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Dealing With Resistance
Dr. Jim Martin, University of Kentucky extension weed scientist located at the UK Research
and Education Center, once again discusses the problem of herbicide resistant weeds in wheat.
Photo by John LaRose, Jr.
Fall Treatments Offer Best Control For Resistance In Wheat
BETTY VALLE GEGG-NAEGER
MidAmerica Farmer Grower
PRINCETON, KY.
Herbicide resistant weeds again took the stage as Dr. Jim Martin, University of Kentucky extension weed scientist located at the UK Research and Education Center, discussed the problem in wheat.
“We continue to try to control them,” he said. “One weed that has been around for a number of years is glyphosate resistant marestail. We know marestail has been a big problem in soybeans since 2001. It can be a problem at times in double crop soybeans, depending on how good a wheat stand we have.”
If you have a really good stand of wheat, that goes a long way to managing weeds in general, especially weeds like marestail because it’s not that competitive a weed. Developing an early canopy of wheat can diminish the problems with marestail that emerges in the spring.
“Unfortunately, we are seeing more and more of the fall emergence of marestail where wheat cannot be real competitive over the winter,” Martin said. “That’s something that we have to deal with. There are some herbicides that research shows will help, one of which is Husky. Since Husky has a four-month plant-back restriction for double crop soybean, growers will need to spray in the fall in most instances.”
ALS resistant common chickweed is another problem showing up in Kentucky, particularly along the southern tier counties between Tennessee and Kentucky. ALS resistant weeds have taken a long time to develop in Kentucky compared to states where wheat is growing repeatedly in the same fields without rotating to other crops. The fact that wheat in Kentucky is grown in rotation with other crops has slowed the development of ALS resistance in weeds such as common chickweed.
“Since ALS resistant chickweed has been confirmed in Kentucky, we have started research on evaluating fall applications, using things like Husky, Starane Ultra, and low rates of metribuzin (Sencor),” Martin said.
Metribuzin is fairly economical, especially when applied at low rates of 2 to 3 ounces/A. A good time to apply metribuzin is in the fall as the wheat is just starting to tiller. One of the reasons for using low rates of metribuzin is to limit the risk of injury to wheat.
Metribuzin labels list varieties based on their tolerance to the herbicide. Unfortunately, these varieties are outdated. Martin said researchers are attempting to evaluate wheat varieties based on their tolerance to metribuzin. However, because of the rapid turnover of varieties, an ongoing commitment is needed in order to provide information on metribuzin tolerance.
“I am working with our wheat breeder and the person who runs the wheat variety trial program to evaluate tolerance of 112 varieties to metribuzin applied at 4 and 8 oz/A,” he said. “This was a non-replicated trial; consequently we needed to temper our faith in the data. What we’ve seen so far is the vast majority of varieties appear to be tolerant to metribuzin. In fact, at the lower rate of four ounces per acre, we didn’t see anything that was of major concern.”
If the rate is bumped up to the high rate of eight ounces per acre, some 10 to 12 percent of those varieties showed some type of response. It was not clear that the response was great enough to affect yield, but it was obvious some varieties were prone to damage.
“Last of all, the weed that is the scourge to a number of wheat growers is Italian ryegrass or annual ryegrass,” Martin reported. “Ryegrass has been a problem weed in wheat for a number of years, but our farmers are doing a decent job in managing it. We have seen a few cases of ryegrass resistance to ALS inhibitors like the old Hoelon and ALS inhibitors like Osprey or Powerflex, but nothing at the level reported in Arkansas and Mississippi.
“We’ve also discovered that we do have a glyphosate resistant ryegrass which is alarming, because we use a lot of glyphosate to manage that weed in our rotations,” he reported.
There is some good news in this. Glyphosate resistant ryegrass appears to be contained thus far. There’s hope that’s the only case that shows up, but time will tell.
“To sum it up, I think we have some good options to manage herbicide resistant weeds in wheat and other crops as long as we diversify our herbicide programs throughout the rotation system,” he summed. ∆
BETTY VALLE GEGG-NAEGER: Senior Staff Writer, MidAmerica Farmer Grower
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