Weed Resistance: Meeting the Challenge Head On
Weed Resistance: Meeting the Challenge Head On
Chuck Foresman
Syngenta sponsored a panel discussion on meeting the challenge of weed resistance at the 2010 Commodity Classic. With the help of University of Arkansas associate professor Nilda Burgos, Michigan State University associate professor Christy Sprague, Farming for the Future winner and grower Mike Flanary of Grossnickle Farms in Walkersville, Md., and Resistance Fighter of the Year winner and agronomy manager Paul Barchenger of Hutchinson Coop in Hutchinson, Minn., we were able to address a wide range of practical questions from the field.
Since you may not have made it to Anaheim for the Commodity Classic, here are highlights of their discussion.
Q: How does a weed become resistant to glyphosate?
Burgos: In any weed population, including the seedbank, there is some frequency of the seeds that are naturally tolerant to glyphosate (or any herbicide). The herbicide application selects for those weeds, which can survive the application and add to the seedbank next year. Using the same herbicide over and over again allows that resistant weed population to increase.
Q: What effect might the lower cost of glyphosate have on glyphosate resistance?
Barchenger: Growers are always looking at their bottom line, but not controlling weeds early in the season can actually cost them more in yield in the long run. I encourage my growers to include residual herbicides and multiple modes of action to control weeds and protect crop yields. That's better than just going with a cheap program.
Sprague: If you look at the long-term economics, you're going to be better off using a pre-emergence herbicide or tankmixing. It may cost a little bit more up front, but when you are harvesting, you'll see better yields.
Q: What are differences across crops in resistance management?
Sprague: Our recommendations in corn are to control weeds pre-emergence or when they are 2 to 4 inches tall to avoid yield losses. Incorporating multiple modes of action is especially important in soybeans, because that's where resistance, including multiple resistance, often seems to develop. In fact, in Illinois there is waterhemp with resistance to four modes of action.
Burgos: The duration of a weed-free period, which is very important, varies across crops. For example, in cotton in the South, one major weed we are fighting is Palmer pigweed. Studies show that if you allow Palmer pigweed to compete with cotton during the first 21 days of the season, it can already hurt yield, even though you can try to clean it up afterwards.
Q: What are common management practices for fields with resistant weeds?
Sprague: Most important is to switch up your management practices. Use different herbicide modes of action. For example, if you have marestail, add 2,4-D to your burndown.
Barchenger: In the field where glyphosate-resistant giant ragweed first appeared, the grower has gone to continuous corn because there are more options to control the weed in corn. Lumax® herbicide has been very effective in this field.
But one thing really gets me - growers have all these nice tractors and gators, but I can't get them to go around the edges of their fields to clean up the weeds in their fencerows. Field perimeter management can really help slow the development of resistance and minimize its spread.
Q: How much more work are some of these practices?
Flanary: I know how stubborn some farmers can be about changing or doing things differently. But on our farm, we switch up our tillage a lot, which helps manage weeds. It's not much harder - in fact, sometimes it's a little bit easier. I can save fuel and time.
Q: How long are resistant weeds in the soil?
Burgos: It depends on the weed species. Pigweed seeds germinate quickly on the soil surface, about 95 percent of them can be gone after a year, and ryegrass is similar. Weeds with larger seeds and hard seed coats, such as barnyardgrass or morngglories, last long in the soil even if they remain close to the surface. But if you bury them, those buried deeper remain viable longer than those closer to the soil surface.
Q: How do weeds affect crop yield?
Sprague: Weeds compete for resources. So for example, common lambsquarters requires 658 pounds of water to create 1 pound of biomass. That's not very efficient, and ragweed is even higher. They are taking lots of the resources your crop needs.
Burgos: Another example is competition for nutrients like nitrogen. In rice, if you have one weedy rice weed for every 3 cultivated rice plants, the weed uses 60 percent of the nitrogen growers applied. They can be very competitive.
Q: How can I know if my practices will be effective and safe for my crop?
Flanary: As a farmer, I use a site that Syngenta has called http://www.farmassist.com/ quite a bit - a couple times a day when I'm spraying. We are out there on our own, and although I could call someone, they may not answer right away. On FarmAssist.com I can check product labels, see how to spray and mix them, and be sure I am using them right. That's been a helpful resource for me.
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