Reduced Rates of Roundup Ultra and Tank-Mix Partners for Herbicide Resistance Management 2001
Project Leader(s): Russell R. Hahn, Department of Crop and Soil Sciences
Type of grant: Pheremones; biorationale; microbiale; conventional pesticides
Project location(s): Findings might be applied throughout the Northeast.
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 Abetween 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 four species have developed in glyphosate-resistant populations after 26 years of glyphosate use. 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 2000 and 2001 at Aurora and Mt. Morris, NY to determine the efficacy of reduced (1/4X and 1/2X) rates of Roundup Ultra in combinations with 1/2X rates of tank-mix partners in Roundup Ready7 corn. Roundup Ultra at 0.5 and 1 pt/A was applied early and mid-postemergence (EPO and MPO) in combinations with 1/2X rates of AAtrex, 2,4-D, Clarity, Hornet, and Scorpion III. Each experiment also included EPO and MPO applications of 2 pt/A of Roundup Ultra alone, a preemergence (PRE) standard treatment of 1.5 qt/A of Bicep Lite II Magnum plus 3.6 pt/A of Prowl, and an untreated check.
There were no differences between the average yield of the EPO and MPO applications in any of the four experiments and the average yield of all Roundup Ultra applications was not different from those of the PRE standard. At Aurora, the average yield of all Roundup Ultra treatments was 128 and 140 bu/A in 2000 and 2001 respectively compared with 130 and 147 bu/A for the PRE standard. At Mt. Morris, the average yield of all Roundup Ultra treatments was 176 and 185 bu/A in 2000 and 2001 respectively compared with 188 and 190 bu/A for the PRE standard. Weed control and corn yields were affected by herbicide treatments in some instances. At Aurora in 2001, control of all species was good to excellent with the reduced rate combinations except for common lambsquarters control. EPO and MPO applications of 0.5 pt/A of Roundup Ultra plus 1 oz/A of Hornet controlled only 63 and 73% of the lambsquarters respectively compared with an average of 96% with the other reduced rate combinations. The average yield for this combination was 126 bu/A compared with 143 bu/A for the other reduced rate combinations. At Mt. Morris, the dominant weeds were redroot pigweed and common lambsquarters in 2000. Pigweed control was good to excellent with all treatments but lambsquarters control was reduced with application of 0.5 pt/A of Roundup Ultra plus 1 oz/A of Hornet. The EPO and MPO applications of this combination controlled 86 and 80% of the lambsquarters respectively compared with an average of 98% with the other reduced rate combinations. Reduced lambsquarters control did not result in yield reductions at this location. In 2001 the dominant weeds were velvetleaf and redroot pigweed. Velvetleaf control was 82% with the 0.5 pt/A rate of Roundup Ultra tank mixes and 93% with the 1 pt/A Roundup Ultra tank mixes. The EPO application of 0.5 pt/A of Roundup Ultra plus 1 pt/A of AAtrex only controlled 55% of the velvetleaf. Pigweed control with 0.5 pt/A rates of Roundup Ultra plus 1 oz/A of Hornet averaged 87% compared with 98% for 0.5 pt/A of Roundup Ultra with the other tank mix partners. There were no differences in yield among the herbicide treatments at Mt. Morris in 2001. These results suggest that the 0.5 pt/A rate of Roundup Ultra may not be reliable in all situations and that Hornet may not be a reliable tank mix partner for common lambsquarters control. On the other hand, the 1 pt/A (1/2X) rate of Roundup Ultra in combinations with 1/2X rate of AAtrex, 2,4-D, Clarity, and Scorpion III did provide control of a broad spectrum of annual grass and broadleaf weeds. Yields of these reduced-rate combinations did not differ from those of the PRE standard and each combination involved herbicides with two or three different modes-of-action. Such combinations would delay/prevent the development of Roundup resistant weed populations.