Macon County, Ga., has been a hot spot for glyphosate-resistant Palmer amaranth pigweed since it was first discovered in the state. So it’s not surprising that growers there are well-versed in the use of alternative management practices to help control the weed pest.
“Glyphosate-resistant Palmer amaranth was found in Macon County, Ga., in 2005,” says Jeremy Kichler, county Extension coordinator. “I’ve tried to help my growers with this issue over the past five to six years. They’ve used alternative management practices, such as managing the seed bank with deep tillage, cultivating, using wick bars, and hand-weeding.
In Macon County this past year, Kichler estimates that at least 80 percent of the cotton acres were hand-weeded at least once. That’s very expensive and time-consuming, he adds, costing from $20 to $70 per acre, depending on the density of the pigweed.
“A lot of growers become frustrated during planting time when they’re strip-tilling and trying to deal with larger Palmer amaranth with their herbicides. So I’m starting to see acres get out of conservation-tillage and into conventional-tillage,” he says.
Producers are looking at irrigated practices to activate residual herbicides, with residual herbicides being the cornerstone of management practices for resistant Palmer amaranth, says Kichler.
“But we also have dryland situations,” he says. “In Georgia during April and May, we have erratic rainfall patterns that make the activation of these residual herbicides somewhat difficult.
Growers are starting to ask a lot of questions about incorporating residual herbicides, they’re asking about tillage implements, and they’re starting to get out their disk harrowers, which wasn’t so common just a few years ago.”
In trials conducted in 2010, the objective of Kichler and other researchers was to determine the impact of a roto-tiller, a field cultivator and a disk harrow on the activity of Reflex herbicide on Palmer amaranth.
“We had three different scenarios including an irrigated field and two dryland situations, which included a moist soil for the incorporation of our residuals, but not having rain to activate our pre-emergence. Then, we had a bone-dry situation with no soil moisture or rain,” he explains.
In the irrigated site, in Tift County, the trials also looked at crop tolerance to Reflex. The plot size was 6 by 70 feet, replicated three times in a randomized complete block design. Phytogen 375 Widestrike Roundup Ready Flex was planted, and Ignite treatments were made on a timely basis to keep the crop tolerance portion of the trial weed-free.
Two herbicide treatments
“We had two herbicide treatments at this site. One included Reflex plus Prowl H20, applying each product at 1 pint per acre, and we had a no-herbicide treatment. Reflex currently is registered as only a pre-emergence application on Georgia cotton.
“We had five application methods — the first a pre-emergence application of the herbicide right after we planted cotton. Then we looked at three different preplant incorporated methods of putting out a residual herbicide. One was using an L-tine roto-tiller before we planted cotton, and another was a field cultivator, incorporating at a depth of about 3 inches for both. We also used a disk harrow to preplant incorporate the Prowl and Reflex, at about a 6-inch depth, and we did that before planting. Then, we sprayed the residual herbicides and immediately came through with a strip-till rig.”
The site was irrigated one half inch before planting, says Kichler. “Then, the residual herbicides that were applied pre-emergence lay there for five days. So we applied another one half inch of water five days after we planted. We wanted to create a good environment for crop injury, as the cotton was emerging at that time.”
The pre-emergence applications of the residuals gave 99 percent control of Palmer amaranth, and the roto-tiller was just as effective at 95 percent. The disk harrow was least effective, giving 71 percent control.
“When we looked at the percent of Palmer amaranth control 38 days after planting, again the pre-emergence application of the Prowl and Reflex was the most effective, but the disk harrow gave us only 3 percent control, probably because the residual herbicides were buried deeper,” he says.
As a county Extension agent, dealing with resistance on a daily basis during the summer, Kilcher says cotton injury concerns are common among his growers.
“Cotton injury from Reflex can be severe if rainfall occurs during emergence. At 10 days after planting, the pre-emergence application of Prowl/Reflex gave us 20-percent injury. When we used tillage to incorporate the residuals, it gave us at least 50-percent less injury. In the pre-emergence treatments, there was more concentration of Reflex in the soil compared to the tillage applications where we diluted the residuals.”
The second experiment, he says, was conducted in Macon County in a dryland situation, with rainfall before the residuals were incorporated.
Good soil moisture
“We had good soil moisture to incorporate, but our pre-emergence treatments did not receive any rainfall for 12 days. So they lay there for 12 days before they were activated. At this site, we had two tillage options, including a pre-emergence application and the roto-tiller. We had two herbicide options — one was no herbicide and the other was Reflex applied at 1 pint per acre. At 14 and 28 days after planting, the preplant incorporated treatment was more effective than the pre-emergence treatment.”
At the third site, also in Macon County, there was no soil moisture, says Kichler. Treatments included no herbicide and Reflex at 1 pint per acre. Tillage included a pre-emergence application of Reflex and also the L-tine rototiller. There were no significant differences between the two applications due to the fact that there was no moisture to aid in the emergence of Palmer amaranth.
“In an irrigated situation, the pre-emergence applications were most effective, but if you have a grower who is concerned about crop injury, a shallow incorporation will reduce control by 9 percent. In a dryland situation with moist soil to incorporate your residual herbicide, but no activating rainfall for your pre-emergence, the preplant incorporated with shallow incorporation treatment will be more effective than the pre-emergence application.
“And if you’re in a bone-day situation, it really doesn’t matter about your application method because Palmer amaranth probably won’t emerge anyway until it rains. The roto-tiller was most effective and the disk harrow was least effective as far as tillage implements go.”