Herbicide season for a lot of Georgia cotton farmers is winding down, and the timeliness of those applications will reveal how much hand weeding they’ll be needing, or if they’ll have a crop worth harvesting.
Stanley Culpepper, University of Georgia Extension weed specialist, conducted a test to see how important a timely first post-emergence herbicide application can be for the control of Palmer Amaranth in cotton.
He compared an auxin-based system (dicamba or 2,4-D system) and a Liberty-based system on a test site at the Sunbelt Ag Expo Farm Moultrie, Ga., located in the southwest part of the state.
“If executed timely, both systems work well to manage Palmer Amaranth in Georgia,” he said.
For both systems, he planted the cotton into weed-free plots and applied as his pre-emergence residual herbicides a combination of Warrant plus Direx at-planting. He then came back and hit part of the test with a timely Post 1 herbicide applications, hitting the auxin-tolerant technology with Roundup plus the auxin and the Liberty tolerant-technology with Liberty, before the tallest Palmer Amaranths in that part of the trial reached three inches, or about 15 days after planting.
At this point, it is important to remember that Palmer Amaranth can grow one to three inches a day in the Southeast during late-season.
For the test, one part of Culpepper’s trial on purpose didn’t receive that timely ‘less-then-three-inch-pigweed’ Post 1 application of either the auxin or the Liberty, and that cotton didn’t get that first post application of either herbicides until eight days after the cotton that did get the timely first applications. For both the timely and untimely systems, a sequential POST application was made two weeks after the first application.
In short, both the timely auxin and timely Liberty systems looked great by mid-July with cotton growing strong and weed free.
But the untimely, eight-day-late auxin system by mid-July was infested with a few large pigweeds, translating into a real-world, on-farm cost, Culpepper said, of about $15 per acre in hand weeding that would be needed to eliminate the weed from the field. The untimely, eight-day-late Liberty system was dominated by pigweeds and might cost more than $100 per acre to hand weed out the escapes, “meaning it would’ve in the real world been a challenge to harvest,” he said.
“Timely applications are the critical link to success with all cotton weed control systems. In this experiment, not being timely reduced our control with each technology but much more so in the Liberty system compared to the auxin system. We are working desperately to figure out approaches to improve the Liberty system because there are many areas across Georgia where auxin herbicides should not be used because of adjacent crops being extremely sensitive to auxin herbicides,” Culpepper said.
Generally speaking, weed control of the auxin system in 2017 “was a great success partly because of good technology but also because of amazing growing conditions enhancing herbicide control. Less control, especially with auxin herbicides, should be expected in 2018 if drier conditions occur,” he said.
If Georgia cotton production is to remain viable, he believes growers must continue to walk the path they've blazed over the last decade to get the edge on herbicide resistances in the state. They can't slide back into bad old habits. It is critical to continue to:
- Start clean with no weeds at planting.
- Apply a tank mix with two residual pre-emergence herbicides using rates that won’t damage the crop.
- Make two-timely POST applications.
- And most importantly run a layby rig including products such as diuron.
“This program approach is absolutely essential for herbicide-resistance management for our farms,” he said.
Culpepper shared another recommendation/question for growers: “For those relying too heavily on glyphosate plus the auxin, I wonder which of you will be the first to destroy the technology for your community by creating additional resistances in Palmer amaranth?”
So, why did Culpepper wait eight days to apply the first post application of auxin or Liberty on the comparison test plots? No real science there. Eight days later was the soonest he could get back to that test plot in Moultrie, Ga., to make the applications of the herbicides on that comparison side of the test.