Burndown Herbicide Selection Before Early-fall Cover Crop Seeding

Weed regrowth and new seedlings in small grain stubble or fields harvested for corn silage should be controlled prior to cover crop seeding to start clean and give cover crops a competitive advantage. Early September seeding windows allow for the use of diverse cover crop mixtures as well as winter-killed species. Whether you plan to use grass cover crops or diverse mixtures, make sure to consider the following issues related to herbicide program approaches.

Burndown programs prior to cover crop establishment?

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 some other no-till weeds.

Sharpen (1 fl oz) or a glufosinate product (32-43 fl oz; Liberty 280) 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 has a 7-day plant-back interval for cover crops and suggests waiting for a half-inch of rainfall prior to seeding cover crops. The use of either Sharpen or glufosinate products prevents legal use of cover crops for forage due to rotation interval restrictions.

Systemic broadleaf products, including 2,4-D ester (1-2 pt) and dicamba (8 oz), may also be considered to improve broad-spectrum weed control prior to cover crop seeding. However, our field trials suggest that their use can significantly reduce the establishment of small-seeded legumes (crimson clover, red clover, alfalfa) and brassica (forage radish) cover crops when applied less than two weeks prior to cover crop seeding. In comparison, we found minimal injury from 2,4-D or dicamba on the establishment of grass cover crops (annual ryegrass, cereal rye) when seeding 7 to 14 days after application. If considering the use of cover crops for forage, consult the label for these products to determine rotation intervals.

Cover crop vs. forage crop?

Cover crops are often 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 on the label, which is 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 or triticale, 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.