Glyphosate resistant weeds increasing
May 18, 2009
Well planned crop management and careful use of glyphosate will be the best defense for producers in Nebraska and South Dakota to avoid development of glyphosate resistant weeds. That’s the recommendation of University of Nebraska-Lincoln Extension Educator Lowell Sandell. With a focus on weed science, Sandell has kept a watchful eye on the glyphosate-resistant weeds developing in states such as Georgia, Ohio, Illinois and Indiana. Missouri is also seeing glyphosate-resistant water hemp. Weed control is becoming an issue in some of those states because resistant species are becoming more common.
“The message we want to convey to producers in the Midwest is that glyphosate has become such a cornerstone of weed control in corn and soybeans that we want to take steps to ensure we continue to receive the maximum benefit from this herbicide,” Sandell says. “We’ve seen the results of overuse and continual use of only glyphosate in other states. Users should make every effort to avoid that kind of use pattern, which could lead to resistant weeds.”
Resistance to herbicides has been a concern since chemicals were used for weed control. Initially, the herbicide kills the plant, but after a number of applications, the plant’s genetics change. Worldwide information suggests more than 150 weed species are resistant to one or more of 15 herbicide families. The only species in Nebraska with confirmed glyphosate-resistance is marestail. A UNL study of a marestail population revealed glyphosate resistance requiring three to six times the normal level of herbicide.
The Weed Science Society of America web site lists weeds resistant to various herbicides by states, which includes atrazine, ALS, AHAS, glyphosate, and 2,4-D.
In southern states, resistant varieties of palmer amaranth, giant ragweed and marestail have emerged in recent years due to heavy glyphosate use. Glyphosate-resistant common ragweed was recently discovered in a South Dakota Charles Mix County soybean field. Seed samples tested at a Monsanto lab confirmed the glyphosate resistance. The weed species in Nebraska and South Dakota posing the greatest risk for developing resistance include marestail, common ragweed, waterhemp, common lambsquarters and kochia.
Resistant weeds often begin as a patch in a field, but they sometimes spread across a field before they’re noticed. Scouting, crop rotation and herbicide-application rotation are the most effective ways to combat resistant biotypes.
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“The first thing a farmer should do is identify the weed species in their fields,” Sandell says. “If these weeds that are at risk of developing resistance are in the field, just using glyphosate every year is not recommended. If it seems that you have to use more herbicide every year to control weeds, that could be a sign of developing resistance.”
Sandell says plants that branch out from lower nodes once the primary growing point is killed could also indicate an evolving resistant species and producers should use alternative control methods.
“You may want to consult with your state or extension weed specialist, collect some seed and offer them for testing,” Sandell says. “There are a lot of factors that affect weed control, such as whether spray particles were too small, enough gallons-per-acre were used or temperatures were too cold when the herbicide was applied. Weeds that don’t succumb to treatment aren’t necessarily glyphosate-resistant.”
Use of an integrated management approach is an important tool in avoiding development of resistant weed species. While economics drive weed control systems, Sandell encourages producers to consider pre-emergent herbicides as part of their program in order to decrease the likelihood that weeds will become glyphosate-resistant.
Sandell recommends use of these management practices:
– Scout fields prior to applying herbicide to identify weed species and ensure the selected herbicide controls the weeds that are present.
– Rotate crops and use of conventional and glyphosate-resistant varieties or Liberty Link varieties.
– Rotate herbicides and herbicide modes of action; avoid using herbicides with the same mode of action in the same field more than once each year.
– Limit the number of glyphosate applications to one or two in a single growing season.
– Apply herbicide when weeds are small and easier to control.
– Use soil-applied pre-emergence at planting as it helps maintain high yield potential and provides flexibility for post-emergence applications and reducing the need for multiple glyphosate applications.
– For post-emergence applications, use mixtures of herbicides specific to actual weed species in the field.
– Scout fields within two weeks following herbicide application to detect weeds that escaped treatment or changes in species composition. Use alternative control methods to prevent seed production.
– Use alternative weed management practices – mechanical cultivation, or spot spraying with alternate herbicides.
– Plant weed-free crop seed and clean equipment before leaving infested fields with or suspected to have resistant weeds.
“It really comes down to knowing the weed species in your fields and recognizing the potential for development of resistant weeds,” Sandell says. “We want to educate people so we don’t have do deal with this issue in our state. If you see indications that resistance is developing, we recommend that you take pre-emptive action.”