Soybean Research

Utilization of Roundup Ready crops in rotations for the prevention and management of herbicide resistance
 Dr. François Tardif, University of Guelph

Many different weed management tactics allow growers to deal with group two resistant pigweed. Research plots were set up at the Woodstock Research Station in 2005 until 2008. The site was infested with group two resistant pigweed in order to determine the best options in a corn-soybeans rotation.

Many growers have told us that their strategy of choice was to grow Roundup Ready crops. We thought that while this is probably an excellent program, other options exist. Therefore, growers would not have to limit themselves to one system.
We set out to compare seven rotations to determine how they would allow growers to manage a field after resistance has appeared. These systems were as follows:

1. Roundup Ready soybeans and corn in rotation with glyphosate as the only herbicide
2. Roundup Ready soybeans with conventional corn (Ultim/Marksman)
3. Roundup Ready corn with conventional soybeans (Pinnacle/Basagran/Assure)
4. Combinations of five pre-emergence herbicides at reduced rates: Soybeans = Dual/Prowl/ Pursuit/Sencor/Lorox; Corn = Frontier/Prowl/ Banvel/Converge
5. Pre herbicides: Lorox/Dual in soybeans and Primextra/Callisto in corn
6. Post herbicides: Ultim/Marksman in corn and Pinnacle/Basagran/Assure in soybeans
7. Pre program (five) with Post rescue if needed.

Results demonstrated that soybean and corn yields did not differ amongst treatments. Over four years, soybean yields averaged 56 bushels per acre and corn 160 bushels per acre.

Weed control was equally good across all treatments except for some grass escapes with treatment four. However their impact on yield was limited. We did not see any increase in weed species over time, even within the Roundup Ready only system. However the Group two resistant pigweed population declined over the four years from 50 to 15 plants per meter squared. Overall this research indicates that growers could rely on several options to manage Group two resistant weeds.