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Virginia soybean growers warned about pigweedVirginia soybean growers warned about pigweed

Roy Roberson 2

June 18, 2008

6 Min Read

Ten dollar a bushel soybeans are a real good reason to avoid Palmer amaranth, commonly referred to as Palmer pigweed. Profit and pigweed simply don’t go well together.

Virginia Soybean Specialist David Holshouser tells his growers in a recent newsletter, “I am not crying wolf.”

Glyphosate resistant pigweed has been documented just across the border in North Carolina, and the problem is headed north, unless Virginia growers take quick and decisive steps to prevent it.

Glyphosate resistance problems have been well documented in North Carolina and Georgia, but may be a bigger, yet under-reported problem in South Carolina. Big increases in wheat acreage and the need for a quick and reliable burndown of wheat point to a possible increase in glyphosate usage in all the Southeastern states.

Gibson, N.C., grower T.G. Gibson says pigweed, especially the resistant types, are a problem no farmer needs. Despite being technologically in-tune with farming and keenly aware of potential production problems, Gibson says a high percentage of his 3,000 acre farming operation has at least some glyphosate resistant pigweed.

“We diligently washed and cleaned equipment when we came from a field we knew we had some resistant weeds — before going to fields where we didn’t think we had the problem. Despite everything we could do the first couple of years, the problem got bigger and bigger he says.”

“First, we noticed individual weeds that we knew were sprayed, but we couldn’t control. Then, these isolated weeds grew in small, oval-shaped groups with no noticeable pattern of movement from field to field. Getting a handle on the problem has been difficult at best,” Gibson adds.

A few miles away in Rowland, N.C., Roger Oxendine admits he didn’t do a good job of cleaning equipment and is sure he helped spread glyphosate resistant weeds around his 8,500 acre farming operation. “Now, we are managing the problem, but when it first started we really didn’t know the risk we were taking by not getting all the weeds and weed seeds out of a field,” he says.

Further south in Bishopville, S.C., veteran farmer William McElveen says rotating chemicals has helped him manage the problem, but contends the best thing he has done is to rotate crops that help him better manage which family of herbicides he uses.

The hard lessons learned by growers to the south could prove to be valuable warnings for Virginia growers. So far, glyphosate resistant pigweed has not been documented there. Keeping it that way is an urgent goal for Holshouser and other soybean researchers and specialists at Virginia Tech.

“We can no longer rely on the cheapest and easiest control strategies for soybean weeds. To prevent, or even slow down the development of herbicide resistance, especially glyphosate resistant pigweed, weed management must quickly become a carefully planned and integrated program that preserves herbicide and genetic technologies,” he says.

The Virginia Tech Specialist says growers should look to the future to prevent Palmer pigweed resistance problems. Multiple tactics are needed to avoid the kind of problems farmers to the south are facing with resistant Palmer pigweed.

Included among these multiple tactics are:

• Cleaning tillage and harvest equipment. Pollen and wind can and do spread resistance, but the quickest way to get into trouble with weed resistance is to spread resistant seed from field to field.

• Diversifying in-season herbicides to incorporate different modes of actions. Simply changing trade names is not adequate, nor is rotating crops and using the same family of herbicides on both crops.

• Closely monitoring fields. Resistance starts with a single plant. Typically, in the early stages, glyphosate resistant pigweed numbers are small enough to allow growers to manually remove these plants from a field. The old adage about the cost of prevention versus cure clearly applies here.

• Completely control resistant-prone weeds. The best results have come in programs that provide some means of killing pigweed when the plants are very small, less than three inches tall. Once these fast-growing weeds (resistant or not) get more than 7-8 inches tall, there are few alternatives for control.

• Remove weed escapes before seed production begins. One mature Palmer pigweed can produce 500,000 seed. If the odds of resistance forming are one in a million, it only takes two plants to produce enough seed to meet the odds. Up the odds to one in a billion, and it only takes 20 mature pigweed plants.

“You don’t want to combine beans or pick cotton in a field that contains even a few mature palmer pigweed,” North Carolina grower T.G. Gibson says.

“A few six-foot tall pigweed that are 6-8 inches in diameter at the base of the plant is just not something you want to deal with at harvest time,” he adds.

To avoid these kinds of problems, Holshouser suggests Virginia growers develop a 3-5 year weed management program that manages weed seed production. Such a program should combine cultural, biological and chemical control strategies, he says.

When growers make changes in cultural practices, like going to strip-till or no-till systems, subsequent changes should be made in weed management strategies. For example, he says, no-till systems over long periods of time may result in increased problems with grass and small-seeded broadleaf weeds. Growers, he says, should anticipate changes in weed spectrums when significant changes in cultural practices are made.

A recent study at the University of Georgia indicates use of heavy winter cover crops may increase control of glyphosate resistant Palmer pigweed by 40-50 percent. Growers should look for opportunities to take advantage of naturally-occurring means of reducing the risk of herbicide resistance.

Taking care of glyphosate resistant pigweed may also prevent another potentially devastating problem for growers in the Southeast — Italian ryegrass. Ryegrass resistance to other herbicides is already a problem in some parts of North Carolina and South Carolina and will no doubt cause some yield losses when growers begin harvesting this year’s wheat crop.

Wheat yields in the Southeast are more competitive with Midwest yields than are soybeans and corn grown without irrigation. With a renewed interest in barley coming from upcoming ethanol plants fueled with barley instead of corn, the opportunity for Southeastern growers to grow winter crops is clearly optimistic.

The other side of that coin is the need to be more careful in what burndown materials are used and how weeds known to be resistant to a family, or families of herbicides are used from one crop to the next.

Soybeans and wheat or soybeans and barley in a double-crop system are a natural combination for Virginia growers. Keeping as many herbicides as possible available to manage resistance will likely a play a big role in how long these grain crops remain profitable in the upper Southeast.

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