Herbicide options for burndown of existing weeds prior to planting of no-till wheat include glyphosate, Ignite, Gramoxone, Sharpen, and dicamba. Dicamba labels have the following restriction on preplant applications – “Allow 10 days between application and planting for each 0.25 lb ai/A used”. A rate of 0.5 lb ai/A would therefore need to be applied at least 20 days before planting. We have, as usual, been receiving questions about the safety and legality of 2,4-D use prior to wheat planting. We do not know of any 2,4-D product label that supports this use of 2,4-D. There is some risk of stand reduction and injury to wheat from preplant applications of 2,4-D.
The primary targets for a preplant burndown in wheat are the small, emerged winter annual weeds that can overwinter and have a negative effect on wheat the following spring. This includes marestail (horseweed), chickweed, deadnettle, annual bluegrass, mustards, etc. Herbicide treatments at this time can also have considerable activity on biennials (wild carrot, wild hemlock), dandelion, and Canada thistle. Although, herbicides are often more effective on these weeds later in the fall. The larger summer annual weeds (ragweeds, marestail, foxtails, etc) are going to die after the first hard frost, and soybean harvest decimates these weeds to the point that herbicides won’t be effective on them anyway. Where wheat is planted into a fallow situation, it may be necessary to target the large summer annuals with herbicide in order to ensure that they do not interfere with planting or wheat stand establishment.
While glyphosate can adequately control small winter annual weeds, it should be combined with Sharpen or dicamba in fields with a history of marestail problems (or in fields downwind of a neighbor’s marestail nightmare). Due to the late soybean harvest, glyphosate and Sharpen may be the better alternative because of the 10-day waiting period for dicamba. Ignite or Gramoxone should also effectively control seedlings of marestail and other winter annuals. Be sure to use the appropriate adjuvants with any of these, and increase spray volume to 15 to 20 gpa to ensure adequate coverage.
This year’s late-season leaf canopy of soybeans may be suppressing winter annuals and reducing populations that develop into late September, compared with years where soybeans have lost leaves already. This may reduce the need for a burndown application in wheat, although often a field that appears to be weedfree from a distance does actually have a lot of small weeds upon closer examination. There are several effective postemergence herbicide treatments for wheat that can be applied in November to control these weeds, in fields where preplant burndown treatments are not used. Effective postemergence treatments include Huskie or mixtures of dicamba with Peak, tribenuron, or a tribenuron/thifensulfuron premix with dicamba, among others. Huskie may be the most effective fall postemergence treatment for control of marestail, where the marestail population is resistant to ALS inhibitors. We discourage application of 2,4-D to emerged wheat in the fall due to the risk of injury and yield reduction.