Still time for herbicides this fall


Rob Leeds - OSU Extension



There is always a constant living in Ohio. The weather will change – you can count on it. One day we are out in our shirtsleeves and the next day it is damp, cold and windy.

Most all of the crops have been harvested and ground work has been off and on due to the weather. There is still time though to apply herbicides yet this fall because the frosts we’ve had have little effect on weeds such as marestail, purple deadnettle, chickweed, etc. We have applied well into December with good results.

According to Mark Loux, OSU Extension specialist in horticulture and crop science, 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 their studies, Loux says that 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.

In fields with grass covers and a history of marestail, it is recommended that for fields with grass to either treat the fields this fall yet, or scout thoroughly for the presence of marestail before making a decision. Keep in mind 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, Loux suggests 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.

Early November application of 2,4-D has not seemed 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.

According to Loux, 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 five 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 six leaves or prior to tillering, or apply more than 1-1/3 pints (1 pound 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 post-emergence to newly seeded grasses or grain species grown as cover crops. Post-emergence applications may be made after seedling grasses exceed the three-leaf stage. Rates of Clarity greater than 16 ounces/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 ounces/A.

• Express (tribenuron) – from “wheat, barley, oats and triticale” section. Apply after the crop is in the two-leaf stage. From “bentgrass, annual ryegrass, etc.” section, apply 0.25 ounces after stand is in the four-leaf stage. Note: The 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 two-leaf stage (three-leaf for oats). Label does not mention grasses per se, but allows use as a fallow treatment in the fall.

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Rob Leeds

OSU Extension

Rob Leeds is the Ohio State University Extension educator for agriculture and natural resources in Delaware County.

Rob Leeds is the Ohio State University Extension educator for agriculture and natural resources in Delaware County.