Early September is an opportunity to incorporate cover crops into your rotation on small grain acres that are not double-cropped with soybean, as well as corn silage acres that are not double-cropped with winter cereal forages. This early seeding window allows for use of winter-killed species, such as forage radish, and likely improves winter hardiness and growth potential of winter-hardy legumes, such as hairy vetch, Austrian winter pea, and crimson clover. If you plan to use grass-cover crops or cover crop mixtures, make sure to consider the following issues related to herbicide programs.
Herbicide Carryover from Corn Herbicides?
We have received a few calls about the likelihood of herbicide carryover injury to fall-seeded cover crops due to dry conditions throughout much of the state in June and July. The widespread rainfall we have received from tropical storm remnants in August should significantly lower the potential for carryover injury to fall-seeded cover crops. For the few regions that remain dry, the potential for herbicide carryover will be highest when seeding small-seeded broadleaf cover crops (i.e., clover species, winter canola, rapeseed, forage radish) on production fields that received applications containing mesotrione or clopyralid, particularly when applied in POST applications. Misapplications or overlapping spray patterns are often an additional contributing factor that increases the likelihood of herbicide carryover.
Cover Crop vs. Forage Crop?
Given low forage yields within drought-impacted regions, cover crops may be seeded as a dual-purpose crop that can be either managed as a cover crop or harvested for forage. Make sure to consider herbicide rotation interval restrictions, which are the amount of time from herbicide application to establishment of subsequent crops. A cover crop that is not harvested can be established legally regardless of herbicide selection. But a cover crop that is harvested for forage must follow rotation interval restrictions on the label. Small grains used as a cover crop, such as cereal rye, are often included on rotation interval tables, but other brassica and legume species are often not. If not listed within the rotational interval table, the most restrictive interval listed should be used. Rotation restriction intervals are designed to prevent herbicide residues within harvested forages from exceeding tolerable levels established during the registration process. So before selecting burndown products prior to cover crop seedings, make sure to reference rotation intervals if your cover crop may become a forage crop.
Burndown Programs for Establishing Fall-Seeded Cover Crops?
In addition to rotation interval restrictions, it is important to understand the potential for burndown products to impact cover crop establishment due to soil residual activity. The simplest and most risk-free approach is to use only glyphosate or paraquat to start clean prior to cover crop seedings. These broad-spectrum products have no soil residual activity. However, additional products are often needed to control glyphosate-resistant species (horseweed, Palmer amaranth, waterhemp) as well as other select no-till weeds.Sharpen (1 fl oz) or a glufosinate product (i.e., Liberty) are options for control of glyphosate-resistant species prior to cover crop seedings. Our field trials suggest that there is limited risk of injury to establishing cover crops when seeding at least 7 days after application of these products. The Liberty label suggests adhering to a 7-day plant-back interval and waiting for a half-inch of rainfall prior to seeding cover crops. Use of either Sharpen or glufosinate products prevents the legal use of cover crops for forage due to rotation interval restrictions.Systemic broadleaf products, including 2,4-D ester and dicamba, may also be considered to improve broad-spectrum weed control prior to cover crop seeding. Our field trials suggest that their use can reduce the establishment of small-seeded legumes (crimson clover, red clover, alfalfa) and brassica cover crops when applied less than two weeks prior to cover crop seeding. In comparison, we found minimal impact on the establishment of grass cover crops (annual ryegrass, cereal rye) when seeding 7 to 14 days after application. If considering use of cover crops for forage, consult the label for these products to determine rotation intervals.
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