Pigweed not only resistance problem

Though glyphosate resistant Palmer amaranth, commonly called Palmer pigweed, has been the big story in the Southeast in 2006, this combination is not the only resistance problem growers will be facing in 2007.

“We have a number of different weeds that have shown resistance to different herbicides in North Carolina,” says Alan York, long-time North Carolina State University weed specialist. “If growers had not had glyphosate, we would have more resistance problems, because glyphosate took the place of a lot of other herbicides,” York notes.

York says Roundup Ready systems are here to stay, but the days of using glyphosate only weed control are gone. Likewise gone are the ease of application and the relatively low cost of weed control, he contends.

In the upper Southeast 2006 was pivotal year in documenting weed resistance problems. Although glyphosate resistant pigweed is the biggest problem, it is far from the only one cotton growers will face in 2007.

Horseweed resistance was first found in North Carolina in 2003, and it continues to be a problem. “Each year we find a little bit more — it is wide-spread all up and down the Coastal Plain of North Carolina,” says York.

The problem with horseweed, he says, is that it has an extended period of germination. It is a winter annual and begins germinating in September. It will germinate until cold weather comes. When temperatures warm up, horseweed will germinate again.

Glyphosate resistant horseweed was first found in Tennessee and growers there commonly use a dicamba burn down material tank-mixed with either glyphosate or paraquat.

North Carolina growers have an aversion to using dicamba products that goes back to wide-spread production of tobacco. Dicamba and tobacco shouldn't even be said in the same sentence, York jokes.

In North Carolina, weed scientists are recommending 2,4-D as part of a burn down program. A pint and a half will probably be needed in early March. Because horseweed has multiple germination times, a residual is needed, otherwise any delay between burn down and planting will likely result in a new flush of horseweed.

York says growers should choose their burn down and residual herbicides wisely, if horseweed is the main target. For example, he says Valor, has no post-emergence activity on horseweed, so 2,4-D or some other burndown material will be needed.

Valor does have excellent residual activity on horseweed and should get a grower into the cotton season weed free, if tank-mixed with a burn-down material.

“If you think you have glyphosate resistant horseweed and you don't have a residual in the mix, you can come back at planting time with a residual, with Cotoran being a logical choice.

“Ideally, if there are not any small horseweed plants up at planting time, Cotoran alone is a good choice. Otherwise, you will need paraquat mixed in with Cotoran,” York says. In reality, few people are going to check for small horseweed at planting, so a good option is to go ahead and add paraquat and crop oil to the Cotoran, York explains.

On a warm day, 60-70 degrees F, Gramoxone, crop oil and Direx will do a good job on horseweed. On a really warm day, 70 degrees F or higher, Ignite at high rates will do a good job. On a cold day, Ignite will not do a good job, according to York.

He stressed that growers remember that in fields with glyphosate resistant horseweed, once cotton comes up, there are few options. Ignite under a hood is about the only option left. If Liberty Link cotton ever becomes popular, Liberty in the crop will do a good job on horseweed.

In 2006, glyphosate resistant common ragweed was reported in a handful of counties in North Carolina. “High rates of Weathermax uglied-up the terminal, but the ragweed didn't die,” York says. He explains that to document resistance, there has to be proof that the resistant trait is heritable.

Common ragweed is not likely to be a big problem for growers in the Carolinas and Virginia, according to York. The biggest problem may be in no-till or reduced-till systems which require a clean field to plant cotton.

In these systems the problem is what to use for burn-down. If ragweed is resistant to glyphosate, the options are limited to dicamba, paraquat and 2,4-D.

Cotoran or Direx should get growers to layby. If not, Envoke is an option. If growers recognize the problem of glyphosate resistant ragweed, it is manageable with timely applications of one or more of a number of herbicides, York says.

Researchers in South Carolina have found glyphosate resistant cocklebur, and are in the process of documenting for certain that it is resistant. Clemson researchers are still conducting greenhouse tests, but the evidence is strong that at least one cotton field in South Carolina has glyphosate resistant cocklebur.

Virginia Tech researchers are at a similar place in time in documenting glyphosate resistant lambsquarters. More of a problem in the upper end of the Southeast, lambsquarters, prior to the introduction of Roundup Ready technology, was a constant problem in a number of row crops.

Glyphosate devastated lambsquarters populations, so many of the herbicides used to target this weed have gone from the market or don't have cotton registration.

For both cocklebur and lambsquarters, arsenicals have been widely used for control. The EPA is actively pursuing removal of arsenicals for use in agriculture. Though other herbicides are available for use in cotton weed control, MSMA and DSMA were staple products prior to the onset of Roundup Ready varieties.

Italian ryegrass is almost certain to have resistance to glyphosate. Researchers in the Delta have not confirmed resistance, but it highly likely. Johnsonngrass and goosegrass have shown resistance to glyphosate outside the U.S.

Common water hemp, a Midwest version of Palmer amaranth, has been verified to be glyphosate resistant in Missouri.

In addition to glyphosate, cases of resistance have been documented worldwide to virtually every family of herbicides used in cotton.

By far pigweed is the most problematic of resistant problems. In 2005, North Carolina weed scientists sprayed seven fields with varying rates of glyphosate, and there were survivors in all the tests.

In the fall of 2006, York's research team took samples of Palmer amaranth, mostly at random, from fields from the Virginia line to the South Carolina line. The intent was to learn where in North Carolina pigweed resistance is a problem. Greenhouse trials from these seed showed varying rates of resistance.

In pigweed plants with low levels of resistance to glyphosate, a double rate of Weathermax stunted the plant's growth by approximately 50 percent. In pigweed plants with high resistance to glyphosate, the same rate of Weathermax had no affect.

The most common result from these tests was to have 20 pigweeds in a planting, spray them with a double rate of glyphosate and 19 of the treated plants die. The survivor showed little or no stunting from the herbicide. “What that tells me is that we have mixed populations of resistant and non-resistant plants. If we don't do anything about the problem, it will get much worse the next year,” York says.

North Carolina grower Bill McGoogan was among the first to report glyphosate resistant pigweed. From a few isolated resistant plants in 2005, McGoogan says roughly 25 percent of his farm now has some resistant plants.

“The first thing most growers turn to when they learn they have glyphosate resistant pigweed is Staple,” York contends. The problem he points out is that Staple is an ALS inhibitor and this family of herbicides is already known to have resistance problems with pigweed.

“In North Carolina, we knew we had some ALS resistant pigweed. Now we have good evidence that we have pigweed with resistance to two herbicides with totally different modes of action. Multiple resistance is bad news,” says York. In this case the two modes of action are glyphosate and ALS inhibitors.

A glyphosate resistant management program in areas of the upper Southeast where documented cases of resistance have been demonstrated is critical for 2007. If left unmanaged, any combination of horseweed, ragweed, lambsquarters, and especially pigweed can take over a cotton field.

“We have been preaching resistance management programs for the past year. Some growers have taken us seriously, but many haven't,” York notes. “In 2006, we got a better idea of the scope of resistance problems, and it really got our attention. Growers who think they have resistance problems will have a number of management options to look at for 2007,” the North Carolina weed scientist says.

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