Herbicide resistant pigweed reality for Carolina grower

Like most good farmers, Bill McGoogan knew about the increasing problem of herbicide resistant weeds. Until 2005, herbicide resistant Palmer pigweed was somebody else's problem, now it's his.

McGoogan, who farms near Lumber Bridge, N.C., first noticed some pigweed in one of his soybean fields that had been sprayed with glyphosate. He went back and sprayed it again with glyphosate, making sure to get good coverage.

“The second time I sprayed it, I knew I had good coverage and nothing happened with the pigweed. I called Monsanto, and they came and sprayed a 3X and a 6X rate of Weathermax, and it still had little effect” the North Carolina farmer says.

In 2005, McGoogan says he didn't see any other real bad spots, though there were scattered Palmer pigweed escapes throughout his farm. Pigweed escapes are not at all uncommon, and he couldn't pinpoint for certain that any of these were caused by herbicide resistance. After finding the resistant pigweed in his soybeans, McGoogan began noticing patches of weeds in neighboring fields.

The soybean field in which he first found resistant pigweed in 2005 had been planted in cotton in 2004, and in previous years, and the primary weed program was either glyphosate alone or glyphosate with Staple. Though not documented, the continued use of Staple and other ALS-inhibiting herbicides may have created some pigweed resistance on his farm to this family of herbicides.

In 2006, the resistant pigweed spread to cotton and other soybean fields. On cotton, he used 24 ounces per acre of Weathermax, plus 1.7 pints of Staple. When that didn't control the pigweed, he put on a second application of 24 ounces of Weathermax, and did not control the weeds.

Normally, he says, one application of glyphosate and Staple would take out pigweed the first time around. Even waiting until the weeds were 12 inches tall, on non-resistant Palmer pigweed, the second application would take care of the problem.

“I stepped up the rate a little bit when I put out 24 ounces, but I've seen neighbor's fields in which they have gone up to 32 ounces per acre, and they still don't get control,” McGoogan says.

In tests in Georgia, University of Georgia Weed Scientist Stanley Culpepper put on three applications of Weathermax at 84 ounces per acre, per treatment and didn't even slow down the fast growing pigweed. It is well documented that no matter how much glyphosate is applied, nor at what growth stage the Palmer pigweed is in, there will be no control, if the plant is resistant to glyphosate.

In addition to competing with cotton or soybeans for nutrients and dramatically reducing yield, Palmer pigweed can be tough to deal with at harvest time. Even if large 3-4 inch diameter, four-foot tall pigweed plants are killed, when the cotton crop is defoliated, it still causes problems with the picker.

McGoogan says large, dead pigweeds are often a bigger problem because they tend to loosen up from the ground and hang up in the picker. The larger the weeds and the later they are killed, the bigger problem they are to deal with at harvest time, he says.

Already looking toward the 2007 season, McGoogan tried three different combinations of pre-plant, residual herbicides in test plots on his farm in 2006. His goal is to help keep pigweed under control for 4 or 5 weeks after cotton is planted with these herbicides.

In these test plots, he used Prowl alone, Reflex alone, and Prowl plus a pint of Direx.

The best combination, he says, was Prowl alone, and he got similar results with Prowl plus a pint of Direx, Prowl alone cost $4-$5 per acre and a pint of Direx adds another $2 to the tab. Reflex did not perform as well in McGoogan's test and cost about $11 per acre.

In the 2006 test, the North Carolina farmer said conditions were nearly perfect for using a pre-plant material. Adequate rainfall after he applied the test plots of Prowl, Prowl plus Direx and Reflex gave each material the opportunity to work. Prowl and other pendamethalin-based herbicides requires adequate moisture to work properly.

In 2007, McGoogan says he will probably go back to more cultivation, use pre-plant residual herbicides, most likely Prowl, then come back at the four leaf stage and apply Weathermax. Within a week or so after applying Weathermax, he will likely come back and cultivate and use a post-directed spray, probably Cotoran and MSMA.

While the added cost of managing glyphosate resistant pigweed won't quite double the cost of weed control, it will add a significant cost to production. In 2007, McGoogan says managing glyphosate resistant weeds is at the top of the list of problems growing cotton in his area of North Carolina.

For farmers who document cases of resistance on their farms, he says, plan on the problem getting much bigger the following year. “I thought, maybe resistance won't carry through from one year to the next, but it does. If you see an isolated area this year, you better count on it being a quarter or half the field the next year,” he says.

“I feel like I know what the problem is now, he says. The problem is I don't know exactly how to solve the problem. I hope farmers will share their experiences, so we can get as much information as possible to deal with the problem,” he concludes.

North Carolina State University Weed Scientist Alan York, says a recent ride through the southern end of North Carolina convinces him that the problem is widespread. As a wild guess, the dean of southern weed scientists says maybe 100,000 acres, mostly in a 10-12 county area in the south and south central part of the State, show definite signs of resistance to Palmer pigweed.

McGoogan, who farms 1,700 acres of cotton, 450 acres of soybeans, 250 acres of wheat and 200 acres of corn in Robison County, N.C., is in the heart of the problem area. Though McGoogan and farmers in that area of the state first reported pigweed resistance in the summer of 2005, York says the problem has probably been there for two or three years.

Resistance always starts with one plant, and a resistant Palmer pigweed plant looks just like a non-resistant plant. There is no way for a farmer to know which is and which isn't resistant to glyphosate until it's too late to control the weed with any chemical material labeled for use in cotton or soybeans.

York says the key for farmers is on reducing pressure on the non-resistant plants by rotating families of herbicides. It will be critical he says for growers to know the active ingredient and mode of action of each herbicide used on the farm and to be careful to avoid over-use of any one family of chemicals.

Pigweed resistance to ALS-inhibiting herbicides is already rampant in the Southeast and trying to control pigweed with Staple for example in cotton, then trying to control weeds in another crop with a different trade name, but the same ALS-inhibiting active ingredient will further increase resistance problems.

Being at the tip of the sword of the Palmer pigweed problem in North Carolina is no badge of honor for growers like Bill McGoogan. He's seen the problem first hand, and now he's looking for answers.

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