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Terminating Winter Cover Crops in Organic Feed and Forage Crops

Terminating Winter Cover Crops in Organic Feed and Forage Crops
By Mary Barbercheck and Kristy Borrelli
Including cover crops in crop rotations is a practice that can benefit all farmers. Some benefits of using cover crops building soil organic matter, suppressing weeds, managing soil fertility, and providing resources for pollinators and beneficial insects. Winter cover crops are planted after cash crop harvest and terminated in the spring before the next cash crop is planted. Winter cover crops are especially important in organic cropping systems because synthetic fertilizers, pesticides, and other synthetic inputs are not allowed. In addition, the use of cover crops in organic production systems is mandated in the Soil Fertility and Crop Nutrient Management (§205.203) and Crop Rotation (§205.205) practice standards of the USDA National Organic Program.
There are many winter cover crop species from which to choose, each with their own management attributes and abilities to provide specific benefits. The selection of the appropriate cover crop species or species mixture in a crop rotation should be based on grower management goals, the timing of cover crop establishment and termination in a crop rotation. General information on choosing appropriate cover crops is available in the Penn State Agronomy Guide , Penn State Organic Crop Production Guide , and USDA SARE’s Managing Cover Crops Profitably.
In conventionally-managed cropping systems, winter cover crops are often terminated with herbicides. Because synthetic herbicides are not allowed in organic production, this option is not available to organic producers. Therefore, this article will focus on ways that organic growers can terminate their winter cover crops before planting cash crops in the spring. In organic systems, the most common methods used to terminate winter cover crops is by winter-kill, mowing, tillage, a combination of mowing and tilling, roller-crimping, and grazing.
Winter-killed cover crops
Cover crops vary in their hardiness over winter. Cover crops that die during freezing temperatures in winter create a mulch-like mat on the soil surface that can help protect the soil from erosion. The absence of a living cover crop to manage in the spring can allow for early planting of another cover crop or a cash crop and provides an opportunity for no-till planting. Planting through residue requires access to a no-till drill or planter. One trade-off with using winter-killed cover crops is that, depending on the cover crop species and biomass produced before winter-kill, very little weed-suppressive and soil conserving residue may remain on the soil surface in the spring. In central Pennsylvania, cover crops that winter-kill include spring oats, several mustard species, and forage and oilseed radish. In areas with colder winters, crimson clover usually winter-kills. Information on the winter-hardiness of different cover crop species can be found in USDA SARE’s Managing Cover Crops Profitably.
Mowing is often used to quickly terminate a cover crop before it goes to seed. Mown residues can be left on the soil surface or incorporated with a disc. Crops can be no-till planted into a mown cover crop, a reduced-tillage option that is usually easier to accomplish than planting into surface mulch created with a roller-crimper. As with any method that leaves residue on the soil surface, mowing can result in cooler, wetter soils than where cover crops have been terminated with tillage and incorporated. There is also a possibility for cover crop re-growth after mowing depending on the species and timing of termination.
The most commonly used type of implements used for mowing cover crops include sickle bar, rotary, and flail mowers.
  • Sickle bar mowers cut close to the soil surface, increasing the likelihood of effective termination. This type of mower does not chop the cover crop but lays it down uniformly over the soil surface, which leaves a slowly decomposing, weed-suppressive mulch on the surface. Viney legumes, such as hairy vetch or field peas, can get wound up on machinery and implements, slowing field operations and leaving an unevenly distributed layer of mulch on the field.
  • Rotary mowers (brush hogs) cut higher than sickle bar mowers and distribute chopped residue over the soil surface. Decomposition of surface residues is usually more rapid after rotary mowing compared to sickle-bar mowing.
  • Flail mowers contain many small doubled-edged knives that uniformly distribute finely cut residue on the soil surface. They generally require more horsepower than rotary mowers and tend to leave more finely chopped, evenly distributed, and quickly decomposed residue on the soil surface compared with sickle bar and rotary mowers.
Tillage or mowing plus tillage
Cover crops are most commonly terminated and incorporated with tillage. Some cover crops can be mown and left in the field and tilled in or harvested for forage and the remaining stubble incorporated with a plow or disc. Incorporation of cover crops into the soil allows rapid decomposition and release of nutrients for the following cash crop. In general, termination by tillage should occur just before or at full bloom to slow decomposition of the cover crop residue and to release nutrients over a longer period of time. Tillage after full bloom may result in cover crop self-reseeding, resulting in volunteer cover crops in the following crop that may act as weeds. In addition, delaying cover crop termination and incorporation can results in a high carbon to nitrogen ratio, which slows decomposition and delays nutrient availability to the following cash crop.
