Is Roundup Ready alfalfa "fool proof"?

Roundup Ready technology was a tool created several decades ago with the first crop, Roundup Ready soybeans, being released in 1996. The technology had been discussed years in advance, and is in wide-use today. There are many crops that now have Roundup Ready technology, including alfalfa (the primary focus of this article, I'll call it RRA), canola, corn, cotton, sugar beets and potentially even bentgrass.

The major draw of this technology is the flexibility it provides. Producers are able to spray when necessary based on weed invasion, and they have what appears to be a "foolproof" plan for in-crop weed control.

However, in the case of alfalfa, it has recently been discovered that it may not be as error-proof as previously thought.

I was fortunate to sit in a meeting last year and hear the story of the discovery of glyphosate injury in RRA. The story goes that in 2014 researchers at University of California, Davis received a call from a producer to come look at his alfalfa field. He had sprayed his alfalfa that spring with glyphosate and noticed that the plants under his wheel-line, which didn't get sprayed, looked much healthier than the plants that did.

After several research projects, and what seemed like some frustration and ingenuity, they think they hit on the problem: spraying Roundup on RRA in the spring, when there is risk of immediate frost, may actually damage the alfalfa. They found that if frost occurred within two weeks after spraying, the alfalfa appeared stunted and chlorotic, and the upper stems actually had a "shepherd's hook" appearance, with the top part of the plant bending.

The symptoms usually appear within 10 days to two weeks after glyphosate application. In some of the worst fields, they were seeing losses of approximately 0.8 tons per acre. The injury only appeared during the first harvest, and by the second harvest there was no difference between sprayed and unsprayed plots.

The combination of glyphosate application, cold temperatures and alfalfa maturity may not be observed very often, lending more reasons as to why this may occur in only some areas.

From this, researchers are recommending that if spraying glyphosate on RRA in the spring, make sure that either the alfalfa is less than two inches tall at spraying, or there is no risk of frost in the near future. It is ideal to spray when the alfalfa is short, as you usually get a better kill on the weeds, which is the ultimate goal.

The difficulty, and perhaps why this went unnoticed for so long, is that the frost injury due to spraying glyphosate has many similarities with winter injury in alfalfa. Now I am much more careful when evaluating fields for winter injury and ask when glyphosate was applied to determine what may be the true cause of spring regrowth problems. In talking with producers, several have thought they may have been dealing with winter injury, and after discussing this, herbicide injury may be a possibility.

The moral of the story is, that even with this amazing technology available, we still have to take care to manage our fields properly as there may still be limitations. With the increased cost of planting RRA compared to conventional alfalfa, we need to protect those plants at all costs.

Another thing to consider is that even using RRA, we still have to remember other parts of integrated pest management, including alternative methods of weed control and rotating herbicide modes of action when possible. It also shows how producers and researchers can work closely together to figure out in-field issues and how important collaborations can really be.

For more information on the evaluation of herbicide injury in RRA, visit the UC Davis Alfalfa page, or feel free to contact Emily Glunk for more information.

Editor's note: Emily Glunk is an Extension Forage Specialist at Montana State University. Her website is animalrangeextension.montana.edu/forage/ and she can be reached at emily.glunk@montana.edu or 406-994-5688.