Fall Herbicide Treatments and Cover Crops

By Mark Loux OSU Extension

There is still time to apply herbicides yet this fall. The frosts that are starting to occur have little effect on the weeds of concern – marestail, purple deadnettle, chickweed, etc. We have applied well into December with acceptable results. Fall treatments are a key component of marestail management programs, and it’s not necessary to spend a lot of money to get the desired result.
• This pertains to marestail infestations in grass cover crops also. Cereal rye can integrate well with herbicides to improve control of marestail, but the help the rye provides is variable. In our studies, the rye has at some sites provided enough additional control that fall herbicides are unnecessary, but there have also been sites where the rye contributed almost no control.
• So in fields with grass covers and a history of marestail, our recommendation would be to either treat the fields this fall yet, or scout thoroughly for the presence of marestail before making a decision. Keep in mind also when making this decision, an early-planted cover that is well-established, uniform, and relatively tall at this time of the year is likely to provide more control of marestail and other weeds than a later-planted cover that is variable in stand and still small.

For grass covers, we generally suggest the application of 2,4-D ester, which is effective on marestail. It’s also possible to use mixtures of 2,4-D with either dicamba, tribenuron (Express, etc) or tribenuron plus thifensulfuron (Harmony Xtra, etc.) to broaden the spectrum of control, since 2,4-D does not control chickweed and can use some help on a few other weeds. These sulfonylurea herbicides have limited activity on grasses and we have used them successfully in the fall on wheat. Not all labels specifically state approval for fall application to cover crops but they typically allow application to cereals or pasture or fallow areas in the fall.
• Application of 2,4-D or dicamba before grass planting or emergence can cause reductions in stand, and the tolerance of emerged grasses increases with size. So where the stand has only recently been planted, consider giving it some time to grow yet before application.
• We have not observed early November application of 2,4-D to cause injury to cereal rye or bentgrass in our research, and minor injury probably does not diminish the value of the cover anyway. Any of these herbicides will likely injure or kill broadleaf cover crops.

A review of a few labels revealed the following information with regard to application to grasses. We assume other product labels for the same active ingredient contain similar wording.

– Salvo (2,4-D) – From “grass pasture” and “perennial grasslands not in production” sections – preseeding applications should occur at least 30 days prior to planting. Do not apply to newly seeded areas until grass is well established (more than 5 true leaves). Addition of surfactant may increase injury to newly seeded grass. From “winter wheat, barley, rye” section – grains are generally tolerant, but more so after they are in the full tillering stage.
– Weedone 638 (2,4-D) – From “conservation reserve program areas” section – Do not apply to young grasses with fewer than 6 leaves or prior to tillering, or apply more than 1 1/3 pints (1 lb ae/A) until grasses are well established, as excessive injury may result.
– Clarity (dicamba) – From the “conservation reserve program” section – Clarity may be applied either preplant or postemergence to newly seeded grasses or grain species grown as cover crops. Postemergence applications may be made after seedling grasses exceed the 3-leaf stage. Rates of Clarity greater than 16 oz/A may severely injure newly seeded grasses. Preplant applications may injure new seedings if the interval between application and planting is less than 20 days per 16 oz/A.
– Express (tribenuron) – from “wheat, barley, oats and triticale” section – apply after the crop is in the 2-leaf stage. From “bentgrass, annual ryegrass, etc.” section – Apply 0.25 oz after stand is in the 4-leaf stage. Note – label states in several places that applying Express with 2,4-D or dicamba actually reduces risk of injury compared with Express alone.
– Harmony Xtra (tribenuron + thifensulfuron) – from “specific use – cereals” section – apply after the crop is in the 2-leaf stage (3-leaf for oats). Label does not mention grasses per se, but allows use as a fallow treatment in the fall.

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