October 2, 2012

With this year's drought conditions there is less residue than normal in many fields. In some, the crop was harvested as a forage or cut as silage, leaving very little residue at all. Without residue to absorb the energy of raindrop impact and keep the wind off the soil surface, soil erosion will be greater and surface crusting could be a problem. Also, without the water-conserving residue mulch, soil moisture losses by evaporation will be much higher.

Figure 1. Research has shown that cover crops (left) tend to use less water than what is lost to evaporation from bare soil (right) and they provide valuable organic matter as they break down in the soil.

To reduce these problems, producers should consider planting a cover crop to help protect the soil. With the early harvest and the open fall this year, there is enough time for a cover crop to be seeded and to have adequate growth to provide some benefits. The growing cover crop will "harvest" sunlight and carbon dioxide that would be otherwise wasted as there is no cash crop growing.

Benefits of Cover Crops

Cover crops can be used for a variety of purposes including protecting the soil, improving soil structure, fixing nitrogen, feeding the soil biological life, and managing soil moisture. A key soil health concept is that there should be something green and growing during as much of the year as possible. This is important to protect and feed the soil system, especially arbuscular mychorrhizae fungi.

Research has shown that while a cover crop uses some soil moisture as it is growing, it tends to use less water than what is lost to evaporation from a bare soil surface. Grasses provide the longest lasting residue cover because they have a higher carbon to nitrogen ratio in their biomass compared to non-grass species. In addition, they improve snow catch in the winter and reduce wind erosion in the spring compared to bare soil. Taller brassicas and broadleafs like rape, mustards, and canola also will stand nicely to reduce wind erosion and catch snowfall, but they provide less residue.

With the drought conditions this year, there may be considerable residual nitrate in the soil following corn production. A cover crop will scavenge the residual nitrate for its growth and store it in a biological form, reducing potential losses. Some producers apply some fertilizer to encourage cover crop growth. This fertilizer and the residual nitrates are recovered later as the cover crop residue breaks down. Producers could use legume cover crops to fix some nitrogen for the next crop and to feed the soil system, but they will need the proper inoculate for the legume species.

The cover crop will add organic biomass both above and below ground and the growing roots will help build soil structure. The fibrous roots of grasses help build soil stability near the surface. The deeper tap roots of broadleafs, especially brassicas, penetrate and open up tight soils, improving infiltration. The finer roots of legumes feed the soil microbes while the tap roots grow downward. Any root growing in the soil helps dewater excess soil moisture, provides some structural stability to the soil, and helps the mycorrhizae fungi recover. When using winter annual cover crops, these benefits can become very important if the next spring is a wet one. If the spring looks like it will be a dry one, the cover crops need to be killed in a timely matter so as not to use too much soil moisture.

Cover Crop Seed Mixtures

Cover crop cocktails, a mixture of several species and plant types, provide different rooting patterns and varying plant architecture to add diversity to the system. The diversity is valuable for building microbial and physical soil function. Mixtures also provide good soil cover across a variety of conditions as the different cover crop types respond differently to varying soil and weather conditions.

As an example, a mixture of a cool season grass (oats), a winter annual grass (cereal rye), a winter annual legume (hairy vetch), and a brassica (winter canola) would provide some quick cover in the fall, some nutrient scavenging, some nitrogen fixation, and more cover in the spring, while helping the soil system. Cover crop cocktails should be used as much as possible to accomplish multiple objectives.

Cover Crop Type and Time of Seeding

Cover crops are usually classified as cool season, warm season, winter annual, biennial, or perennial and as grass, legume, brassica, or other. Often times, cocktail mixes of the various types are used to ensure success or to achieve a variety of goals as the diversity of the mix adds to the benefits. Selecting a single cover crop, the seeding rate, or the amount of any type in a mix would depend on the goals and the time of seeding. (See Table 1 for some suggestions.) Most cover crops need at least 30 days of growth to start being effective and many should have 60 days of growth or more for the full benefits.

As examples, a cool season grass like oats would work if seeded as soon as possible after harvest since it will continue growing after a light frost and be killed by a hard frost closer to November. A winter annual grass like cereal rye or winter wheat could be seeded later, become established in the cool fall and continue growing once the spring warms up. However, it would have to be killed before planting the next crop. Mixing oats and rye would give some quick growth in the fall, some additional growth in the spring, and improve the soil building.

Further examples include seeding a cool season legume like forage peas or lentils now as they can survive a mild frost but will later be killed by a hard frost. Seeded later, a winter annual legume like Austrian winter peas or hairy vetch would be better, but would have to be killed in the spring before planting the next crop. Mixing a grass with the legume would provide more cover and increase the diversity.

While most brassicas are cool season annuals, they are most effective if they have 45 to 60 days before a killing frost to allow the tap root to penetrate deeper into the soil. They shouldn't be used as a single species cover crop because the residue breaks down rapidly, they don't fix nitrogen, and they aren't very mycorrhizae friendly. Many producers include them in cover crop cocktails as the seeding rate is fairly low and the benefits of the large tap root are great, if the killing frost comes later in the fall. However, after mid-September, the odds of getting much growth aren't very good.

If unable to seed any cover crops in the fall, a cover crop seeded in the early spring would still provide benefits. For a spring seeding, consider cool season grasses and legumes, like oats and forage peas, to help build the soil and manage soil moisture. Often these spring cover crops are seeded as soon as the soil conditions allow, before the spring rains really start. Brassicas, mustards, and other cool season broadleafs can be used in the spring, but they won't develop the large tap roots as if they were seeded in the early fall.

Frost seeding of oats, mustards, vetches, or clovers may also be an option, once the temperature drops and the soil firms up. These cool season cover crops will then germinate and grow in the spring, providing some cover and feeding the soil system. However, they may start growing early and be killed by a frost later because of the lack of residue to help protect them from temperature fluctuations.

Proper timing of a herbicide application will be needed to reduce the risk of using too much soil moisture in the spring. Tillage to kill the cover crop often negates the cover crop benefits and destroys soil structure. The tillage also reduces the residue cover and plants weed seeds. Thus, cover crops are best suited for no-till or ridge-till conditions.

Paul Jasa
Extension Engineer