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Reduced Rates of Roundup Ultra and Tank-Mix Partners for Herbicide Resistance Management 2000

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

Abstract:

Since Roundup (glyphosate) herbicide was introduced in the mid-1970's, it has become the most widely used herbicide in the world. Until recently, most of this use was as "between crop applications". The introduction of Roundup Ready soybeans and corn in 1996 and 1998 respectively has drastically changed the way this herbicide is used. Prior to this new technology, Roundup was used in combination with other weed management practices, including herbicides with different modes-of-action. With Roundup Ready crops, the temptation, and in many cases, the recommendation will be to use this non-selective herbicide alone in some cropping systems. The development of glyphosate-resistant weed populations has been limited compared to resistance problems with other classes of herbicides. Only two grass species (rigid ryegrass and goosegrass) have developed glyphosate-resistant populations after 25 years of glyphosate use. In both cases, multiple glyphosate applications were made annually. As glyphosate becomes more widely used than in the past, and with the possibility of multiple applications within a single growing season, selection pressure for developing glyphosate-resistant weed populations will increase. There is little evidence to date that residual herbicides are needed for Roundup Ready soybean or corn weed control programs in New York, so the temptation for growers will be to use Roundup alone. Preliminary results from 1999 showed that 1/4X and 1/2X rates of Roundup Ultra in combinations with a 1/2X rate of Scorpion III (a mixture of flumetsulam, clopyralid, and 2,4-D) provided excellent control of annual weeds, including velvetleaf which is not always well controlled with Roundup alone.

Field experiments were conducted in Cayuga and Livingston Counties in 2000 to determine efficacy (weed control and grain yields) of early and mid-postemergence (EPO and MPO) applications of reduced rates (1/4X and 1/2X) of Roundup Ultra in combinations with 1/2X rates of Weedar 64 (2,4-D), Clarity (dicamba), Hornet (flumetsulam and clopyralid), and Scorpion III in Roundup Ready corn. Each experiment included EPO and MPO applications of the full rate (2 pt/A) of Roundup Ultra alone, a preemergence (PRE) standard treatment of 1.5 qt/A of Bicep Lite II Magnum (S-metolachlor + atrazine) plus 3.6 pt/A of Prowl (pendimethalin), and an untreated check.