Sufficient time between cover crop incorporation and cash crop planting is needed to allow decomposition of the cover crop residue. Decomposing residues in tilled soil can attract seed corn maggot flies that oviposit in the soil. The resulting maggots feed on the cash crop seed and can reduce germination. There are many variables that impact organic matter decomposition but in general, growers will need to wait two to four weeks before planting the subsequent crop.
Cover crop roller-crimpers are generally made from a hollow steel drum or cylinder 1 to 2 feet in diameter with blunt blades or knives arranged on the cylinder that crimp or crush the stems of the living cover crop, which kills it. Roller-crimpers can be front- or rear-mounted on the tractor. Terminating susceptible cover crops with a roller-crimper leaves an intact mat of soil protective mulch oriented in the direction of planting. This unidirectional uniform surface mulch can suppress weeds and facilitate no-till planting. In contrast to mowing, there can be less risk of cover crop regrowth when it is roll-crimped at the correct time, the intact residue decomposes more slowly than chopped residues, improving weed suppression with a uniform mat of residue on the soil surface.
Cover crop varieties and timing of termination are critical components to the successful use of a roller-crimper in an organic system. Cover crop rollers can be effective for terminating annual crops including cereal grains, such as cereal rye, wheat, oats, and barley as well as annual legumes and other forbs. Annual legume cover crops such as hairy vetch, winter pea, and crimson clover can also be terminated with a roller-crimper. In general, biennial and perennial cover crops, are not recommended for termination with a roller-crimper. Some cover crops not controlled by the roller-crimper include alfalfa, red clover, canola, and annual ryegrass.
Timing of termination is a critical aspect for successful termination of cover crops with a roller-crimper. Regrowth of the cover crop can occur if roller-crimping occurs before the cover crop reaches a phenological stage at which it can be effectively killed. Cereal grain crops can be killed with a roller-crimper when the anthers were clearly visible and shedding pollen. Legume cover crops, such as hairy vetch can be controlled when small pods are visible (early pod set) on the upper nodes of the plant counting down from the top. If roller-crimping is delayed, cover crops may set seed and become a weed in the following cash crop. Weed suppression may be incomplete if the rolled mulch cover crop biomass is insufficient to completely block light on the soil surface. Small grain cover crops must reach early flowering to be effectively terminated with roller-crimpers. For more information on using a roller-crimper to terminate cover crops, see Terminating Cover Crops with a Roller Crimper in Organic Grain Rotations and Cover Crop Rollers for Northeastern Grain Production .
Grazing can be an effective method for cover crop management depending on the cover crop’s purpose and infrastructure available for managing grazing animals. Grazing can add value to cover crops, making it more attractive to farmers to plant them. Nutrient management can be improved because urine from grazing animals soaks into soil quickly, reducing the likelihood of large ammonia volatilization losses. Grazing removes a substantial portion of the plant aboveground biomass and may interfere with the goal of enhancing soil organic matter. If crops are intended for human food, producers should consult with their organic certif­ier prior to grazing any cover crops. For more information on using grazing to terminate cover crops, see Grazing Cover Crops .
Termination Methods by Cover Crop Species
Not all termination methods can be used in controlling all cover crop species. Some cover crops can be rolled, mowed or winter-killed to allow organic no-till planting, while others require tillage or undercutting. For example, cereal rye is most effectively mow-killed or roller-crimped at flowering. If mowing or rolling is too early, the plant re-grows or stands back up readily. Annual ryegrass, grown as a winter or summer annual, cannot be mow-killed or roller-crimped and must be terminated with tillage. Crimson clover is easy to kill by mowing at full bloom and self-seeds readily if allowed to stand past peak bloom.
If planting only a legume, the best time to kill the crop for maximum Plant Available Nitrogen (PAN) is at the budding growth stage. PAN from cereal grain cover crops is low to negative (ties up N). At cereal grain tillering, PAN can be zero to slightly positive. Cover crop mixtures can be more difficult to manage than single species stands. For mixtures cereal grain-legume mixtures with low percentages of legumes, the cover crop should be killed before cereals reach boot stage so that the PAN is not tied up by the cereal’s carbon. If a cover crop mixture is 75% legume, it will behave like a pure legume stand. If using a roller-crimper to terminate cover crop mixtures, all species in the mixture should be susceptible to termination by roller-crimping, and all should reach a stage at which they can be effectively terminated in a similar timeframe. 
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