Roundup Ready alfalfa promises weed-free hay in Southwest fields

Texas Agricultural Experiment Station scientists here recently evaluated several lines of new, transgenic alfalfa that are tolerant to the widely used, broad-spectrum herbicide glyphosate, the active ingredient in Roundup.

“This technology, similar to that used in Roundup Ready corn, soybean, canola, and cotton, is proving effective in overcoming difficult-to-control weeds in alfalfa,” said Vincent Haby, Experiment Station soils scientist based at the Texas A&M University System Agricultural Research and Extension Center at Overton.

For the last decade, Haby has been working to make alfalfa a viable crop for the acid soil conditions of East Texas. Haby's work has shown alfalfa could yield a return on investment as high as $300 per acre, higher than practically any other agronomic crop in East Texas.

The largest cost of producing alfalfa is in its establishment, and the gradual thinning and invasion of stands by weeds and grasses can reduce stand life to four or five years. A combination of herbicides may be required for control.

The genetic modification of alfalfa makes it tolerant to the glyphosate in Roundup, potentially making weed and grass control simpler and cheaper.

Haby's work is in its third year. Under a cooperative agreement with Monsanto Chemical Co., the developer of the Roundup Ready technology, Haby and research associate Allen Leonard first tested 17 varieties, recording yield and other characteristics.

From Haby and Leonard's data, as well as that from other researchers around the country, Monsanto selected several promising lines. Last year, the research team performed tolerance level tests on a single line. The idea was to examine in detail how different rates and number of Roundup applications would control weeds without any damaging effect on the growing alfalfa. This year, the tests are specifically targeting control of bermudagrass in alfalfa stands.

Bermudagrass question

In East Texas, it's not just broadleaf weeds that threaten an alfalfa stand, it's also grasses such ryegrass and encroaching bermudagrass. There's some question whether glyphosate will kill bermudagrass with any reliability at the proposed labeled rates for weed control in Roundup Ready Alfalfa. And the proposed labeled rates are not likely to be increased because residual levels of glyphosate cannot be allowed to exceed those set by the EPA.

Under current law, genetically modified crops are subject to strict review by several federal agencies. The U.S. Department of Agriculture oversees seed shipping and field-testing. The U.S. Food and Drug Administration evaluates food and animal feed safety issues. The Environmental Protection Agency establishes, among other things, residue tolerances for food and feed crop commodities.

In the case of the new alfalfas, EPA has established a level for residual glyphosate in hay. To avoid any problem with residuals, the EPA labeled rates are no more than 1.5 pounds acid equivalent in a single application per harvest, and no more than four such applications during a growing season.

“According to Monsanto, residue data currently under review at EPA indicate that glyphosate residues in Roundup Ready alfalfa made into hay following the proposed maximum application rates would not exceed the established tolerance,” Haby said.

In response to these issues, Haby and Leonard are trying various timings of Roundup with an eye on bermudagrass control. They are also testing another grass herbicide, Select, by itself and in conjunction with Roundup. In another strategy, they are examining the effect of adding 2 percent ammonium sulfate fertilizer to increase herbicide activity. It's a proven technique, but the question is whether the boost of control will be enough to kill the bermudagrass.

Timing difference

Timing of the application may also make a difference.

If the herbicide application is made too soon after a harvest, the bermudagrass won't have enough leaf area to take up the herbicide systemically. If the application is made too late, the alfalfa will form a canopy and shade the bermudagrass from the spray, Leonard explained.

Results from the current tests should be available early next year, but it's likely to be several years before glyphosate tolerant alfalfas reach the marketplace, Haby said.

“Before this technology is released, test results must undergo an extensive regulatory review,” he said.

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