2007 Weed Management Research at the University of Minnesota

By David Boehm

Growers looking for grass weed control in wheat have some good options, but that does not mean differences do not exist in herbicide application timing and resistance management.  Research results from Dr. Beverly Durgan, University of Minnesota Weed Scientist, compare the difference in wild oat and foxtail control with various herbicides.  Durgan, who spoke during the Small Grains Update Meetings, said wild oats are cool-season weeds that germinate earlier in the spring when soils are above 40 degrees F. Cool and early seasons can be more problematic for good wild oat control if many weeds have yet to germinate when herbicides are applied. Wild oats that emerge before or with wheat are the most competitive. “If you are trying to apply herbicide early or at reduced rates, you need to look at when weeds emerge,” Durgan said.  Her research suggested that only 50 wild oat weeds per square yard can account for 18% yield loss, adding that 20 plants per square yard should be controlled as early as possible.

In 2007, Axial, Discover, and Puma, plus their adjuvants, gave wild oat control from 93-98%. These herbicides have the same mode-of-action, ACCase inhibitors. Axial and Discover showed very little antagonism when mixed with broadleaf herbicides, while Puma showed some reduction in wild oat control when mixed with some broadleaf herbicides, which resulted in reduced grain yields. Everest herbicide provided less wild oat control which resulted in less yield.    “Everest is an ALS herbicide and is effective in controlling ACCase resistant wild oats,” Durgan reported. She said ALS chemistries do not always have the same wild oat control as the ACCase chemistries, and get less with tank mixes, but she feels growers “need to think about managing resistance.” 

Her 2007 data showed less wild oat control and reduced yields with ALS chemistries including Everest, Silverado, and Rimfire. No herbicides tested caused plant injury. Durgan said that wild oats need to be actively growing so early applications where temperatures are close to freezing at night will reduce control.  Wheat injury is not a major concern with most of the labeled grass herbicides.  However, the addition of broadleaf herbicides has the potential to increase crop injury.      

Durgan also presented research results for the control of foxtails, which are warm season grasses that germinate at 50-55 degree F soil temperatures, which is generally later than wild oats. Durgan revealed that lower populations of foxtails, such as 50 plants per square yard or less may not need to be controlled if they emerge after the crop and with good crop stands.  She added that foxtails do not always cause yield loss but “do you want weeds when you are harvesting?” she asked.  Aside from yield loss, weed control also reduces weed seed populations. More than 50 plants per square yard or greater can cause 4-5% yield loss.  Wheat yields and foxtail control with Axial, Discover, Puma and Everest were not significantly different in her study.  Everest had slightly less foxtail control but did not reduce yields or affect harvest.

Durgan has also been studying reduced herbicide rates due to grower requests.  “I have to give a disclaimer,” she said, “as reduced rates are off-label use, leaving the risk to the grower.” Durgan warns growers not to reduce rates unless they have very good growing conditions, the correct wild oat staging, and the proper tank mixes. Some broadleaf herbicides such as Bronate Advance and Huskie can cause reduced wild oat control. Reduced rates did not affect wild oat control in 2007 with Axial, Discover, Puma, or Rimfire.  This was helped due to early wild oat emergence.  Discover and Puma had reduced yields when wild oats were sprayed at larger stages. When you compare herbicides for reduced rate effect on grain yield, Rimfire was significantly less than Axial, Discover, or Puma. Her experience suggests that application rates should not be reduced with Everest, Silverado, or Rimfire for wild oat control.

In regard to foxtail control, Rimfire had significantly less control overall, but there was no difference between rates or timing for Rimfire. Axial, Discover, and Puma showed no significant differences in yield at different timing or rates.

Durgan also reported results from studies done on the new Huskie broadleaf herbicide from Bayer CropScience.  Huskie is a post-emergence broadleaf herbicide for wheat and barley that has a new mode-of-action called a pigment inhibitor.  Durgan said the “weed leafs will turn yellow to white.”  She added that it “may affect the crop a little bit.” The chemical compound, pyrasulfotole, does have some soil activity but according to Durgan “it shouldn’t be a problem unless you apply late in the summer.”  Huskie with adjuvants gave complete control of lambsquarters, Pennsylvania smartweed, wild buckwheat, and wild mustard.  She reported other broadleaf weed control such as waterhemp, hairy and eastern black nightshades, redroot pigweed, Russian thistle, common ragweed, and common mallow. Grain yields were the same as Harmony, Bronate Advance, Widematch, and Clarity.

The complete study and other herbicide results can be found at http://appliedweeds.cfans.umn.edu. 

Boehm is regional business manager for AgriPro Wheat. He is a member of the Minnesota wheat checkoff-funded Minnesota Small Grains Research and Communications Committee, a MN Crop Improvement Assoc board member, and has a master’s degree in plant breeding from NDSU.