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While February ended on a cold note, we had a preview of spring with a few days of warm weather.
Spring will arrive soon, and many are getting ready to plant. Spring also means cover crop termination, which may be challenging given the low inventory of glyphosate (Roundup). Therefore, there may need to be some prioritization of glyphosate supplies in case things are tight.
Glyphosate is the most effective burndown herbicide for winter grains such as wheat, triticale and cereal rye.
A 0.75 – 1.0 lb acid equivalent per acre rate with appropriate adjuvants (surfactant + AMS) generally provides good control. Optimal conditions for termination are several sunny and warm days (warmer than 55 degrees) where night temperatures exceed 40 degrees. To avoid product antagonism, do not tank-mix glyphosate with liquid N fertilizer, such as UAN or clay-based herbicide formulations, which include some atrazine and metribuzin products.
Paraquat (Gramoxone)-based burndown programs can be an alternative to glyphosate, but they may be less consistent across all plant growth stages.
Tank-mixing paraquat with a triazine herbicide, such as atrazine or metribuzin is preferred as it will improve burndown activity. Carrier volumes greater than 20 gpa and flat fan nozzles are essential for achieving adequate coverage, particularly in thick cover crops.
Glufosinate (Liberty) and Group 1 grass herbicides, such as Select and Assure are unlikely to be effective burndown alternatives for controlling winter grains, especially if they are to be applied in cooler temperatures.
If faced with limited glyphosate supplies in soybeans, it would be best to use glyphosate in the pre-plant burndown, following by a timely post-emergence application of a Group 1 herbicide for controlling small summer annual grass weeds.
Corn herbicides that have activity on small summer annual grass weeds, such as Accent and Basis Blend and Group 27 products such as Impact, Shieldex and Laudis will not provide good control on winter grains that survive a burndown pass. Therefore, optimize glyphosate or paraquat burndown programs to control winter grains if glyphosate is in short supply.
Chopping winter cereals prior to flowering is generally not an effective method of termination and spraying winter cereals with glyphosate and paraquat prior to forage harvest is illegal. Therefore, terminate winter cereals harvested for forage with a post-harvest herbicide application. Glyphosate is generally more effective than paraquat for post-harvest termination of winter cereals. It can be applied shortly after harvest if there is enough green leaf tissue at the base of plants to absorb the herbicide and temperatures are above 55 degrees. Temperatures below 50 degrees will slow the activity of glyphosate and may reduce herbicide efficacy. If using paraquat, it is best to delay termination until adequate regrowth is present.
By now, I hope that no one will be surprised if their herbicide of choice is hard to find. Fortunately, given a little tweaking and a different arsenal of products, there are still options for getting things done.
Zach Larson is the Penn State Extension agronomy educator for Blair County. He can be reached at 814-414-0582.