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Herbicide resistance: A 2008 reality

First came the horses, then the rags and now the pigs says long-time University of Arkansas Weed Scientist Ken Smith.

Speaking at the recent Belt Wide Cotton Conferences, Smith explained that horsenettle first showed resistance to glyphosate in 2003. The following year ragweed was documented as being resistant to glyphosate.

In 2005, the worst imaginable scenario began to play out in a small cotton field in southeast Georgia, where Palmer amaranth pigweed proved untouchable with rates of 96 ounces per acre of glyphosate.

Smith contends weed resistance to herbicides — not just glyphosate — is the most critical issue he has faced during his career. In hindsight, he says, we probably should have seen this coming. Insecticide resistance should have been a bigger danger signal, because by 1976 there were over 300 insects with some resistance to different insecticides, Smith adds.

The reality, Smith says, is that going into the 2008 growing season we have more than 300 weed species documented to have resistance to 19 different families of herbicides. For cotton growers, the worst possible combination is pigweed resistance to glyphosate, because of the proliferation of Roundup Ready cotton varieties, which has changed the whole face of cotton farming since the late 1990s.

“When Roundup Ready crops came out, any rate of glyphosate would kill pigweed. Then, I thought what a gift. Now, I think it wasn't a gift so much as it was a loan,” Smith says.

Roundup Ready cotton varieties allowed growers to expand cotton acreage and dramatically reduce input costs. “There is no going back to the pre-Roundup Ready era of growing cotton in the United States. We have to manage glyphosate resistance,” Smith says.

In too many cases, across the Cotton Belt too many growers have chosen to ignore the problem. “When you stick your head in the sand, Smith says, you leave a valuable part of your anatomy exposed.”

Don Parker, who works with the National Cotton Council in Memphis, adds the problem with weed/herbicide resistance isn't limited to one crop and it's not a new discovery and not limited to one state. Herbicide resistance is real and must be addressed, he says.

The National Cotton Council is dedicated to educating farmers to recognizing herbicide resistance and to help them develop resistance management programs specific to the conditions on their farm.

On the National Cotton Council's Web site, there is a link to ‘weed module’, which will help growers plan herbicide resistance programs.

Stewart Weaver farms with his father, H.E. Weaver. They farm nearly 5,000 acres of cotton, soybeans, wheat and grain sorghum near Edmonston, Ark. Starting in 2005, Weaver began an aggressive herbicide resistance management program that so far has been successful in warding off glyphosate resistant pigweed — a problem that plagues some of his neighbors.

They plant cotton on 38-inch rows. All their cotton is furrow irrigated. The cotton land is bedded up in the fall. They come back in the spring, knock the top out of the bed and plant. In the past they relied heavily on glyphosate for herbicides, until they began seeing resistance in 2003 with marestail.

“We had a mess in 2003. We finally found and refurbished our old cultivator and plowed the weeds under as best we could. Then, we came back with a pound of diuron and a pint of crop oil. We figured if nothing else we would burn the weeds back and manage as best we could,” Weaver recalls.

“In 2004, we called the University of Arkansas and got in touch with Ken Smith and Bob Scott, and they came out and set up trials on our farm. At their suggestion, we added 2,4-D and later in the season dicamba for burndown. Later we added Dual to help with grass and pigweed and a tenth of a pound of Envoke. All this was in addition to our standard treatment of glyphosate,” Weaver says.

“The 2004 treatment worked really well for resistant marestail, and we believe helped slow down development of glyphosate-resistant pigweed,” he adds.

“In 2005, we started with eight ounces of dicamba to allow us to move away from glyphosate. We used 20 ounces of glyphosate and came back with 10 pounds of Envoke, applied with a hooded sprayer. Envoke did a good job of managing the resistant marestail.

“In 2006, we applied 16 ounces of glyphosate. We moved to Flex cotton that year. We came back with two more glyphosate applications and a quart of Caparol. In some fields with resistance problems, we added 1.5 ounces of Valor.

“In 2007, we added Valor to our burndown program. We added Dual to our second glyphosate application to help with grass and pigweed.

“The key is when you suspect you have resistance, jump on it and don't let it get out of hand,” Weaver stresses.

“We have had production problems with cotton before — herbicide resistance is just another bump in the road. Cotton growers are very resilient, and I am confident with the number of tools we have to fight herbicide resistance that we will overcome this problem, too,” Smith concludes.

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