Roy Roberson 2

July 25, 2008

8 Min Read

T.G. Gibson is a life-long farmer — and a good one. Being a good farmer, having a degree in agriculture from North Carolina State University, and carefully monitoring all the production practices on his 2,500-acre row crop operation prevented him from having to deal with glyphosate-resistant pigweed.

Gibson grows cotton, soybeans, corn and wheat, finishes out 12,000 to 13,000 head of hogs per year and operates six poultry broiler houses. He farms near Gibson, N.C., and has been farming for a living since he was a teenager, taking a couple years off to complete NCSU's two-year program in agriculture.

Glyphosate-resistant Palmer pigweed started as a small, almost insignificant problem in one remote field on his farming operation. Now, he says, 70 percent of his 2,500-acre operation has pigweed with some degree of resistance.

For Gibson, glyphosate-resistance problems started on a farm that he took over four or five years ago. Prior to Gibson farming it, the land had been used to grow Roundup Ready soybeans or cotton — consistently over a number of years. Like other farmers, Gibson had no idea glyphosate-resistant weeds existed when he picked up the new land.

“When it first started, we sprayed this particular field with glyphosate, and we noticed some of the pigweed didn't die. The weeds grew in more of an oval than a circular pattern and were scattered, seemingly randomly across the field,” Gibson says.

“For example, in a 20-acre field in the early development of glyphosate-resistant pigweed, we would see two oval-shaped patches, roughly the size of a pickup truck. We conducted our own test in cooperation with our county Extension agent.

“We sprayed these suspect weeds several times with high rates of glyphosate. We knew it wasn't a wash-off situation, and we knew it wasn't weather-related. In short, we knew we had a problem,” Gibson says.

The field in which Gibson first noticed resistant weeds is isolated enough, and he is careful about cleaning equipment when it goes from one farm to another. He doesn't think he spread the problem by moving resistant pigweed seed from farm to farm.

“It has spread throughout our farm, but it had to spread by pollen or by continued weed selection for resistance,” Gibson adds.

Gibson's story is far from unique in what Alan York, NCSU weed specialist, calls the “hot counties” in North Carolina. York says this five- to six-county area was the first in the state where Palmer pigweed resistance to glyphosate was documented.

Because of good farmers like Gibson, the problem has not gotten significantly worse in this area of the state, according to York.

“We have been picking glyphosate-resistance up in fringe areas around where the hot counties were. So, it is either spreading or it is being recognized in more areas. Within the hot areas, I don't think it is any worse, probably because growers are trying to deal with it better,” York explains.

Though the original hot spots have been managed to some degree, the problem continues to grow and may get worse before it gets better, according to York.

“Currently, we have Palmer amaranth with resistance to both glyphosate and ALS inhibitors. And, in the spring of 2007, we picked up Italian ryegrass resistant to AC-Case inhibitors (Hoelon, Axial) and ALS inhibitors (Osprey). So far, this particular problem seems to be confined to a couple of counties.

“We are becoming more concerned with the potential for resistance to PPO inhibitors. So far we have none. But, with widespread use of Valor and Reflex (or other products containing fomesafen), it would not be unexpected to find it. That would be serious since we depend heavily on PPO inhibitors to help us with glyphosate-resistant weeds,” York says.

Economics is another concern for the spread of herbicide-resistant weeds. With grain prices at record high levels, it seems farmers can afford to spend more money for herbicides and other crop production inputs. However, farmers have similar risks for profit and loss — they can make more money than ever, but they can lose more money than ever.

York has a positive take on farmer response to herbicide resistance and believes high grain prices will help the overall weed management program for most growers.

“Fortunately, soybean prices are at a historical high. Soybean growers now, as a whole, are much more likely to use another herbicide in addition to glyphosate than in previous years. Frankly, cost does not matter where resistant Palmer amaranth is already present. You control pigweed, regardless of cost, or you don't harvest,” he says.

“Cotton prices are nothing to brag about, but our growers are using a lot more pre-emergence herbicide than in the past few years. Again, if a grower has resistant Palmer pigweed, he must get most of it pre-emergence or he doesn't get it.”

York, who spends tireless hours conducting grower meetings and answering e-mail and voice mail questions about weed resistance, gives most of the credit for reducing the risk of herbicide resistant weeds to farmers.

