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Cover crops are mostly planted for adding benefits to a field, and are not harvested for their seed, fruit, or forage. Cover crops are terminated before planting production crops to avoid becoming weeds and/or hinder crop production.
Timing of removal is specific to each cover crop species and farming conditions. Cover crop termination four to eight weeks prior to commercial crop planting can allow soil warming, soil water replenishment, and residue drying and decomposition.1 Termination that occurs less than four weeks before commercial crop planting may provide the benefits of increased cover crop biomass, soil and water conservation, and possible nitrogen sources.
Nitrogen release and commercial crop yields can be affected by the timing of cover crop removal. Early termination of small grain cover crops can result in more rapid decomposition of the residue due to the narrow carbon to nitrogen ratio of young plant tissue. Carbon to nitrogen ratios of 30 to 1 and greater are reached at the flowering stage of small grains.2 This wider ratio may be desirable for increasing above-ground biomass production and residue coverage.
The crop being planted following a cover crop should be considered when terminating cover crops. For example, termination of winter-hardy cereals including cereal rye, triticale, and wheat should be timed to allow at least a two-week window prior to corn planting to avoid a yield reduction.3
Cover crops may include grasses, broadleaves, legumes, or any combination of these. The four methods used to terminate cover crops are: winterkill, tilling, mowing, and applying herbicides. A survey of farmers in 2014 found that approximately 48 percent of cover crop growers applied herbicides, 21 percent used tillage, 20 percent depended on winter kill, 10 percent terminated cover crops with mowing, 1 percent reported using a roller-crimper, and 6 percent used some other method to terminate cover crops.4 Winterkill is a method in which the cover crop is left until it is terminated by a hard freeze. This method is only possible in northern climates and for certain cover crops that are susceptible to the first hard frost (temperatures below 25° F) such as turnip and radish.2 While tillage is the second most common method of terminating cover crops, the use of tillage can be expensive and can reduce the benefits of the cover crops.
Mechanical. Aside from tillage, other mechanical methods of cover crop termination include use of a roller-crimper or mower. Roller-crimpers (mechanical rollers) can be used to kill tall-growing cover crops by breaking or crimping the stem. This method can be effective when used on cover crops at the flowering stage or later.1 Mow-kill is a method of cover crop removal that is effective on some species. Cover crop regrowth and residue distribution should be evaluated when mowing is used for termination.
Herbicide. Effective herbicides come in contact with the target plant, are absorbed at the leaf surface, and move to the site of action at a lethal dose. The following factors should be considered when selecting herbicides for cover crop termination:
Figure 1. Control of annual ryegrass in the spring should begin before the first node is just starting to form.