The No. 1 question Dr. Charlie Cahoon gets from farmers now that commodity prices are low and budgets are tight is “will a one-pass program work for me to control weeds?”
Cahoon, North Carolina State University Extension weed specialist for corn and cotton, says he understands farmers want to do all they can to save money, but he notes that research conducted last year in North Carolina found corn yields had a 13- bushel advantage with a PRE followed by a POST treatment compared to POST only.
“A 13-bushel yield increase will more than pay for your residuals, especially here in the Blacklands where you don’t have as many options,” Cahoon said at the Blacklands Farm Managers Tour at the Coastal Carolina Gin in Fairfield, N.C.
In corn tests last year, Cahoon and his team applied Bicep II Magnum or no pre-emergence herbicide, followed by various POST herbicide treatments. Cahoon says not only did the PRE followed by POST yield greater, but growers would have greater flexibility in POST herbicide choices where a PRE was used. “If our Bicep PRE was activated, we could easily achieve the same level of weed control with Roundup plus Atrazine POST versus more elaborate POST tank-mixtures,” says Cahoon.
Cahoon also pointed out that due to the hot, dry weather during May this year growers probably paid extra for early season weed competition.
“If you allow early season grass to compete with corn, you’ve probably lost more than you think. I’ve been a big proponent of a PRE followed by a POST. You don’t have to get too fancy here in the Blacklands. If you have a good PRE program, you probably can get by with Roundup and atrazine POST,” Cahoon said.
Weed management practices in the Blacklands are different than other parts of the state due to high organic matter in the dark black soils. “Herbicides are very tightly tied to organic matter. When herbicides are bound to organic matter they are not in the soil water. If they are not in the soil water they can’t work on the weeds,” Cahoon explained.
“It’s easier to get residual herbicides to work in sandy loam soils, but that isn’t the case in high organic matter soils. “Prowl is useless on high organic matter soils, but it will work on sandy loam soils. The HPPD inhibitors have limited residual activity in high organic matter soils,” Cahoon added.
Moreover, Cahoon said PPOs don’t offer as much residual control in the Blacklands as they do in other parts of the state. Atrazine does have some favorable characteristics to work in high organic matter soils, but as a weak base, atrazine loses some of its effectiveness in the lower pH soils of the Blacklands compared to its efficacy in other soil types.
“What we are left with, and this is scary, is Warrant, Dual, Outlook and Zidua. These Group 15 herbicides have the most favorable characteristics for working on these Blackland soils. What’s alarming is pigweed in the Midwest and Mid-South have confirmed resistance to this mode of action. If we get resistance to the Group 15 herbicides in this part of the world, I don’t know what we’re going to do from a residual standpoint,” Cahoon said at the Blacklands tour.
Cahoon said Roundup still works on redroot pigweed, but it is only a matter of time before we see Roundup-resistant redroot pigweed. He said this is a cause for concern in the Blacklands because redroot pigweed is ubiquitous in that part of the state.
In a corn trial conducted on Blacklands soil, Cahoon noted Outlook applied PRE controlled annual grasses and redroot pigweed five weeks after planting better than Warrant, Dual Magnum, Zidua, and Atrazine. “Outlook has been overlooked in the Blacklands. When you look at the herbicide from a chemistry standpoint, it actually has favorable characteristics for working on these extremely high organic muck type soils,” Cahoon said.
Finally, Cahoon pointed out that common ragweed continues to be a major problem in the Blacklands. Atrazine in the tank is a must for control in corn.
“Common ragweed is resistant to Roundup. Outlook, Dual and Warrant are useless on ragweed. They’re not going to help you. You need atrazine in the tank to pick up some of that resistant ragweed. The other thing about ragweed is the weed emerges relatively early compared to something like pigweed. Atrazine has residual activity as well as POST activity, especially if the ragweed is small.
“If you are bedding land up early, you may have some ragweed emerged by corn planting. We need to be sure to clean those small ragweed up at planting to start the season off on the right foot” Cahoon said.