There’s no one-size-fits-all cover crop, and selecting the right cover crop mix may seem a bit overwhelming.
Depending on the mix, cover crops can provide many benefits below the soil’s surface and above. It just depends on what your goals are, said Anthony Bly, soils specialist with South Dakota State University Extension.
“Understanding what you want to do with cover crops is very important,” Bly said. “By identifying your goals, landowners are able to select the most appropriate mix that will provide the greatest benefit and comply with herbicide laws without spending more than necessary or having a negative impact on next year’s crop.”
Goal: Build organic matter
Overall, cover crops enhance soil health because they extend the amount of time a living root is on the landscape, Bly said.
“The living root is probably the most important reason to plant cover crops,” he said.
In a typical cropping system — corn, soybeans, wheat or oats — there is a segment of time when there is no living root in the soil. By extending the timeframe a living root is in the soil, cover crops also increase soil organic matter, said Jason Miller, conservation agronomist for South Dakota Natural Resources Conservation Service.
“Plants harvest sunlight. When you grow plants longer, it increases organic matter on the surface and in the soil,” Miller said. “This does not happen overnight. We don’t increase organic matter by 1% a year. So, your goal to increase organic matter needs to be a long-term goal.”
Full-season cover crops tend to build soil organic matter quicker — as long as they are not hayed off.
“Anytime we remove residue with machinery, we are likely going backward in the overall system from the standpoint of organic matter,” Miller said.
However, results differ if cattle graze cover crops. Miller explained that in most cases cattle recycling cover crops increases the amount of organic matter built up in a season.
Goal: Break up soil compaction
Although it may seem like tillage breaks up compaction, the weight of the implement combined with the physical force of iron dragging through the soil actually creates compaction, Bly said.
Cover crops help break up the “tillage pan” and more.
“As the roots grow through the tillage pan, they fracture it,” Bly said. “As aggregation takes place along the living root, it offsets the negative effects of compaction.”
The aggregation, or clumping of soil caused by living roots, allows for increased water infiltration and air movement through the soil.
Although all cover crops aid in breaking up soil compaction, Bly said some are better at breaking up compaction than others.
“We tend to think of tap roots as doing a good job with defined compaction layers,” Bly said. “The radish has the largest tap root, but others to consider include sunflowers and other oil seeds like rapeseed, canola, soybeans as well as clovers.”
He added that fibrous root systems do well with soil with thicker compaction layers near the soil surface.
“Sorghum sudangrass is probably the best. Other millets are good as well as cereal rye and triticale,” he said.
Goal: Weed suppression
To understand how cover crops aid in weed suppression, Eric Barsness, conservation agronomist for South Dakota NRCS said it is a good idea to take a few minutes to consider where weeds flourish.
“During the growing season they germinate and grow where the crop canopy is open, allowing sunlight to get to the soil, or after harvest if residue is not covering the ground,” he said.
Cover crops suppress weeds by providing soil cover.
Cereal rye is Barsness’ top recommendation for a cover crop species to suppress weeds in a corn-soybean rotation.
“In addition to providing quick soil cover, this species also has some weed control tendencies,” Barsness said.
For best results, Barsness said to aerial seed cereal rye into standing corn the last week of August or first week in September at a rate of 60-pounds-per-acre. Drill soybeans into rye the following spring. Spray down the rye right after planting.
“The rye allows for a nice mellow seedbed,” he said.
Goal: Extend grazing season
Certain cover crop mixes work well to extend the grazing season and increase available forage, explained Garretson, South Dakota, farmer Austin Carlson.
“Prior to utilizing cover crops, we were always running short on grass mid-to-late August,” Carlson said. “Cover crops allow us to rest our pastures and put our cattle to work spreading their manure for us.”
Match the season with the cover crop mix, Carlson said.
“Why plant a warm season cover crop in the late fall?” he said.
Goal: Nitrogen fixation
Nitrogen fixation occurs when legumes absorb nitrogen (N2) gas from the air and with the help of rhizobium bacteria, their root nodules convert the N2 gas from the atmosphere into a readily available form plants can utilize.
Because each legume depends upon a specific rhizobium bacteria to convert atmospheric nitrogen into a form that is readily available to plants, Bly encouraged growers to consider purchasing inoculated cover crop seed.
“Our soils are full of native rhizobium, but not necessarily specific to the legumes you are planting. The rhizobium bacteria allows the biochemistry process to take place in the root nodule,” Bly said.
Seed companies selling legume cover crops, like clovers, sun hemp, cow peas and vetches, will also have inoculants available.
Goal: Wildlife habitat
Habitat loss is a big issue for wildlife — especially pheasants and other birds. Cover crops can help, Barsness said.
“Certain cover crop mixes can provide that needed habitat for birds to nest, catch insects and seek refuge from prey and weather extremes,” Barsness said.
When selecting cover crop mixes for wildlife habitat, Barsness said it is important to select a mix with an abundant mix of plants with strong stalks.
“You want them to stand up over the winter and not lay down flat,” he said. Examples include Ethiopian cabbage, millets and flax.
In addition to taller, strong-stalk species, ground cover is also important to ensuring plentiful insects and nesting habitat.
“Cereal rye, winter wheat and triticale go deeper into the fall, provide good cover and grow nicely in the spring,” Barsness said.
For all purposes, diversity is key.
“Some think that if they have a diverse cover crop mix it becomes more expensive and complicated. I have found just the opposite,” Barsness said. “The more species in a blend, the better the blend does because the species complement each other.”