By Sjoerd Willem Duiker, Penn State University Extension
Wheat and barley harvest is underway in many parts of the Northeast U.S. and oat harvest is not far away. If you do not plan to double crop with soybeans, there is a great window of opportunity to establish a cover crop.
Cover crops can be used for different reasons, such as: provide soil erosion protection; improve soil health; alleviate compaction; absorb nutrients from manure; control weeds; fix atmospheric nitrogen for the next crop; harvest for hay (lage); and as a grazing resource. The reason for using a cover crop will determine which species or mixture of species you choose, as well as how you manage it.
For example, farmers who use a cover crop only for soil protection and improvement typically look for economical options in terms of the seed costs or seeding rate. In contrast, farmers who count on harvesting or grazing a cover crop are typically willing to pay more for the seed and often use a higher seeding rate to obtain large yields of high feed quality.
Farmers who count on nitrogen fixation may be willing to pay more for a cover crop because they can reduce nitrogen fertilizer applications for the following crop. At this time of the year you have many options for cover crop species:
Summer Annual Grasses
Summer annual grasses such as sorghum, sorghum/sudangrass, sudangrass, pearl millet, Japanese millet, should be planted as soon as possible. These grasses grow fast at warm temperatures and in the presence of moisture so should be planted as soon as possible.
They will winterkill at temperatures below 32 Fahrenheit. They provide excellent soil protection until that time, improve soil health and alleviate soil compaction through their fibrous root systems, absorb nutrients from manure, compete vigorously with weeds, and can be used to be harvested or grazed.
Cool Season Annual Grasses
Cool season annual grasses such as oats and annual ryegrass. Oats and annual ryegrass can be planted until mid-September to provide soil protection, improve soil health, absorb nutrients from manure, compete vigorously with weeds, and can be used to be harvested or grazed. Both provide very good forage in the fall and large quantities of it.
Oats will die at about 17 Fahrenheit, which usually occurs in December. Annual ryegrass can survive the winter in the southern part of the northeast if its height is kept below 8 inches when it goes into the winter.
Winter Hardy Cool Season Annual Grasses
These are our ‘traditional’ cover crops, and include wheat, barley, triticale and rye. However, it is not recommended to plant them before September.
Summer Annual Legumes
Summer annual legumes fix atmospheric nitrogen but are not recommended for many other purposes. They include cowpeas, sunnhemp and forage soybeans that should be planted as soon as possible because they thrive at warm temperatures.
Make sure to inoculate cowpeas and sunnhemp with cowpea/peanut rhizobium inoculant, and soybeans with soybean inoculant. Both fix their atmospheric nitrogen. Forage soybeans are viney types that produce much more biomass than our grain soybeans so are more suited to improve soil health produce biomass.
Cool Season Legumes
Cool season legumes are often included in mixes to fix nitrogen as well. They include Austrian winter pea, crimson clover and hairy vetch. They need to be established before mid-August in central, and before early September in the southern part of northeastern U.S. to make it through the winter.
In fact, it is better to wait until August to plant them or they may start to become very rank in the fall and then winterkill. In fact, it is better to plant spring peas now because, although they will die at about the same time as oats, they will put on more growth than the winter varieties.
Other broadleaves include buckwheat, rape, and radish.
Buckwheat is a fast growing broadleaf that will flower in 6 weeks at which time it should be terminated to avoid it from setting live seed (if that is a concern). It is a great weed suppressor due to its fast growth. Buckwheat can be grown in low-fertility soils.
Rape is winterhardy in the southern part of northeastern U.S. if established before September 15.
Radish dies at 17 Fahrenheit like oats and can be established right now so it puts on a lot of growth. Brassicas like rape and radish thrive in high-fertility situations. They have taproots that can create large macropores in the surface 4 to 8 inches of the soil.
The species we have reviewed can be used in mixtures. In fact, there are many advantages to using mixtures instead of single species. You look for species that complement each other for the purpose sought.
Although it has become popular to promote ‘more species is better’, we have found that that is not always the case. Some species can outcompete and suffocate all others.
An example is radish planted at 10 lbs/A at the end of July, or rye planted at 1-2 bu/A in September. The radish will suffocate all other species in the fall, and because it winterkills, the field will be bare in the spring. In this case it would be important to drastically reduce the seeding rate of radish in the mix to only 1-2 lbs/A.
Rye planted at 1-2 bu/A will result in a mix totally dominated by rye in the spring. Therefore, it is also important to reduce the rye seeding rate in the mix or use another species such as wheat or triticale.
More information about species selection and how to compose mixtures can be found in the Cover Crop Chapter of the Penn State Agronomy Guide.