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Preventing herbicide resistance is best cure

Princeton, N.C., farmer Dan Kornegay has farms in North Carolina and South Carolina and his home farm sits in the middle of some of the most severe pigweed resistance in the country.

Despite his geographical disadvantage, the young North Carolina grower says resistant weeds have been a manageable problem.

The Kornegay farming operations in North and South Carolina include several hundred acres of tobacco and sweet potatoes and several thousand acres of grain crops and cotton, plus a livestock operation. His father and grandfather are both still active partners in the farming operation and also manage a farm in Tennessee.

The South and North Carolina farms are approximately 100 miles apart, but share many of the same challenges — especially in keeping weeds in check.

“We first started seeing pigweed resistance on a small scale at our North Carolina farm 3-4 years ago. I suspect it was a problem at our South Carolina location a little earlier than that,” the Carolina grower says.

Like many growers in his area of North Carolina, Kornegay has cut back cotton acreage significantly over the past few years. At one time they planted 3,000 acres of cotton, compared to 500 acres in 2009.

“When cotton acreage started declining and growers began growing more soybeans, then followed soybeans with soybeans — that’s when we first started seeing significant weed resistance problems,” Kornegay explains.

“Too many people treated soybeans like a secondary crop. They tried to produce beans with the least amount of inputs possible and glyphosate for weed control had been a low cost reliable herbicide on cotton, so it was natural that a lot of farmers relied on it to control weeds in soybeans,” Kornegay says.

“We stayed on the same type herbicide program in our beans as we had used in cotton. We used two pre-emergence herbicides on our soybeans — this year Valor and Prefix on our beans. We also used Prefix postemergence on some fields. We feel like cotton acreage is going to come back somewhat, so keeping our cotton land that is planted in soybeans clean is good investment,” he explains.

“Though this is strictly just an observation, it seems we see more resistance problems on land on which chicken litter has been used for fertilizer. Whether pigweed seed can remain viable and pass through the digestive system of a chicken, I don’t know, but it sure seems that we have more problems following litter,” he adds.

Roy Gorena, a regional sales representative for Syngenta says a recent farmer cooperative test in which poultry litter was used in an isolated field that had never had resistant weeds convinced him there is a correlation between litter use and herbicide resistant pigweed.

“This particular field had no history or resistant weeds, yet the year after litter was used, we found resistant pigweed and other resistant weeds. The only plausible explanation is that viable pigweed seed from the litter pile germinated and contained glyphosate resistance,” Gorena says.

“It seems to me that pigweed is more of a problem on lighter, sandy land. Most of this type land that we have is planted to tobacco or sweet potatoes, and we just don’t put litter on our lighter land.”

Whether poultry litter is directly correlated to increased weed resistance is open for debate. That herbicide resistant weeds continue to increase in severity in North Carolina is not.

Al Wood, a North Carolina State Extension agent in northeastern North Carolina says resistant weeds have spread slowly, but surely northward and are becoming a real problem for many of his farmers. “It’s like the genie is now out of the bottle,” Wood says.

Recently retired North Carolina State University Weed Scientist Alan York has called glyphosate resistant Palmer amaranth pigweed the biggest threat to cotton production in the Southeast since the boll weevil. York, who now works for Monsanto, the industry leader who brought the first herbicide tolerant plants to the market place says, “If you could play God and you wanted to invent the perfect weed, Palmer amaranth would be an ideal blueprint to follow.”

While Kornegay doesn’t disagree with the dire predictions for rampant herbicide resistant weeds, he does contend managing the problem is economically feasible and avoiding it in large part is possible.

“Even though I think there is a correlation between the use of litter and the increase in herbicide resistance on some of our land, it is sometimes more cost effective to use the lower cost fertilizer source and manage the weeds. Obviously, you have to use some common sense, but when it gets down to dollars and cents, sometimes litter is a better option than higher priced conventional herbicide,’ the North Carolina grower says.

“Even when we were growing large acreages of cotton, we didn’t rely solely on glyphosate for weed control. We use Reflex pre and a lay-by residual herbicide such as Caparol,” Kornegay explains.

“Now that cotton acreage is down, it is easier to use rotation to keep resistance problems from spreading. We never grew much cotton behind cotton and we don’t grow soybeans behind soybeans. Fortunately, we grow a diversity of crops, so it’s easier for us to switch crops around than it is for growers who grow only two or three crops.

“Sweet potatoes are a big crop for us and the tillage and multiple cultivations as well as the herbicide application methods are different. We hand pull pigweed escapes in sweet potatoes and we use a rope wick. Also, corn has become a big crop for us to alternate chemistry's such as Atrazine and others,” he adds.

“We use Valor with our burndown treatment, get Prefix on pre-emergence and handle escapes — 4-5 inches tall — with Reflex or Flexstar. We try not to depend much on ALS herbicides because of possible resistance developing. And, with vegetable crops and sweet potatoes, we have to rotate different families of herbicides because of re-entry issues.”

Managing weeds for Kornegay has always been a complicated system of burndown, pre-emergence and postemergence herbicide use — even before glyphosate resistant pigweed became an issue. In effect, the Kornegay farming operation was on a herbicide resistance management program long before herbicide resistance became a wide-spread problem in southeastern North Carolina.

The newest herbicide resistant threat comes from glyphosate resistant marestail. In the past two years, it has popped up in scattered fields across their farm, Kornegay says. Like resistant pigweed, the North Carolina grower says, herbicide resistant marestail can be a problem, but it is a manageable one.

“We use a post harvest burndown behind corn and this helps with marestail and Palmer amaranth seeding out,” Kornegay notes.

Multiple mode of action herbicides, two pre-plant and/or pre-emergence herbicides, timely postemergence herbicide application and good crop rotations, he says, are critical to avoiding and managing resistant weeds.


TAGS: Management
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