Chapter 16

The Use of Cover Crops to Manage Soil

Termination Strategies for Cover Crops

Developing a termination strategy for cover crops is an essential part of the planning process to ensure there is no interference with the planting or management of the subsequent cash crop whether it be field crops, tree, vines, or vegetables. Effective control or suppression of the cover crop generally is necessary because if not managed it will compete with the cash crops for moisture and nutrients. In a dry year, cover crops can rob crops of valuable soil moisture. In other years, they may also compete for other resources such as nitrogen if not managed properly. Cover crop termination influences soil temperature, soil moisture, and the amount of nitrogen fixed by legumes or scavenged by grass as well as subsequent nitrogen release through cover crop decomposition and nitrogen uptake by the cash crop.

Carbon-to-Nitrogen Ratio

The carbon-to-nitrogen ratio (C:N ratio) of the cover crop at termination influences whether nitrogen will be immobilized or released. In general, mineralization or release of nitrogen occurs when the C:N ratio is below 24:1, while immobilization or sequestering of nitrogen usually occurs when the C:N ratio is above 24:1. Cover crop residues with very high C:N ratios may also immobilize some soil or fertilizer nitrogen. The C:N ratio of small-grain residues is dependent on its maturity. Early termination of grass cover crops results in a smaller amount of residue with a low C:N ratio typical of young plant tissue. This results in rapid decomposition and limited ground coverage. Killing small-grain cover crops after flowering results in significantly more biomass and a C:N ratio usually exceeding 30:1.

Timing of Cover Crop Termination

When determining an ideal cover crop termination date, producers need to consider the growing season of the cover crop, soil moisture, soil temperature, nitrogen management, allelopathy/weed suppression, and equipment under a variety of different climatic patterns and crop rotations. Cover crop termination dates are often dictated by the target date for planting the subsequent cash crop. A general rule is to terminate winter cover crops two to four weeks prior to the anticipated cash crop planting date. Terminating a cover crop two to four weeks before cash crop planting allows residues to dry out and become “brittle.” This enables planting equipment to easily cut through the residue.

Cover Crops as a Nitrogen Source

Cover crops that include nitrogen-fixing plants such as vetch, clover, peas, and beans can contain substantial amounts of nitrogen. However, a cover crop’s ultimate nitrogen contribution to the crops depend on the mass of cover crop produced, the types and relative amounts of legumes and nonlegumes it contains, and how the cover crop is managed. If a cover crop is going to be terminated the maximum addition of nitrogen from legumes occurs when the legumes are blooming.

Cover Crop Termination Methods

The method of cover crop termination is also related to the purpose for implementing cover crops and the equipment available. Poor termination can result in cover crops becoming “weeds” and impeding cash crop germination, and in turn, significantly reduce yields and profit. Termination methods for cover crops are: mowing, tillage, roller-crimping, winter-killing, and herbicides. There are advantages to all methods. Not all termination methods can be used in controlling all cover crop species. For example, annual ryegrass, grown as a winter or summer annual, cannot be mow-killed or roller-crimped and must be terminated with tillage.

Mowing

Timely mowing can be an effective tool to terminate or control the growth of some cover crops without disturbing the soil or using herbicides. Mowing is an effective method for rapidly terminating a cover crop at its appropriate growth stage before seed development. Winter hardy annuals can be effectively killed through timely mowing—barley, cereal rye, and hairy vetch. Many forbs and brassicas can also be effectively terminated through mowing. However, as they also tend to winter kill, such interventions are typically not necessary. Mowing is generally less effective than tillage or roller-crimping, with often unpredictable effectiveness and regrowth of some species.

Roller-Crimping

Cover crops can be killed using a roller-crimper. As the name suggests, the roller-crimper is a large, heavy cylinder, with metal flanges coming off at an angle that are attached to a roller drum, 1 to 2 feet in diameter (Figure 16.18). As it rolls, it pushes down the cover crop and “crimps” the stems to kill the crop. Rollers can be front- or rear-mounted on a tractor. The cover crop needs to be tall enough for the roller-crimper to be effective. Terminating susceptible cover crops with a roller-crimper leaves an intact mat of soil protective mulch oriented in the direction of planting.

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 disk. Timing is very important, as the soil must still be moist enough to easily till-in the crop. Cover crops with extensive root systems, such as annual ryegrass and perennial clover, like sweet clover and red cover, are notoriously difficult to terminate and may require multiple tillage passes. Incorporation of cover crops into the soil allows rapid decomposition and release of nutrients for the following cash crop.

Winter-Killing Cover Crops

The concept of winter-killing cover crops involves the strategic planting of a cover crop that will be reliably killed by temperature shifts as seasons change. Cover crops should be fall planted early enough to accumulate adequate top growth before being killed by freezing temperatures. Instead of living cover, they provide dead mulch through the winter months. Most often spring oats, buckwheat, hairy vetch, rye, radishes, field peas, and sorghum fill this need (Figure 16.20).

Herbicide Control

When selecting herbicides for cover crop control, consider the following: the cover crop and weed species that are present, their growth stage, the crop to be planted, weather conditions at application, and whether the chosen herbicide has contact or systemic properties. Cover crop varieties will differ in their susceptibility to herbicide. Red clover and vetch are notoriously difficult to terminate. Winter wheat is more difficult to kill than cereal rye. Also consider that cover crop mixtures of grasses and broadleaves can be challenging to terminate and generally require a multi-faceted herbicide program as each species will respond best to a different herbicide and will be at a different phase of development.

Grazing Cover Crops

The cover crops mix planted and timing of the grazing are critical. For the cover crop to be terminated, it has to be at the correct stage of its life cycle. Usually this means it has finished pollinating. High stock densities are required to trample the cover crop. While high stock density grazing can significantly reduce opportunity for the cover crop to interfere with the subsequent cash crop, it will not create or cause complete termination of the cover.

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