Glyphosate resistant weeds compound drought damage

Despite the best efforts of scientists, farm commodity groups and virtually everyone associated with agriculture in the Southeast, the spread of glyphosate resistant weeds, particularly Palmer amaranth, continues to devastate cotton and soybean production in relatively small pockets of land from southern Virginia to southern Georgia.

Combining one of the worst droughts on record and the continued movement of herbicide resistant plants proved to be a bad duo for Southeastern farmers. Though still sporadic enough to have only minimal impact on overall production, in areas where resistance is widespread, crop yields plummeted across the region.

North Carolina State University Professor and recognized expert on glyphosate resistant pigweed, Alan York, says Palmer amaranth has a number of characteristics that make it an “ideal” weed. “There are some five dollar terms for this, but basically Palmer amaranth is a tough critter,” York says.

One characteristic is that Palmer pigweed is able to deal with high temperatures much better than most crops. It can continue to photosynthesize well at temperatures where crops are shutting down, York explains.

Similarly, Palmer pigweed appears to be quite efficient in water usage. Obviously it will grow better with good moisture, but it can handle drought better than crops.

Despite the advantages of pigweed versus cotton and soybeans, York says “overall, I think the resistant Palmer amaranth, and Palmer amaranth in general, was somewhat less severe this past summer than in 2006. That is based on general observations as I traveled over North Carolina. I can't separate effects of weather from potentially better management programs by the growers; I expect both were involved,” he adds.

“The biggest thing I noticed was that continued germination throughout the 2007 season (which is one factor making Palmer hard to deal with) was less this year, and I attribute that to the bone dry conditions,” York says.

Since it was first documented in August 2005 in cotton and soybeans in Georgia, recognition of the problem has spread across Dixie. Whether that is due to actual increase in acreage of glyphosate resistant Palmer amaranth or to better recognition of the problem is not certain. One thing for certain neither cotton nor soybeans can compete with Palmer pigweed.

Though the glyphosate-Palmer pigweed problems are the most publicized, these aren't the only ones farmers need to consider for 2008. There are a number of plants linked to herbicide resistance and all take one of two routes to make plants resistant to a specific herbicide:

  • The genetically engineered plant produces a new protein which detoxifies the herbicide.
  • The protein in the plant which is normally the target of the herbicide's action is replaced by a new protein which is unaffected by the herbicide.

Though science is progressing rapidly to develop new herbicide resistant crops, there are four combinations currently in widescale use worldwide:

  • Glyphosate used on corn soybeans, cotton, canola and sugarbeets.
  • Glufosinate used on corn soybeans, cotton, canola, rice and sugarbeets.
  • Bromoxymil used on cotton.
  • Sulfonylurea used on cotton and flax.

The problems growers are facing with glyphosate resistance, primarily on pigweed, but to a lesser degree on lambsquarters, cocklebur and marestail should not be a surprise considering the high percentage of Roundup Ready varieties planted in the Southeast. In North Carolina alone, over 95 percent of cotton and soybean acreage and over 50 percent of corn planted in 2007 were Roundup Ready varieties.

The continued growth of glyphosate resistance in Palmer amaranth is a complex problem for growers, but its spread can be described as a simple numbers game. If the odds for a 40 acre field of cotton that is treated year after year with glyphosate to develop resistance is one in a million, it takes two or three Palmer pigweeds to produce a million seeds. If the odds are a billion to one is takes 20-25 mature Palmer pigweeds to produce a billion seeds.

Predicting where resistance will and won't occur is difficult according to York. However, he says growers seem to be taking the problem seriously. “I have nothing definitive to back me up (no surveys, etc), but based on comments from growers, I think many of them are taking the threat seriously. Overall, I think cotton growers are doing so more than soybean growers. Most of the cotton growers with problems are trying to get some additional chemistry in the system. Some are doing a little, some are doing a lot more,” he says.

“The primary thing I noticed soybean growers doing this year was spraying Harmony GT after the glyphosate didn't work. In most cases, the Harmony GT worked pretty well, but we need to keep in mind that Harmony GT is an ALS inhibitor. We already have a fair amount of ALS resistance in Palmer, and continued reliance on ALS inhibitors is simply going to select for more resistance. Where we do not currently have ALS resistance, Harmony GT is a good way to salvage a bean field, but it is not a long-term solution.

The increase in corn acreage in the upper Southeast (40 percent increase in North Carolina) may bode well for cleaning up some acres infested with glyphosate resistant Palmer amaranth. Palmer is generally easier to deal with in corn than soybeans, and certainly easier than in cotton, according to York.

“Palmer amaranth will give you a new appreciation for atrazine” York says. “I noticed some pretty nasty corn fields scattered around, but overall, not a high percentage were overrun with Palmer” he adds.

York contends planting corn and following a good program in corn would help growers hold down some of the Palmer amaranth but, unfortunately, the worst problems with Palmer are on the lighter, sandier soils where corn does not fit.

Researchers throughout the Southeast continue to monitor glyphosate resistance in cotton and soybeans and most agree the key to managing the spread of the problem is to rotate chemistry. Glyphosate resistance is not a problem that strikes cotton or soybean fields in a vacuum, it is a cumulative problem that occurs over many seasons, over many crops and over a continued use of a target specific herbicide.

A number of herbicide resistance management herbicide programs are available to cotton and soybean growers. With a projected increase in soybean acreage in the Southeast in 2008, it is particularly important for growers to know the family of chemical herbicides used in all their crops and to rotate these, regardless of the crop being grown over a period of several years.

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