Farmers in the Southeast took advantage of low global stocks of soybeans and subsequently good prices to plant more beans in 2009. Similar planting levels, possibly up somewhat, are expected for 2010.

For many areas of the Southeast, including soybean production areas throughout the Carolinas and into Virginia, herbicide resistant weeds continue to be an ongoing problem that weed scientists project will get worse unless farmers begin to make management decisions geared toward solving the problem.

Timeliness has never gone out of vogue in weed control, but it will become even more important in 2010 as growers see more and weed species with tolerance to multiple herbicides.

According to North Carolina State Agronomist David Jordan, the timeliness issue is critical in the Tar Heel State as more and more growers revert to older chemistry to fight weeds resistant to glyphosate, ALS and PPO chemistry.

“When we first started using glyphosate-based systems a decade or so ago there was a lot of flexibility on how big you could let a weed get before treating it. Glyphosate resistance changed that window of opportunity and when having to move to some of the older chemistry and new chemistry, like Ignite, you have to be more timely,” Jordan says.

Palmer pigweed is a particular challenge, especially because of the ‘super weed’s’ ability to develop resistance to glyphosate.

If a grower lets pigweed get to 12-15 inches tall, it will be a real challenge for Ignite, any of the PPO inhibitors or any of the older ALS-inhibiting herbicides. Even pigweed at 8-10 inches tall is borderline for these materials. On the other hand, 3-4 inch tall pigweed are still manageable with Cobra, Blazer or some of the other PPO-based herbicide.

Once resistance is established in a field, it is very difficult to eliminate it. There are ways to manage the problem and reduce the risk of getting herbicide resistance, and the best strategy is to rotate chemistry.

In soybeans, Jordan conducted a series of tests to look at two ways of reducing the threat of resistance and managing it when it occurs.

The first way is to put out a pre-emergence or incorporated herbicide, depending on what crop is grown and how it is grown. Most folks don’t want to do this — it’s time consuming, expensive and depends a lot on getting moisture at the right time.

Jordan put together a test using the major labeled soil-applied herbicides — some of which were popular 12-15 years ago, but may have not been used much the past decade. With each of these herbicide systems, he came back with Flexstar, compared to glyphosate.

The other approach to get multiple modes of action is to tank-mix other herbicides in a glyphosate-based system. Despite the much publicized resistance problems now facing glyphosate herbicides, this chemistry is still very effective on a wide range of weeds.

In on-farm testing near Elizabeth City, N.C., the North Carolina State researcher got excellent weed control from several different herbicide systems.

Prowl at two pints per acre, applied June 20, followed by Flexstar at 1.25 pounds per acre applied on July 15 provided outstanding weed control.

Prefix, a combination of Reflex and Dual has become a popular treatment on soybeans in the upper Southeast. In Jordan’s on-farm test in northeast North Carolina, Prefix provided good weed control without Flexstar or glyphosate. Jordan says that under other growing conditions or soil types or weather conditions, the same treatment may not work so well. He urges growers to look at comprehensive statewide tests before making decision on which materials to use.

“In some cases you get good rainfall and good activation of these pre-treatments and get fast growing soybeans up quickly, you may not need post-applied herbicides. In other cases when conditions aren’t so ideal, you are going to need one or more of these materials,” Jordan cautions.

Authority is another material that did well in the North Carolina test. It was applied pre-emerge at 12 ounces per acre on June 20. This herbicide contains an old familiar herbicide — metrabuzin, plus a different, newer chemistry sulfentrazone.

Authority MTZ controls a variety of broadleaves that have exhibited resistance to glyphosate and/or ALS herbicides. In addition, when used as part of a fall burndown program, it provides residual control of a number of small-seeded broadleaves and winter annual weeds including henbit, chickweed, pigweed, marestail, lambsquarters and waterhemp, according to product manufacturer FMC.

Python and Sencor under the test conditions did a good job, but benefitted from a post-emergence application of Flexstar or glyphosate to clean up. Flumetsulam is the active ingredient in Python and metrabuzin in Sencor, so two different chemistries to fight resistance.

Dual-Magnum at a pint and a half per acre did not do well in the northeast North Carolina grower field and did not provide adequate pigweed control by itself. Valor at 2.2 ounces applied on June 20, plus Flexstar at 22 ounces per acre did a good job, but showed some crop damage.

Outlook, a chloroacetamide-containing herbicide, is another option that did well in Jordan’s on-farm test. Chloroacetamide herbicides such as Outlook control susceptible weeds by inhibiting the synthesis of very long-chain fatty acids in the plant, thus preventing root and shoot growth.

Typically, weed seedlings exposed to soil applications of Outlook die before or shortly after they emerge. Outlook, which is a popular herbicide choice in potato production, will not control weeds already emerged.

Envive also did well in the North Carolina test. It contains Valor, thirinuron and disulfuron. Some grass came through in the test, but pigweed control was good. Add glyphosate or Flexstar to the application and Envive provided excellent control of broadleaf weeds.

Envive is a combination of three slightly different modes of action. It is a spring — applied, pre-emergence, herbicide premix that includes chlorimuron (Classic), flumioxazin (Valor), and thifensulfuron (Harmony GT).

Envive is designed to provide some burndown and residual weed control when applied two weeks before planting up to three days after planting (before soybean emergence).

Intro, previously known as Lasso, looked good at three quarts per acre for pigweed control. There was not much need for Flexstar or glyphosate at this particular location in North Carolina.

Jordan says the choice of the older herbicides used in soybeans to diminish glyphosate resistance problems is not as important as using different modes of action. Knowing which of these materials will work best in your particular operation is critical to both optimum performance from the herbicide and optimum crop production.

“In North Carolina, we are beginning to see resistance in PPO inhibitors, like Reflex. Cobra and Blazer. We already have ALS resistance and glyphosate resistance. In some areas of the Midwest they are beginning to see quad resistance — or resistance to four different families of herbicides — in water hemp,” Jordan adds.

Knowing families of herbicides and modes of action of herbicides, not trade names, is going to be an ongoing challenge for soybean farmers in the Southeast in 2010 as they struggle to manage an ever-growing weed resistance program.

e-mail: rroberson@farmpress.com