Winter is the worst time for weeds in alfalfa and understandably so. Winter is when alfalfa plants go nearly dormant in most of the country, growing extremely slow unable to provide enough competition for weeds. According to Earl Creech an Extension agronomist at Utah State University there’s three new herbicides to consider using this winter. “The very most important thing to control weeds in alfalfa is to promote a dense, vigorous stand,” Creech says. “If you do that you’ll be dollars ahead.” If you still need herbicide assistance to keep weeds under control, he recommends looking at these three options:
Gramoxone (paraquat) – This post emergence herbicide can be applied between cuttings. “The action is very quick,” Creech says. “Often visible in a matter of hours.” The chemical becomes inactive once it comes in contact with soil so there’s no residual benefit. According to Dwight Lingenfelter with Penn State Extension, Gramoxone is effective at killing chickweed, henbit, deadnettle, and mustard species. There are two main concerns with this herbicide: resistance and toxicity. According to Creech there are cases of resistance if you use this product for too many years in a row. In addition, it’s a restricted use herbicide and is orally toxic. “This one makes me the most nervous because it’s so dangerous,” he says. Sharpen (saflufenacil, Group 14) – This chemical is absorbed by the roots and the leaves, and is labeled for use in dormant alfalfa only. It will not control grasses, it’s a broadleaf product and Lingenfelter says it effectively manages chickweed, henbit, and marestail. However, it does have some pre-emergence activity on some weeds. Sharpen has to be applied 90 days before the first cutting in northern areas, but in Southern areas it can be applied at 75 days before the first cutting. Also, it can be applied to alfalfa mix fields. According to Creech it will kill some weeds that Roundup struggles with. Shark/Aim (carfentrazone, Group 14) – This post-emergence herbicide can be applied when alfalfa is dormant or between cuttings. According to Lingenfelter, it works best to tackle broadleaves such as chickweed, mustards, lambsquarters, velvetleaf, pigweed, and Star-of-Bethlehem. “There’s a little alfalfa injury when it’s applied,” Creech explains. The chemical doesn’t control grasses and fields shouldn’t be irrigated immediately before or after applying the chemical to avoid damage to alfalfa. Data from North Dakota State University shows the costs of these chemicals on a per acre is almost identical. “We can base our choice on performance and the weeds we’re trying to control and not cost,” he says.