Glyphosate-resistant horseweed was by far the No. 1 weed control problem in west Tennessee this year, according to weed scientist Larry Steckel with the West Tennessee Experiment Station. Steckel says he received his first call on the pest in February and received his most recent call in July. “It’s a big problem in no-till, which is the practice on about 80 percent of our cotton acres.”
The good news is that scientists have been figuring out some control methods for resistant horseweed, although this year, the weed threw producers and researchers a curve ball.
“We’ve gotten very good control of horseweed with dicamba, or a program centered around that herbicide, going with 8 ounces of dicamba mixed with glyphosate, Gramoxone or Ignite. This season was not the case. We saw a lot of failures in a lot of cases.”
Steckel says much of the blame for these failures were due to environmental conditions. “It seemed like we had our June weather in April. The first week of April, temperatures were in the 90s and it was very dry. We had not encountered those types of conditions at burndown before.
“Most of the applications we make at that time, the weather is cold. Coverage was not an issue, but we were only getting foliar uptake of the dicamba, we weren’t getting root uptake.”
Another problem is that this year, horseweed got a good head start on growth before control measures began. “Horseweed was growing in our fields in August, September and October of 2005. That was the horseweed we were trying to control.
“The taproot was large and the plant was living off the taproot after it had been sprayed with dicamba. When the dicamba wore off, it started to regrow.”
Steckel has seen populations building everywhere. “I have found 20 horseweeds per square foot. We were getting 80 percent control, but that isn’t good enough when you have that type of population.”
Steckel recommends going with an earlier burndown to control smaller weeds. “I’d much rather us go in March, instead of April. It’s likely that we could get another flush, but we can do something with the smaller horseweed.
“There are a lot of positives to a March application. No. 1, in some fields where we have huge populations of horseweed, we can bump the rate. The label on dicamba says you need 21 days for every 8 ounces of dicamba, plus an inch of rainfall. The inch of rainfall is critical.
“We can go out in early- to mid-March and bump the rate up to 12 ounces, and we shouldn’t have any problem coming back with cotton. Plus when we spray in March, we have a much better chance of getting that inch of rainfall than we would in April. We have a little more time to work with.”
Fall applications of Valor are another alternative. “According to the research we’ve done, it’s kept the horseweed out of the field until April. It can keep the ground too clean, and on some of our hills in Crockett County and other places, you may see too much washing. In some of our river counties where we’re flat and there’s not quite the erosion issue, it might be a fit.”
An early spring application of Valor for horseweed “is very good from a residual control point of view. For small-seeded broadleaves like horseweed and pigweed, it’s outstanding. Two ounces can take you into April and May. It did a little better than I expected on grasses. But I think that is unique to this year.”
There is good news on one weed that has shown resistance to glyphosate — Palmer pigweed, according to Steckel. “We haven’t seen resistant weeds move much. They are in the Crockett County area in a field or two, but nothing has shown up in Lauderdale County.”
Steckel says that 22 ounces of glyphosate “is not giving us complete control on some of the resistant biotypes of Palmer pigweed. But in some of the areas where we have Flex cotton, we can go up to a quart, and it’s taking care of it. So going forward, Flex cotton is going to help us on our pigweed management.”
Steckel stresses that growers should incorporate other herbicides into the mix “to get in some other modes of action out there. We cannot afford to get Palmer pigweed here like they have in Georgia. As long as we can make glyphosate work for us, we have to do that.”
Some of the practices growers use to manage resistant horseweed can also apply to Palmer pigweed, according to Steckel. “Valor is right at the top of the list. Dual and Prowl can give us good residual on those small-seeded broadleaves. A lot of growers this year put Dual in with their glyphosate applications. It did a very good job and I’m encouraged that we have that in the mix.”
Post-directed applications of Valor can also help provide some residual control of pigweed, “as long as you have a good tractor driver and someone who knows how to run a hooded sprayer. You can’t be sloppy with Valor on the hoods.”
Liberty Link cotton, which uses Ignite herbicide to control a broad spectrum of weeds, will work well on Palmer pigweed “if you catch it small. If they get any size on them, 3 inches to 4 inches tall, oftentimes you’ll burn the top, then you’ll get re-growth.
“You’ll need some sort of two-pass program, maybe three. It’s not as effective as glyphosate and that’s the bottom line right now.
“If you can manage your cotton and jump on Palmer pigweed when it’s small, you can do it. We have a lot of acres of cotton out there and getting around timely to all those fields is going to be tough. On a hot day, Palmer pigweed can grow 2 to 3 inches.”
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