Exploring advantage, disadvantages of summer cover crop options

According to Iowa State University Extension, warm-season annual grasses, such as sorghums, millets and teff grasses, are usually planted from mid- to late-May through early July or when the average four-inch soil temperature is 65°F and increasing. They are typically used for two to three months during summer and autumn. Most are ready for first harvest or grazing about 50 days from emergence, and if managed properly, often result in more than one forage crop per year.

June 7 of this year, Penn State University Extension provided a list of potential summer cover crop options to consider this summer.  They posted this in Seeding Practices, Soil Health on the Extension website, https://extension.psu.edu/summer-cover-crop-options.

Sudangrass is a standard summer cover. Seeded at 30-50 pounds per acre, it tolerates heat and produces large amounts of organic matter (8,000-10,000 pounds per acre of dry matter). It needs warm soil to germinate and so it is usually planted between June and August. After six to eight weeks, producers will likely want to mow it (18-30 inches tall). This will keep it from becoming too woody. Mowing will also cause a flush of food for soil organisms as roots temporarily die back. Some varieties have been shown to suppress root-knot nematode and soil-borne pathogens when green material is mowed and quickly disked into the ground.

Those planning to try to use sudangrass or others below as a biofumigant should make sure they know the details to make this effective. However, there are disadvantages. Large amounts of plant material, which are good for building soil and suppressing weeds, can be difficult to integrate into the soil or cut through with a no-till drill. Another disadvantage is that sudangrass can produce toxic levels of hydrogen cyanide and nitrate when young, drought stressed or just after a frost. Do not graze at these times.

Pearl Millet
Pearl millet is much less commonly used as a summer cover, but one that has good potential. Two years ago it was planted by Penn State University Extension on a farm in Southeast Pennsylvania July 21 at 11 pounds per acre and got good growth. It dealt with severe drought and heat. It is also a warm season grass, so wait for the soil to reach 65-70° F before planting. Target seeding rate is 10-15 pounds per acre drilled a half-inch deep. It is often not thought of to fertilize cover crops, but the recommendation is to fertilize at 70% rate of corn.

This nitrogen should be captured by the millet and released again as the residue breaks down for the next crop. It is also recommended to mow millet when it reaches two-and-a-half feet. It can reach 12 feet high if not mowed. Advantages include: potential to suppress soil-borne disease, high organic input, drought resistance and winter kill. Disadvantages include: more expensive seed and difficulty of incorporation.

Buckwheat is another summer standard. It is known for quick growth that suppresses summer annual weeds and improves soil aggregation. Drill buckwheat at 50 pounds per acre (broadcast 70 pounds per acre) one inch deep. Mow or disk less than ten days after the beginning of flowering (35 days). Make sure to not plant after using herbicides with the following active ingredients on preceding crops: atrazine, imazethapyr, halosulfuron, fomesafen.

Sun Hemp
Another interesting summer cover crop is Sun Hemp. Sun Hemp (Crotalaria juncea, not related to other hemps) is a fast growing tropical legume. It will not do well in the fall. It needs heat. Advantages include: substantial biomass, the ability to fix nitrogen and potential to kill soil-borne diseases and nematodes.

But the cost is generally prohibitive. Drilled at 30-40 pounds per acre with seed cost of over $6 per pound, it can be more than $200 per acre to seed. This might come down over time if they are successful in producing seed in the south. Currently, most of the seed available is from Hawaii.

Other cover crops to consider for a summer window include: sunflower; Japanese millet - very fast growth to maturity, but ability to be weedy; cowpeas; yellow sweet clover; and red clover.

Margaret A Smith, PhD. and Albert Lea Seed Forage Agronomist, writes in Albert Lea Seed Agronomy Blog May 26, 2022: “Where forage is still needed, the best option to supplement alfalfa yields at this time is a warm-season, annual forage grass.”

She says that warm-season, annual forage grasses do best when seeded 1/2” - 1-1/4” deep. They can be no-till drilled into the alfalfa following the first hay cutting. Attempts to broadcast on an undisturbed soil surface and dragging to incorporate, or seeding with a Brillion-type seeder will be much less effective.

Smith notes that Teff is an exception; it has very small seed, so needs to be planted only 1/8”-1/4” deep and needs a firm seedbed. A Brillion-type seeder can work fairly well in an established hay field.

According to Iowa State University Extension, Teff, or summer lovegrass, is becoming a relatively popular choice for a summer grass hay crop. Teff is a warm-season, annual grass that has grown reasonably well in Midwest locations. It establishes relatively quickly and is harvestable in 45-50 days, with multiple harvests possible. To maximize regrowth, leave at least four- to five-inch stubble height at harvest. Seed sources are limited and the small seed requires a well-prepared seedbed for successful establishment of the crop.

“If alfalfa stands are at less than 50%, adding grasses to the stand will increase nitrogen fertility needs.” Smith states. “Warm-season grasses will use about 30 pounds of nitrogen per ton of forage dry matter produced. Some of that will be provided by the alfalfa, with less from more depleted stands. We suggest applying 30-50 pounds nitrogen at planting and, if moisture conditions are favorable for regrowth, follow up after the first grass cutting with an additional 30-40 pounds/acre.”