- Steckel says resistance to glyphosate is developing at the rate of at least one new weed per year.
- Weeds with documented resistance to glyphosate include horseweed, pigweed, giant ragweed, common ragweed, common water hemp, Italian ryegrass, goosegrass and johnsongrass.
- While the list is a cause for concern, glyphosate-resistant Palmer pigweed is still the primary driver in weed control.
The complete answer for controlling resistant weeds in cotton will not be poured from a jug, according to University of Tennessee weed scientist Larry Steckel. Speaking at the 2012 Beltwide Cotton Conferences in Orlando, Fla., Steckel says a “back to the future” approach is required on today’s Mid-South weed-prone farmscape, including the use of hooded sprayers, directed sprays, residual herbicides, moving to Ignite-based systems and cover crops.
Steckel says resistance to glyphosate is developing at the rate of at least one new weed per year. Weeds with documented resistance to glyphosate include horseweed, pigweed, giant ragweed, common ragweed, common water hemp, Italian ryegrass, goosegrass and johnsongrass.
While the list is a cause for concern, glyphosate-resistant Palmer pigweed is still the primary driver in weed control.
“Many times, it is the only weed in the field. It will determine the success or failure of your weed control program, and perhaps determine whether or not you harvest your crop. Going with a ‘back to the future’ approach with hooded sprayers and directed applications is the only way to manage resistant weeds long-term.”
Steckel said new Willmar hoods “are terrific tools, particularly for managing Palmer pigweed. They are designed for a contact herbicide like Gramoxone, or Valor, MSMA. We really need to employ them, whether you’re in a Roundup Ready system, a LibertyLink system or a conventional system. We need a different mode of action or two underneath that hood to help us manage resistance.”
For glyphosate-based systems, Steckel recommends using a residual like Reflex early, “then plant clean, apply residuals and try to overlap glyphosate and Dual and use the hooded sprayer.” He recommends a similar residual approach for Ignite-based systems, except that Ignite is sprayed over-the-top.
Steckel noted that Ignite-based systems are growing in popularity across the Southeast and Mid-South. In fact, these systems were used on 57 percent of cotton acres in Tennessee and 52 percent of cotton acres in North Carolina. “Where we end up I don’t know. Ideally, I’d like to see a 50-50 split where you could rotate back and forth and help prevent resistance developing to Ignite.”
With the increasing use of Ignite-based systems, it’s more important than ever to flag fields to insure that the correct herbicide is used, Steckel said. “The University of Arkansas is encouraging this program. A red flag on the border of a field is means the cotton is conventional, a green flag is LibertyLink cotton and a white flag is Roundup Ready cotton.
“The Ignite- and Roundup Ready-based systems have to be managed differently, but the biggest factor is to get good coverage,” Steckel said. “Use flat fan nozzles, and go back to 15 gallons per acre. We have to, especially on borderline weeds that are too big for Ignite.”
Steckel is also worried about the development of glyphosate-resistant goosegrass in the Mid-South. “The bottom line is that the herbicides we used 10 years to 15 years ago, like Fusilade and Select, could come back into play. But those products did not control goosegrass when it was 3-tillers or larger. Guess what? It still doesn’t control goosegrass larger than three tillers. Timeliness, just like it is on Palmer pigweed, is going to be essential.”
There are no new technologies on the horizon that can do what the Roundup Ready system used to do, Steckel said. “Nothing will be able to provide the efficacy on large Palmer like Roundup Ready did. In the next 10 years to 15 years, we’re going to have to manage it with the older herbicides, the preplant and pre-emergence herbicides and the new herbicide-technology traits.”
While the new traits, such as dicamba- or 2,4-D-resistance, won’t be a silver bullet for control of resistant weeds, “they will be very much needed,” Steckel said. “We will be able to do a better job of managing pigweed than we can today. We can kill that 5- to 8-inch pigweed very effectively with 2,4-D and dicamba. Hopefully, they will come in the nick of time. We’re relying a lot on Ignite and we’re relying a lot on these pre- herbicides, and I think we’re going to start to see resistance.”
Cover crops can also provide an answer, noted Steckel. “It’s actually a pretty good mode of action for Palmer pigweed, which needs three things to germinate, light, water and heat. If you can get some shade on the ground, you’re going to curtail germination.
Steckel says research could reveal new practices for control of resistant weeds. “There are a lot of things we need to take a look at, like row-width, or maybe getting back to some cultivation.”