Weed Resistance: A Numbers Game
For each herbicide and weed species a probability exists that a naturally occurring resistant plant may be in a population, which could lead to a resistance problem. As I mentioned earlier this week in my post about
other resistances, if there is a large population of weeds, there is a probability that a small portion of that population is naturally tolerant (think morningglory's natural resilience to glyphosate). According to an
article in the
Wisconsin Crop Manager Newsletter last spring, the probability is higher for some weeds like horseweed or waterhemp than others. Think about it-your chance of winning the lottery goes up the more tickets you buy, so the larger the population of weeds you spray within a field, the greater the chance of a resistant weed within that population.
Reducing the risk of resistance-getting the numbers in your favor-can happen in a few ways. One is to rotate glyphosate use with other herbicide modes of action, so that weeds are not sprayed with glyphosate more often than other herbicides. Using other herbicides in the same season as glyphosate, like a pre-emergence herbicide, or tank mixing another herbicide with glyphosate reduces the pressure on a single chemistry doing all the work. Growers should be selective when choosing an additional herbicide for a particular field. Examine which weeds are the toughest to control and add a herbicide that best attacks those weeds.
Reducing the risk of glyphosate resistant weeds also makes sense economically. By reacting to glyphosate-resistant weeds once they appear in a field, growers have already lost money. Yield loss has already occurred, and the population of resistant weeds has become well established. A proactive strategy, including ideas mentioned earlier, helps preserve that yield potential and alleviate the pressure of relying on a solitary control measure like glyphosate.
Do you think your risk is high or low for developing glyphosate-resistant weeds? Why do you believe so? What practices could help to lower the risk of a developing resistant population, or reduce an existing resistant population?