Timing, species choices key to cover crop success

Agent outlines pluses and minuses of multiple species

Ray Mueller


Dane County Extension Service crops and soils agent Heidi Johnson is confident that her recommendations on the rankings for choices of cover crops will turn out to be the best choices in most situations.

That's because the recommendations are based on controlled research findings with cover crops with proven success in many counties around the state in recent years, Johnson assured attendees at the Fond du Lac County annual agronomy field day held on Montsma Farms.

Those cover crop species include tillage radish, crimson and Berseem clovers, red clover, triticale, and grass species planted after the harvest of corn silage, Johnson pointed out. Related projects have the interseeding of cover crop species into standing corn and soybeans, the effect of herbicide carryover on cover crops, and the aerial interseeding into seed corn fields, she added.

Purpose of cover crops

When discussing the topic with farmers, Johnson likes to ask why they intend to grow cover crops. If there's a hesitancy to answer or if erosion control is not cited as a major purpose, she has doubts about the situation.

Trying to solve soil compaction problems is not a valid reason to grow cover crops, Johnson suggested. She listed scavenging nutrients, growing nitrogen for use by a subsequent crop, improvement of soil health, and growing a forage for livestock feed as acceptable purposes.

Credit for what's been learned about growing cover crops in Wisconsin belongs to farmers who can be described an innovators, the agricultural research station at Marshfield, and farmer-led watershed protection groups, Johnson stated.

When selecting a cover crop, have a defined goal, fit the crop into the planned crop rotation, have an idea on what constitutes success, and control the costs, Johnson advised.

An early wave of cover crop promotion involved the growing of tillage radish – an effort which Johnson believes was oversold and which did not recognize or acknowledge that the providing of nitrogen to a following corn crop is not happening.

Small grain, grass rankings

The virtual one-half of the growing that remains after the harvest of winter wheat offers many cover crop options, starting with the growing of oats and spring barley, Johnson indicated. Those two small grains grow until a hard freeze, generate lots of residue, control soil erosion, and are suited for no-till practices, she observed.

Winter rye and winter triticale are other choices for planting in August and even September after winter wheat and which will provide a thick cover and begin to grow early in the spring, Johnson pointed out. The challenge with them comes at that point because they need to be terminated by late April unless the intent is to harvest them for forage, she noted.

At the bottom of Johnson's list among the grasses is annual ryegrass. It has the potential to act as a weed in the following year if it overwinters, particularly if its biotype is resistant to the glyphosate herbicide that would be used to kill an overwintering stand in the spring, she explained.

Legume lineup

Although the seed is expensive, legumes grown as cover crops offer a choice of harvesting as forage or to produce nitrogen for the next year's crop, Johnson indicated. To provide enough spring residue, they are often planted with a mix of grasses and/or brassicas and are a good fit in crop rotation of wheat, corn, and soybeans, she noted.

Among the clover varieties, Berseem is a fast grower provided there is enough moisture and is a species that will winterkill, crimson is more tolerant in dry conditions, is sensitive to improper soil pH, and will grow early in the spring if it survives the winter, and medium red clover is a short-lived perennial best suited for planting with a spring cereal grain or frost seeded in late winter into an established cereal grain, Johnson stated.

Field peas don't get much of a nod from Johnson because they're a cool season annual and fare best when seeded as a companion to a spring cereal grain that allows the peas to climb and minimize their lodging.

Brassica observations

For the brassica cover crops, Johnson lists radish (tillage, groundhog, oilseed, forage, and NitrDaikon varieties), turnips, and rapeseed but she warns that they should never be grown by themselves because they don't provide enough residual cover for controlling erosion. They have value by diversifying crop rotations, particularly in increasing the range of soil microbial activity, she notes.

Despite some promises and claims to the contrary, the research with radish shows that they do not solve soil compaction problems nor are they contributors to nitrogen for the next year's crop (usually corn), Johnson reported. It's true that radish take up as much as 100 pounds of nitrogen per acre but most of this disperses or leaches when the foliage rots during the spring thaw, she pointed out.

With radish, select varieties that are slow to bolt and have low portions of hard seed to avoid having volunteer plants the next year, Johnson advised. Turnips usually die during the winter but rapeseed is likely to survive into the spring and will have to be terminated with a herbicide, she added.

After corn silage

Because of their “awesome growth in the fall,” spring barley and oats are ideal choices as cover crops in southern Wisconsin following the harvest of corn silage provided that they can be planted by September 15 to 20, Johnson stated.

Do not consider planting legumes, radish, or brassicas that late, Johnson warned. An option with them or grasses is to consider aerial seeding a couple of weeks before the corn is harvested for silage, she noted.

After soybeans and corn grain

Cereal rye and triticale, which has better feed value than cereal rye, can be planted until about mid-October in southern Wisconsin, Johnson continued. Because both are winter hardy, with the rye being more so, they will need to be terminated in the spring either with a herbicide or by harvest before seed heads appear, she cautioned. In addition, be aware of any restrictions for harvesting a forage for feed stemming from herbicides that were applied during the previous two growing seasons, she stated.

Because of the limited time remaining in the growing season, Johnson does not recommend planting legumes or any of the brassicas following the harvest of soybeans. The same is true for oats, barley, and annual ryegrass because they are not likely to produce enough foliage that will serve as residue to deter erosion in the spring, she pointed out.

Both after soybeans or after corn grain if the stalk debris has been removed, Johnson does not recommend planting legumes or any of the brassicas. At most, she suggests experimenting with a timing schedule or particular species or combination of them on only a few acres rather than a whole field for one's first venture into growing a cover crop.

Related questions

Two related questions are the availability of nitrogen from the plant residue and any allelopathic effect (failure of other species to germinate or grow) on other species from cereal rye, Johnson indicated. She said research shows that the nitrogen taken up in rye foliage is not likely to be available to corn in the first half of its growing season and that there is a possibility of allelopathy in a few cases not only from rye but also with all small grains.

How much should farmers spend on a cover crop? In today's agricultural economy, Johnson proposes limiting it to $20 per acre but adds that “what farmers are willing to spend” is also part of the equation. On that point, she commented that the rather generous cost-sharing for growing cover crops provided through the Natural Resource Conservation Service might be misleading farmers on how much to spend.

With oats, bin run seed could be used in place of certified seed for a cover crop while a forage variety oats would be a good choice for boosting bio-mass growth in short period of time, Johnson observed. No matter what cover crop species is grown, the previous application of manure provides “a huge benefit,” she promised.

Soil health factor

There is very little research data on whether there are benefits to soil health from growing particular species of cover crops or green manure crops, Johnson reported. She indicated, however, that there can be merits in having high bio-mass species add organic matter and recycle nutrients, in how the fibrous roots of cereal grains and forage grasses can improve soil structure and alleviate a bit of soil compaction, and in how a multi-species mix might result in a synergy to provide biological and physical benefits to soil health.

In response to a question, Johnson said no problems have been identified with field tile due to cover crop plant roots and that on that point “they're no different than any other crops.” She suggested that any problems might be due to the tile themselves.

Johnson can be contacted by e-mail to Heidi.Johnson@ces.uwex.edu or by phone to 608-224-3716. Project summaries for all the cover crop research that she referenced are available on the http://fyi.uwex.edu/covercrop website.

Heidi Johnson