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Trials probe Roundup Ready alfalfa

Alfalfa will be the next herbicide resistant crop for California, following cotton and corn.

A team of University of California researchers is getting the first field look at Roundup Ready alfalfa, and there are production similarities to cotton and corn. However, there is a dramatic difference.

How do you take out an alfalfa stand once its commercial viability is diminished? That is usually three to four years in the central and southern part of the state. It can be longer in the state's shorter growing seasons where there are few annual cuttings.

You cannot kill Roundup-resistant alfalfa with Roundup.

University of California Fresno County Farm Advisor Kurt Hembree said the herbicides 2-4D Amine and Clarity followed by disking will successfully take out Roundup-resistant alfalfa, based on his trials at Kearney. None alone would do the job, according to Hembree.

It may be three or four years before Roundup-resistant alfalfa is commercially available in the state. A team of UC researchers has been given herbicide-resistant seed by Monsanto to evaluate ahead of commercial release in the state's four alfalfa production regions.

Target time

And as with herbicide-resistant cotton and corn, the earlier weeds can be treated with Roundup, the better the results, according to Hembree.

“At the three to four leaf trifoliate stage seems to be the target time to get good weed control,” Hembree told producers at the recent alfalfa meeting at Kearney Ag Center, Parlier, Calif.

There is sufficient weed growth at that alfalfa growth stage to get effective weed control, yet the alfalfa has not reached a height that the plants shade out the weeds and reduce herbicide efficacy.

Also, at the seven to nine trifoliate leaf stage, Hembree said an application shuts down the alfalfa's growth. It does not kill the mature growth, but it definitely stops growing.

Growers can expect some stand loss from applying Roundup in herbicide-resistant stands because seed lots may not be 100 percent herbicide resistant. Hembree said the experimental seed received by UC researchers had stand losses of up to 12 percent from a herbicide application over a 25-pound per acre seeding rate. However, he added, Monsanto expects to bring that potential stand loss down to 3 to 4 percent in commercial seed lots.

Weed species shift?

Hembree said Roundup-resistant alfalfa would benefit growers in helping them control weeds like nutsedge and winter annuals. However, the flip side of these herbicide-resistant crops is that there could be a shift in weed species and development of weed resistance, warned Hembree.

“There are weeds Roundup is weak on — nettles, cheeseweed and filaree — but a tank mix of Roundup and Pursuit could be a good combination when those weeds are present,” Hembree said.

The other major agronomic issue is isolation. Herbicide-resistant alfalfa must be buffered away from seed fields to preclude cross-pollination.

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