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C.O.R.N
Newsletter
2006-06
March 20, 2006 -
April 4, 2006
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Evolving Lambsquarters and Giant Ragweed Control Problems – What’s the Cause?
by
Mark Loux
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Across Ohio and the rest of the Midwest, growers and dealers report that lambsquarters has become more problematic in Roundup Ready soybean fields. We have also observed more giant ragweed in Roundup Ready soybean fields recently, and especially compared to the first 5 or so years after their introduction when these fields were largely weed-free at the end of the season. We know that some instances of poor control can be attributed to generally poor management of glyphosate, due to the tendency for some growers to over-simplify their approach to weed management in order to reduce the number of applications and costs. Lambsquarters and giant ragweed can both be generally tough to control with a single postemergence treatment. Growers who omit preplant burndown treatments, apply when weeds are large and old, and use rates too low for the weed size and age, place themselves at risk for control failures. The accompanying article describes management strategies to help ensure consistent control of these weeds, and the rest of this article outlines out thinking on the evolution of resistance in these two weeds.
We believe that some populations of giant ragweed and lambsquarters are evolving to have a low level of resistance to glyphosate. We expect to see changes in the response of our weed populations to glyphosate based on our intense use of this herbicide, and glyphosate-resistant marestail is already abundant in Ohio. While we typically observe about an 8X level of resistance to glyphosate in marestail, our greenhouse research indicates that the level of resistance in lambsquarters and giant ragweed is more on the order of 1X to 4X. So, plants can survive glyphosate rates up to 3 lbs ae of glyphosate per acre, but they usually suffer substantial injury even at lower rates. We also observe considerable variation in the response of these populations among greenhouse experiments. This might not be unexpected for a low level of resistance, but it makes characterization of the resistance difficult, and also makes it difficult to determine the implication of resistance under field conditions.
In field research we conducted in 2005, the suspect giant ragweed population we were working with was difficult to control. In a small plot study in an area of the field where the population was densest, we achieved better than 90% control only when we applied 3 lb ae/A of glyphosate followed by another 1.5 lb ae/A or when we applied Flexstar (1 pt/A) or FirstRate (0.3 oz/A) followed by 1.5 lb ae/A of glyphosate. In a large-plot study using large application equipment, we achieved an average of at least 90% control when we made two postemergence applications at labeled rates, but the most effective control occurred when we applied 1.5 lb ae/A first, and followed with another 0.75 lb/A (as compared to the other way around). We observed a considerable number of plants that were not completely killed in the large plots, and many of these appeared to have produced viable seed.
We conducted field research at six sites where we suspected lambsquarters to have a low level of resistance. At these sites, we were able to achieve excellent control with two postemergence glyphosate applications, but at several sites we did find a few plants that, although small, were able to produce viable seed. All of our lambsquarters plots received two applications, but we did evaluate control after the first application. Our ratings at that time, along with observations of control in the rest of the field at harvest time that was treated only once, led us to conclude that control would have been less than acceptable at several sites had we not treated twice.
So, what does it all mean? Our working theory at this point is that populations of lambsquarters and giant ragweed in some fields are becoming less sensitive to glyphosate. We consider this to be a low level of resistance, since it has developed over time with repeated use of the same herbicide, and control with glyphosate was initially good in these fields. What are the implications of a low level of resistance for users of Roundup Ready systems? We have tentatively concluded the following:
- while some of the blame rests with mismanagement of glyphosate, the evolution of a low level of resistance is likely to be contributing to the problems that growers are experiencing with control of lambsquarters and giant ragweed;
- continued intensive use of glyphosate, an increase in the acreage of Roundup Ready corn, and a failure to integrate glyphosate with other herbicides will cause resistance to be more widespread, and also drive the level of resistance higher within populations.
- the expression of the low level of resistance is variable, so that the resistant populations may be controlled under appropriate management of glyphosate (small plants, high enough rate, multiple applications) and when environmental conditions are favorable. Conversely, these populations are more likely to be poorly controlled when growers mismanage glyphosate (large, old weeds, low rates, no burndown, etc) or when environmental conditions are unfavorable;
- growers observing poor control of lambsquarters or giant ragweed should not assume that it is the result of poor environmental conditions or other factors alone (although these can be part of the problem). Management strategies in subsequent years should be altered in fields where problems have occurred, with the assumption that a low level of resistance could be present (see accompanying article for effective management suggestions);
- growers who mismanage glyphosate will end up with more instances of poor control where there is a low level of resistance. This poor control can result in increased weed seed production, which increases the number of resistant plants the following year. So, mismanagement of glyphosate can speed the development of resistance problems;
- properly managing glyphosate can slow, but will not prevent the development of resistance. Growers also need to implement practices that reduce selection pressure (i.e reduce the reliance on glyphosate alone for weed control), such as rotation of Roundup Ready systems with non-Roundup Ready systems, use of tillage or a multiple-herbicide burndown program to start the crop weed-free, use of preemergence herbicides, and combination of glyphosate with other postmergence herbicides. |
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