By Mark Loux, Extension Weed Scientist
Following the article about marestail control in wheat stubble, a number of people asked about options where cover crops are going to be planted later this summer. The options can be fewer for this situation, especially for small-seeded broadleaf cover crops, for which the residual activity of several herbicides can affect the ability to get an adequate stand when planted too soon after application.
Where there is an issue with residual activity of a herbicide used for marestail control — 2,4-D, dicamba, or Sharpen — planting a grass cover crop instead of a broadleaf crop usually reduces risk of problems. Information on various herbicides follows, although some herbicide labels are less than clear on this.
Glyphosate. No waiting period for any cover and no residual soil activity, but won’t control marestail, of course.
Gramoxone. No waiting period for any cover and no residual soil activity, but needs considerable help from another herbicide in the mix to adequately control marestail.
Glufosinate (Liberty, etc.). No residual soil activity, so would be safe to plant any cover crop. The label allows use in fallow situations but does not directly address cover crop planting following application in fallow. Rotational crop guidelines on the label appear to specify 70-180 days between application and cover crop planting, depending upon species, which is not due to any possibility of stand loss. Interpretation directly from Bayer is that as long as the cover is not fed or grazed, it is not considered a crop, and the 70- to 180-day wait does not apply here.
2,4-D. Ester has shorter soil residual than amine. Generally safe to plant anything 30 or more days after application. Grasses will be safer than broadleaf crops when planted less than 30 days after application.
Dicamba. Soil residual is rate dependent. For small grains and grasses, the minimum interval between application and planting is 15 days per 8 fluid ounces applied (so 16 ounces = 30 days). Labels do not appear to address the waiting period to plant broadleaf covers.
Sharpen. Has soil residual that is rate-dependent, and is primarily on broadleaf plants. The label allows pre-plant/pre-emergence use on small grains and for cool-season grass establishment, so cereal rye and ryegrass could be planted safely following use in wheat stubble. For all other cover crops, the label specifies 1 or 2 months between application and planting for 1 or 2 ounces per acre, respectively. Potential for stand reduction is noted on the label and the cover crop cannot be grazed or fed. The label does not technically appear to address cover crop planting following use in wheat stubble, but instead assumes use of Sharpen in a crop that goes to harvest followed by cover planting, or interseeding of the cover into a crop. We assume that this discrepancy doesn’t matter as long as the prohibition on feeding or grazing is followed.