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Herbicide-resistant weeds plague cotton, peanut growers

The spread of ALS-resistant, dinitroanalin resistant and glyphosate resistant weeds is dealing more misery to peanut and cotton growers throughout the South in 2007.

Decreases in cotton acreage in the South seem to have heightened the spread of resistance problems with a number of ALS-inhibiting herbicides in peanuts.

The return to dinotroanalin, or DNA or yellow herbicides, used in part of avoid ALS resistance, also has exposed resistance to this family of older herbicides.

DNA-based herbicides, including Prowl, Sonolan and Treflan, have been dogged by a multitude of problems in corn in the Midwest for many years. Currently, six species of weeds, including Palmer amaranth in South Carolina, have documented resistance to DNAs.

In the lower Southeast thousands of acres of peanuts were planted well into June because of the intense drought across the region. In many cases delay in planting gave weeds a head start, forcing growers to rely heavily on herbicides that work well to control a broad spectrum of broadleaf weeds — unfortunately the product of choice is often acetolactate synthesis inhibiting herbicides, commonly called ALS-inhibitors.

University of Georgia Weed Scientist Eric Prostko, speaking at the recent Southern Peanut Growers Association meeting in Panama City Beach, Fla., urged growers to know more than the trade name of herbicides. It is critical, he says, to know the mode of action of different families of herbicides.

“With over 20 different ALS-inhibiting herbicides on the market, it is too easy for a grower to alternate chemicals with different trade names, thinking he is doing a good thing by rotating herbicides. If he uses seven different herbicides on four different crops and all the herbicides are ALS-inhibitors, he is greatly increasing the risk of resistance,” Prostko points out.

The newest family of herbicides, the protoporphyrinogen oxidase inhibitors, commonly called PPOs, like the ALS-inhibitors work well on a broad spectrum of weeds, are environmentally and human friendly. So far, the PPO products, including Reflex, Cobra, Valor, Ultra Blazer, Aim and Storm have not had resistance problems, but over-use is a good way to get there.

Growing peanuts in a no-till or reduced-till system has created some additional weed management challenges for growers. In many cases cover crops are used and the least expensive, most effective herbicide for burndown of cover crop stubble is glyphosate.

In cotton and soybean growing areas, with heavy use of Roundup Ready technology, the threat of developing pigweed, horsenettle, lambsquarters, ragweed and cocklebur resistance, or antagonizing an existing problem is significant.

Research conducted in 2006 by Stanley Culpepper at the University of Georgia and John Wilcut at North Carolina State University shows that weed competition and interference directly correlates with yield loss in cotton. Cotton yields are reduced exponentially when weeds are allowed to germinate and compete season-long, emphasizing the importance of residual herbicides and alternate modes of action to reduce the likelihood of weed escapes from ineffective applications of glyphosate alone.

Peanut weed research at North Carolina State University correlates with the cotton research findings.

Staying weed free for as little as 20 days after planting made a significant difference in peanut yields in studies conducted by North Carolina State researchers.

Results of these studies are sure to increase the use of pre-emerge herbicides in both peanuts and cotton. Which of these herbicides to use and what results to expect are critical factors in weed resistance management.

University of Georgia Graduate Student Andrew MacRae conducted tests in 2006 to determine the density of glyphosate-resistant Palmer amaranth required to reduce cotton yield, as well as the effect of establishment time on yield.

Six- to eight-leaf Palmer amaranth plants were hand-transplanted into plots, which had been maintained weed-free with pre-emerge and postemerge herbicides. The plants were cultivated from a field with known resistance and treated with glyphosate at 22 ounces two weeks prior to transplanting.

Treatments included several plant densities and multiple establishment times to mimic weed escape scenarios following herbicide application.

Results confirmed that Palmer amaranth establishment at the three- to eight-leaf stage of cotton significantly reduces yield. One plant per 20 feet of row resulted in a 6-7 percent yield loss, while losses skyrocketed when weed density was increased to 10 plants per 20 feet of row. At 10 plants, yield losses reached 60 to 68 percent.

Conversely, MacRae calculated the yield increase when weed establishment is delayed until cotton is more mature and less susceptible to weed competition. MacRae measured a three percent yield increase for every cotton leaf stage that Palmer amaranth establishment is delayed at densities of two, three and five plants per 20 feet of row.

In treatments with weed densities of 10 plants, the yield increase was further enhanced to 4.5 percent per leaf stage.

Tests in North Carolina, conducted by North Carolina State Weed Scientist John Wilcut showed an increase in cotton yield of nearly 700 pounds of cotton per acre when fields were kept weed free for six weeks after planting, compared to fields that remained weedy 2-10 weeks after planting.

“I recommend to all growers that they use residual herbicides at planting, whether they have confirmed resistance or not,” said Wilcut. “To overcome herbicide resistance, growers are going to have to overcome checkbook resistance. Although it may cost you more in inputs, incorporating alternate modes of action and residual herbicides into your weed management program will help you preserve yield. In most cases, the profit earned by preserving yield will offset the cost of herbicides.”

“I consider glyphosate-resistant Palmer amaranth to be the greatest threat to U.S. cotton production since the boll weevil,” said Wilcut. “We must manage as though we already have resistance to prevent the spread of this devastating weed.”

In herbicide trials, Wilcut found the best pre-emergence treatment for weed control is a tank mixture of Reflex and Prowl. Wilcut noted that Reflex should provide 100 percent control of emerged Palmer amaranth less than two inches in height, further demonstrating the importance of controlling weeds while they are small.

Prostko points out it will cost growers approximately $42 to control resistant pigweed, compared to $25 per acre to reduce the chance of developing resistance.

In North Carolina, Wilcut says, “Reflex is an important component of a Palmer amaranth control program, and is also effective on common ragweed, wild poinsettia and yellow nutsedge. “Growers can generally expect 4-6 weeks of residual control on ideal soils.”

Though pre-emerge-applied broadleaf herbicides give some control of troublesome weeds, Prostko contends the big advantage of using these materials is that they buy a grower time.

He adds that mechanical cultivation, despite more than double the cost of diesel fuel in the past years, is still an option.

Despite well documented problems with glyphosate resistant weeds, this herbicide continues to be among the most cost effective to growers in the Southeast, especially when tank-mixed with other materials in controlling non-resistant weeds.

In fields with documented glyphosate resistant pigweed Wilcut notes Dual Magnum, a biosynthesis inhibitor, when applied pre-emergence, can improve management of these weeds.

Biosynthesis inhibitors must be applied to glyphosate-resistant Palmer amaranth before emergence because just a few days can be important for season-long control, Wilcut says.

Research by the North Carolina State scientist has also shown biosynthesis inhibitors to be effective against goosegrass, crowfootgrass and crabgrass and will help reduce the likelihood of grass resistance, which Wilcut contends is probable in coming years.

There is little doubt that weed resistance to herbicides will continue to be a bigger problem each year for row crop farmers, unless they understand the different modes of action and follow documented resistance management programs developed by researchers, like Stanley Culpepper and Eric Prostko at the University of Georgia and John Wilcut and Alan York at North Carolina State, who have been working on the problem for a number of years.

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