By: Mark Loux, OSU Extension Weeds Specialist
Managing cover crops in a year like this can challenge even those with the most experience. A few suggestions regarding termination of covers:
• Increase glyphosate rates to compensate for larger size, and consider applying alone or just with Sharpen. Mixing glyphosate with other herbicides or ATS can reduce its activity on grass covers, especially when large. Herbicides that can antagonize glyphosate include 2,4-D, metribuzin, atrazine, and flumioxazin and sulfentrazone products. Sharpen has not caused a reduction in glyphosate activity on grass covers in university research. One approach would be to apply the glyphosate or glyphosate/Sharpen first, wait a few days, and then apply residual herbicides.
• For covers that contain legumes, it will be generally still necessary to include with the glyphosate a growth regulator herbicide such as 2,4-D, dicamba, or clopyralid. Sharpen will not be effective enough unless canola or rapeseed is the cover targeted. Clopyralid is very effective on alfalfa and clover. The exception here is hairy vetch which is easily killed by herbicide or even just being run over once it flowers.
• Our experience with relatively small (less than 2 feet) covers is that they do not interfere with the activity of residual herbicides. We are somewhat unsure about the effect of taller covers on residual herbicide activity, but assume it could be reduced. This may be a situation where the residual could be omitted from the burndown and then included in an early POST treatment. Some considerations here:
1. A large dense cover does provide fair to good early-season control of weeds on its own;
2. We have herbicides in corn and soybeans (depending upon which traits are planted) allowing us to obtain POST control of many weeds, so the residuals could be omitted entirely, but this probably won’t work in nonGMO or Roundup Ready soybeans;
3. Many covers are variable in size and density, and fields can have areas where the cover is not providing any control of weeds – omitting PRE residual herbicides in these areas can be a mistake;
4. Most PRE corn herbicides can also be applied early POST, but most PRE soybean herbicides cannot, including anything containing metribuzin, flumixazin, sulfentrazone, or saflufenacil. Soybean herbicides with substantial residual activity on at least some weeds, which can be applied POST, include Scepter, imazethapyr (Pursuit), FirstRate, metolachlor, Zidua/Anthem, Warrant, and Outlook. Due to widespread ALS resistance or just a narrow spectrum of broadleaf weed control, none of these have any residual activity on marestail or ragweeds; so
5. A two POST application approach may be more effective in soybeans than trying to make residual herbicides work.
• This may be a situation where planting green is better than killing the cover ahead of planting and allowing the cover to start to die and degrade. Rationale for this is that a live cover can continue to use soil water and help create conditions fit for planting, while a dead cover can trap moisture and prevent soil conditions from drying as fast. One option here is to delay burndown until the crop has been planted, or even until crop has emerged. Ability to do this in soybeans would depend upon which herbicide resistance traits were present – more complex trait systems would be more effective.
• We generally do not recommend using Gramoxone to control large covers or large weeds. Gramoxone is most effective on small weeds and covers (6 inches tall or less). Where Gramoxone might fit would be in a situation where we are able to spray first with a systemic product like glyphosate, but not able to plant for another week or two, and there is additional weed emergence or an incomplete cover crop termination. This would create a need for a quick burn down with a second burndown pass just before planting, and Gramoxone could be used in the second burndown (with some metribuzin ideally).
• Grass covers that are no completely killed by an initial herbicide applications can be controlled with glyphosate POST. Planting Enlist or Xtend soybeans will provide for the most effective POST options to control legume covers that escape the initial burndown, although high rates of glyphosate can also work. Glufosinate also has some activity on legumes but will be more variable – use high rates and spray volumes and take steps to otherwise maximize coverage and activity.