By Emily Unglesbee
DTN Staff Reporter
COLUMBIA, Mo. (DTN) -- University of Missouri weed scientist Kevin Bradley isn't a short man, but the Palmer amaranth plant in his right hand towered nearly one foot over his head.
"Gain a little respect for the fact that it's taller than I am, and it's got a base that's almost as thick as my arm," Bradley told a group of farmers and ag industry representatives gathered for University of Missouri Extension's annual Pest Management Field Day on July 16. "This is the number one weed to watch for in the Midwest. But at this point in time, I don't believe we can shut the barn door anymore -- it's out, and I don't think we can contain this beast."
In the past decade, southern and Midwestern weeds like Palmer amaranth, waterhemp and marestail -- the spindly, pineapple-like annual sometimes known as horseweed -- have rapidly developed resistance to commonly used herbicides like glyphosate. With chemical solutions from companies still two to three years away, farmers may need to consider experimenting with other tactics like tillage and cover crops to combat herbicide-resistant weeds, Missouri scientists explained at the field day.
TOYING WITH TILLAGE
In a multi-state tillage study by MU graduate student Jaime Farmer, deep tillage methods have shown a significant decrease in weed pressure compared to no-till or less invasive tilling methods.
Quite simply, the deep tillage method buries and suffocates the bulk of the weed seed bank, Farmer explained. Samples from the different fields in the study showed that no-till fields held up to 95% of their weed seedbank in the top inch of soil. By comparison, fields turned over with a mobile plow had only 15% of their total weed seeds in the top inch of soil.
Both Farmer and Bradley stressed that using a mobile plow to control weeds should be a last ditch tool that is used sparingly.
"Waterhemp or Palmer pigweed seeds have a lifespan of 4 to 5 years in the soil at most," Bradley explained. "If you have a resistant (weed) nightmare, it is one option to turn the soil over once and leave them down there. Four or five years later, that population is done. And if you don't let any more weeds go to seed, then you've dealt with your weed seedbank."
In Owensboro, Ky., Paul Winkler's corn and soybean fields haven't seen a plow since 1984, and he doesn't want to reacquaint them anytime soon.
"The most important piece of equipment I have on my farm is my sprayer," he told DTN during the field day tours. "I hope I don't ever have to go back to tillage."
Bradley doesn't want him to either.
"I don't want a no-tiller to move away from no-till -- I'm just presenting all the options," Bradley said of the tillage study. "For the no-tiller who's going to stay a no-tiller, it's going to be all about pre-emergence residual herbicides and rotating modes of action, and cover crops can help. That's the best we have to offer."
Fortunately, another study from MU graduate student Cody Cornelius showed some promising weed suppression from cover crops. The study looked at the influence of cover crops like Italian ryegrass, hairy vetch, tillage radish, cereal rye and crimson clover on winter and summer annual weed densities.
"From a weed control standpoint, cereal rye definitely stands out, whether it be in a cereal rye-hairy vetch mix or cereal rye by itself," Cornelius concluded.
Two aspects of cereal rye make it a good candidate for weed suppression, he added. "Research has shown that cereal rye has an allopathic ability, which means it releases some gases that prevent other weeds from emerging. So we have that and then we have the aspect of good ground cover."
Italian ryegrass also showed good weed suppression for winter annuals, but Bradley said farmers should be wary of that crop.
"Ryegrass is one of the world's worst herbicide-resistant weeds," Bradley noted. "It is glyphosate-resistant in a lot of states and a lot of countries." Seed sellers may claim the rye they have is not Italian ryegrass, but often it is.
Growers must be careful to burn cover crops down early in the spring before they get too big, and picking the right herbicide can be tricky, Cornelius said.
In 2013, Bradley tested the results of a number of burndown herbicide treatments for cover crops, and the results can be found here: http://goo.gl/….
Residual herbicides can affect the success of a cover crop, Cornelius added. For example, his study found that tillage radishes saw a 50% to 60% stand reduction when planted after products like Python, Flexstar and Pursuit.
Growers should also be aware that many herbicides are not labeled for subsequent cover crop plantings, said Leslie Lloyd, an agronomy service manager for Syngenta.
To see the University of Missouri study on weed suppression from cover crops, visit this website: http://goo.gl/….
To see the results from the University of Missouri's study on tillage and other herbicide-resistant tools, visit this website: http://goo.gl/….
Emily Unglesbee can be reached at firstname.lastname@example.org
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