applications, grain corn yields were reduced from 133 bu/A for EPO application to 112 and 110 bu/A for the MPO and LPO applications. As in 1998 and 1999 there is still little evidence that residual soil applied herbicides are needed in Roundup Ready corn weed control programs. The 2000 results provided additional evidence that delayed application of these non-residual postemergence herbicides can result in significant yield losses. These potential yield losses 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 application timing since this is a factor that can, to a large extent, be controlled. Corn growers who use Roundup Ready corn hybrids should plan to spray EPO when weeds are 2 to 4 inches tall. If residual herbicides are used in these programs, no more than a one-half rate should be used.

As in previous years, LPO application of Roundup Ultra provided excellent (99%) wirestem muhly control. LPO application of Liberty herbicide provided good (90%) in-season wirestem muhly control but may not provide long-term control since there is little or no translocation of Liberty into the underground reproductive structures (rhizomes) as there is with Roundup Ultra. This research supports the recommendation in the Cornell Guide for Integrated Field Crop Management for use of a one-half rate of an appropriate PRE residual herbicide program followed by a sequential application of Roundup Ultra for wirestem muhly control. The PRE residual herbicide(s) suppress annual weeds and the postemergence application of Roundup Ultra can be timed for maximum effect on the wirestem muhly.