“Increased use of pre-emergence materials is partly the result of us pushing these herbicides, but I also think more and more growers are genuinely trying to avoid problems. Some incentives offered by industry in both cotton and corn this year have also encouraged growers to use more pre-emergence or preplant herbicides. The combined efforts are working,” he says.

For Gibson, controlling pigweed was not an option — he had to do it to continue growing cotton and soybeans. “When you go into a field with a cotton picker and you have 6-foot Palmer pigweed that are the size of a seedling pine tree at the base, you've got serious harvest issues. The bottom line is you can't grow cotton, soybean or any crop in competition with mature pigweed.”

Gibson says the problem with glyphosate-resistant pigweed has to get better, otherwise he will be forced to look at other cropping options. In cotton, especially, mature pigweed jams the head of the picker and simply won't feed through a picker. Combined with competition-related yield losses, glyphosate-resistant pigweed and cotton is just not a sound economical situation, the North Carolina grower adds.

To combat the problem in cotton, Gibson began applying Reflex in a band. “We had a nice clean band, but pigweed in the middles caused big problems. Once these weeds get much taller than 6 inches, they seem impossible to control.

“This year we will go with a pint of Direx and a pint of Reflex broadcast to give us control in the middles. Before we get to the four or five true-leaf stage we will come back with an over-the-top tank mix application of Dual and glyphosate. Then, we will come back with a layby formulation of another family of herbicides,” Gibson explains.

“We want to mix up the families of herbicides as much as possible. We have to look at the problem across all crops to avoid overuse of any family of herbicides.

“In soybeans, we have played around with row patterns to try and shade out some of the pigweed. We looked at 20-inch rows, but have gone back to 30-inch rows, so we can strip-till. We haven't seen the pressure from herbicide-resistant pigweed in soybeans that we've seen in cotton, but it is still a serious issue. We could go over-the-top with more herbicide options than in cotton, which is probably why the problem was less in beans,” Gibson says.

Throughout North Carolina and Virginia, no-till and strip-till are becoming more and more common. “We are pretty heavy into no-till or strip-till, and that has put pressure on other weed species, like horseweed,” York says.

“Even with conventional planting, almost no one is cultivating corn, soybeans, or cotton. That puts more reliance on herbicides, and hence more selection pressure on weeds.

“We have seen cotton acreage drop and corn and soybean acreage increase over the past few years. While rotation is good, and while it can help in resistance management if we take advantage of the opportunity to get other chemistry into the system, for the most part acreage shifts have been driven by market forces far more than a desire to manage resistance.

“So, the primary thing our growers are doing to control resistant weeds or to try to avoid getting resistance is to incorporate other modes of action into the system.”

Glyphosate-resistance is the major problem facing upper Southeast soybean and cotton growers, but it far from the only threat, according to York.

“We have glyphosate-resistant common ragweed in at least three counties. If history teaches us anything, we would expect a good chance of resistance in ragweed. We are keeping our eyes open for glyphosate-resistant lambs-quarters and Italian ryegrass. So far none has been found, but those are likely candidates.”

Do any changes in cultural practices or herbicide use create an environment in which other weeds, such as cocklebur or sicklepod, will be more likely to develop resistance to current herbicides? “As of now, we have no resistance in sicklepod and the only resistance in cocklebur has been MSMA (Roundup Ready cured that) and ALS inhibitors (which Roundup Ready also cured).

“We watched resistant horseweed or marestail mushroom in Tennessee and Arkansas. For the past three years, I have been warning our growers that it was time to change our ways. Horseweed has seed that are readily spread long distances by wind. It has hit us in a big way this year. It is all over eastern North Carolina, and it is worst in the northeastern part of the state,” York says

“In northeastern North Carolina some counties have a lot of it. My windshield survey indicates it is present in at least 30 percent of the fields in some counties. The time has come to assume any horseweed in eastern North Carolina is glyphosate-resistant and act accordingly,” he adds.

Acting accordingly has been a farming way of life for Gibson since he started farming as a teenager. Managing herbicide-resistant weeds is just one more management challenge and one in which he and other farmers in south central North Carolina seem to be getting the upper hand.

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