Chapter 6

Cover Crops for Organic Farms

Termination of Cover Crops

Effective termination of the cover crop generally is necessary because if not managed they will compete with the cash crop for moisture and nutrients. Commonly used mechanical methods include tillage, rolling-crimping, mowing, undercutting, or selection of species that winterkill or have a short life cycle. Each method has its disadvantages and limits. For example, winterkill (the cover crop is terminated by a hard freeze) is only applicable to certain crops and climate regions; mowing is limited to certain cover crops and crop growth stages. Tillage can be expensive and can negate the benefits of the cover crops, as well as the benefits of minimum/no-till production systems.

Termination Time for Cover Crops

Timing of cover crop termination is specific to each farming condition. Timing can affect soil moisture and temperature, tillage and planting operations, weed suppression, and nitrogen release from legume decomposition. Regardless of the method of termination, weather conditions may prevent cover crop termination at the time planned. Cover crop termination four to eight weeks prior to commercial crop planting can allow soil warming, soil water replenishment, and residue drying and decomposition. Termination that occurs less than four weeks before commercial crop planting can help provide benefits of increased cover crop biomass, soil and water conservation, and possible nitrogen sources from legume cover crops.

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. When using winter-kill cover crops, the key is to get them in the ground early enough to be of the most benefit before they are killed off by cold temperatures. This works exceptionally well when a crops are harvested with plenty of growing days left in the season to accumulate adequate top growth before being killed by freezing temperatures. Instead of living cover, they provide dead mulch through the winter months. Planting or transplanting of early spring crops can follow after mowing and/or strip tillage.

Cover Crop Termination with Tillage

Cover crops are most commonly incorporated with tillage. Tillage not only controls cover crops, but also incorporates them into the soil, allowing them to degrade quickly and release nutrients for the primary crop. Early or delayed incorporation can have negative consequences. As a general rule, tilling is recommended just before or at full bloom. This results in slower decomposition of the cover crop residue and a release of nutrients over a longer period of time. If any tillage occurs after bloom, the cover crop may reseed itself. Furthermore, waiting too long to incorporate the cover crop also results in higher carbon to nitrogen ratios, which slows decomposition and delays nutrient availability to the following cash crop.

Rolling-Crimping

Cover crops can be killed using a roller-crimper (See Figure 7.1). As the name suggests, the roller-crimper is a round drum, one to two feet in diameter, with equally spaced blunt blades or knives arranged around the drum. 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 the tractor. Roller crimpers come in a variety of designs. Straight bar rollers have good killing action and are easy to make, but transfer vibrations to the operator and require slower tractor speeds. Curved or spiral blades on the roller drum enable the roller to stay in constant contact with the ground, thus allowing faster speeds and reduced vibration, but also have reduced crimping action.

Mowing

Mowing cover crops is more common in reduced-tillage systems where growers want to utilize the cover crop residue as mulch or when large amounts of cover crop biomass are present that would challenge their tillage equipment if it were not first mowed. Mowing terminates the crop quickly but there is a possibility for cover crop re-growth depending on the species and time of termination. In humid climates, mowed residues break down rapidly, negating some of the benefits of keeping the soil surface covered.

Rotary Mowers

Rotary mowers (brush-hogs) clip higher than sickle bars and usually distribute vegetative residue throughout a wider area (See Figure 7.2). They distribute the mulch unevenly and chop it up so that decomposition is rapid and soil coverage is short-term.

Sickle Bar Mowers

Sickle bar mowers are fairly effective cutting close to the soil surface increasing the chances of a good kill (See Figure 7.3). Sickle bars do not chop up the cover crop but rather lay the cover down uniformly over the soil surface—an important characteristic in weed suppression. A major problem with sickle bars is that some viney legumes, hairy vetch or field peas, get hung up on the machine, slowing mowing operations and leaving a layer of very uneven mulch on the field.

Flail Mowers

Flail mowers contain many small doubled-edged knives that uniformly distribute finely cut residue on the soil surface (See Figure 7.4). As a general rule, flail mowers are more effective than sickle bar or rotary mowers at chopping up cover crops and evenly distributing the residue.

Undercutting

Undercutting involves using of specialized sharp steel bar and a roller (See Figure 7.5). An undercutter is most often used on deep-rooted winter annuals, severing the crop’s roots from its tops. The unit is primarily suited to bed production systems. A rolling basket is positioned to the rear of the blades for depth adjustment and to flatten the severed cover crop. A big advantage of the undercutter (and the V-blade) is that it achieves a good kill while not chopping the cover crop, resulting in a more persistent, weed-suppressive mulch.

Termination Methods by Cover Crop Species

Not all termination methods can be used in controlling cover crops. Some cover crops can be rolled, mowed or frost-killed to allow organic no-till planting, while others require tillage or undercutting. For example, winter rye is most effectively mow-killed at flowering. If mowing is done earlier, the plant re-grows readily. Annual ryegrass (Italian ryegrass), grown as a winter or summer annual, cannot be mow-killed and must be tilled in as green manure. Crimson clover is easy to kill by mowing at full bloom and it self-seeds readily if allowed to stand more than a few days past peak bloom.

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