Conditions that Might Hamper Cover Crop Development

The key to successful cover crop use is to develop adequate biomass prior to termination in the spring. Good establishment is dependent on the planting method, timing, and success of germination. Fall growth can be affected by nutrient availability, the influence of residual herbicides used in the prior crop, and weed interference.

📸: BASF Contributing Writer/Featuring Amaranthus weed escaping (L). Treating with diphenyl-ethers late in the season can set up carryover problems. High soil pH (lime piles R) can extend sulfonylurea herbicide activity and injure cover crops (Filson, IL).

Herbicide Carryover: An Occasional Threat to Cover Crop Establishment

There are a few crop-cover crop combinations that might raise concern about herbicide carryover affecting establishment of a cover crop. The most vulnerable rotation would be a broadleaf cover crop planted into soil containing ALS inhibitors, PSII inhibitors, PPO inhibitors, or HPPD inhibitor residues.

📸: BASF Contributing Writer/Featuring Broadleaf cover crops. These may be more difficult to establish than grasses and are more sensitive to herbicide residues (Champaign County, IL).

• Most broadleaf cover crops would be planted ahead of corn, likely following soybeans or another broadleaf crop. Although the use of ALS herbicides in soybeans is currently less frequent, any of the POST-applied Imidazilinones (Imazethapyr, Imazaquin) would make clover, vetch, or brassica establishment difficult, particularly since for those species to be successful, they must be planted early in the fall.

• Chlorimuron-containing products can have an extended half-life in high-pH soils and be problematic for not only cover crops but also next year's corn crop. PSII herbicides are less of a concern since they would be applied PRE in soybeans and have significant time to degrade before cover crop seeding.

• Metribuzin products have a relatively short half-life and are likely at sub-injurious soil levels by early fall. The PPO herbicides used as late-post treatments in soybeans may pose a significant risk to cover crop establishment. These herbicides may be used as a type of rescue treatment where herbicide-resistant Amaranthus species (water hemp and Palmer amaranth) have escaped the primary PRE and POST treatments. HPPD inhibitors are primarily used in corn and would only be a concern if a broadleaf cover crop was inserted into a continuous corn rotation.

Recent improvements in weed control have been obtained by adding a residual herbicide to the final POST application. Typically, a group 15-shoot growth inhibitor such as acetochlor, metolachor, or dimethenamid (Outlook) is used, and in dry late-season soil conditions, it may break down slowly and injure early-seeded cover crops.

Not only is cereal rye the most robust-growing grass cover crop that can be planted the latest and germinates easily, but it is also the least likely to be affected by soybean herbicides in the soil.

Which Cover Crops Are Most Sensitive to Herbicide Carryover?

Various university studies conclude that tillage radish and Austrian winter pea are the most sensitive to herbicide carryover and that hairy vetch, cereal rye, and winter wheat are the least sensitive.

The most vulnerable situations are when broadleaf cover crops are inserted between back-to-back corn crops where mesotrione, clopyralid, or topramezone have been used or when late-season rescue treatments using diphenyl ethers in soybeans are used ahead of both broadleaf and grass cover crops.

📸: BASF internal photograph/Featuring weed escaping in soybeans.

Weed Competition: Is It a Problem in Cover Crops?

Winter annual weeds may be germinating concurrently with your seeded cover crop, but this may not be a negative depending on your cover crop goals. If your intention is to create dense biomass, then some weeds may fill in voids, particularly when combined with slow-growing cover crops. If nitrogen fixation is the main goal, then intermingled weeds are a negative as they compete with the planted legume. Perennial weeds or herbicide-resistant winter annuals such as marestail may necessitate herbicide use prior to seeding. Careful attention should be paid to termination timing in the spring to ensure that control is achieved before a large seed rain from maturing winter annual weeds occurs.

Rare Cases For Cover Crop Fertilization

In generally well-fertilized or otherwise nutrient-rich soils, fertilizer is not required for cover crops. Two of the stated goals of cover crops—N fixation and Nitrate-N retrieval from the soil—run contrary to feeding a cover crop with N. An exception might be for rye on sandy soils following a large corn crop where biomass production is desired. With grain production not an issue, small nutrient deficiencies are inconsequential to the goal of your cover crop.

Coming next: Is it too late to plant cover crops?

BASF provides the information in this article as a service to its customers; however, the views expressed by guest writers are their own and do not necessarily reflect the views of BASF.