By Greg LaBarge, CCA, Ohio State University Extension
What do marestail, cressleaf groundsel, purple deadnettle, wild carrot, birdsrape mustard, poison hemlock, dandelion, annual bluegrass, and Canada thistle have in common? These winter annual, biennial and perennial weeds are effectively controlled with fall herbicide applications. So before you pack the sprayer away in the barn, check for these weeds in your just harvested corn or soybean, emerging wheat, and pasture or hay fields.
Commonly asked questions about fall herbicides are how late in the fall can herbicides be applied? At what point is it too cold to apply? Dr. Loux has applied well into December under some very cold conditions and still obtained effective control of winter annuals. He suggests applying before Thanksgiving and aiming for a stretch of warmer weather if possible, but the effective treatments should work regardless. After extended periods of freezing weather, perennials such as dandelion, thistle, and dock shut down, resulting in reduced control.
For 2023 corn and soybean fields, don’t make it too complicated or pricey. Keep in mind that the primary goal is to control emerged weeds. No single herbicide will get all species, but several relatively low cost two-way mixtures work. Dr. Loux’s philosophy is to start with 2,4-D and then add another herbicide for more comprehensive control. Herbicides that make the most sense to add to 2,4-D based on his research: glyphosate, dicamba, metribuzin, simazine, Basis (and generic equivalents), Express (and generic equivalents), Canopy/Cloak DF or EX, or Autumn Super. These options allow corn or soybeans to be planted the following year with these exceptions: simazine — corn next year; Canopy/Cloak — soybeans next year; Basis – possibly restricted to corn based on rate and geography.
Generally, he does not see the need for three-way mixtures, but a low rate of glyphosate added to a two-way mix to target grass or improve activity on perennials is justified. A two-way combination of glyphosate and Sharpen could also be used, but he sees Sharpen as having more utility in marestail control programs when used in the spring.
Wheat fields not treated with burndown herbicides at planting may also be subject to infestation with winter annuals and dandelion. Several effective postemergence herbicide treatments for wheat can be applied in November. Fall-applied herbicides can control these weeds (especially dandelion) more effectively than spring-applied, with less risk of crop injury. The most effective postemergence treatments include Huskie, Quelex, or mixtures of dicamba with either Peak, tribenuron (Express, etc.), or a tribenuron/thifensulfuron premix (Harmony Xtra, etc.). He discourages the application of 2,4-D to emerged wheat in the fall due to the risk of injury and yield reduction.
Cressleaf groundsel in hay or silage is toxic to animals, and infested areas of the field should not be harvested and fed. The weed becomes evident in hay fields when it becomes taller than the alfalfa/grass and develops bright yellow flowers in May. The solution is scouting hay fields in fall and early spring to determine the presence of cressleaf groundsel. Then, spray plants when they are small and still susceptible. Expect groundsel to be more of a problem in new August seedings since it would be emerging with the new stand of alfalfa/grass. In alfalfa, dormant applications of Metribuzin or Velpar, a fall or early spring application of Pursuit or Glyphosate applied to Roundup Ready alfalfa varieties or as a spot spray are options.
Control Cressleaf in pure grass stands gives you more herbicide options that are more effective. Any treatment containing 2,4-D should be effective in grass hay or pasture. Apply 2,4-D (1 quart per acre) in late October or early November. Low-volatile ester formulations can be more effective than amine formulations, but the latter is less likely to volatilize and injure nearby sensitive broadleaf vegetation.
For weed identification information, videos, and more specific recommendations for fall weed control, see https://go.osu.edu/weedfall.