By Alisha Bower

Diverse cover crop mixtures or “cocktails” entice farmers with the promise of greater ecosystem benefits for their farm than a monoculture of one cover crop variety. In theory, carefully selecting species that serve complementary functions takes full advantage of the potential of cover crops. For example, mixing N-scavenging and N-fixing plants maximizes nutrient availability in the soil. But these big mixes can be pricey, especially if they include nitrogen-fixing legumes. So when is it cost-effective to invest in these more expensive cover crop mixes?

New research from Penn State University put diverse cover crop mixtures to the test to answer this question. In a study recently published in the Agronomy Journal and authored by Ebony Murrell and colleagues, 10 different cover crop treatments were examined, combining or relying solely on cereal rye, oats (grasses), red clover, Austrian winter pea (legumes), forage radish, or winter canola (brassicas). Over three years in a wheat-corn-soybean rotation, these 10 cover crop trials were seeded after corn in September or October and after wheat in August. Results show that diverse mixtures, while very beneficial when planted in August after a small grain like wheat, are not achieving their potential when planted after corn (September-October).

Comparing cereal rye to mixtures
In the trial, cereal rye was the consistent star performer for spring biomass, regardless of when it was planted. When it was included in a diverse mixture planted in August, though, canola, winter pea and red clover maintained decent populations in the spring. When those same mixtures were planted in September or October, diversity in the spring cover crop stand was nearly nonexistent. In short, diverse mixes are effective when planted in August, but don’t perform much differently than cereal rye monoculture when planted in September or October.

Crop adviser and farmer Will Glazik from Arrowsmith, Ill., has found this to be true on the ground. He says, “If farmers are planting after corn in September or October, I recommend straight cereal rye or maybe a simple mix — cereal rye with radishes if we’re planting in September or cereal rye with rapeseed if it’s a later planting in October.”

When it comes to legumes, Glazik is cautious, “I try to keep legumes out of it so people don’t get their hopes up and are let down. Legume seed is expensive! Legumes really need to perform to make it worth the investment. Anymore, I only recommend legumes when there’s wheat in the rotation, and covers are underseeded or planted in August after harvest. Using this method, some farmers can completely cut out nitrogen fertilizer applications, especially if they let the legume mix grow for another year.”

Gary Cooper, a farmer at Brook, Ind., agrees: “With legumes, the big thing is you have to be patient if you’re going to count on it for nitrogen. It’s not fixing until it’s blooming, so if you kill it on the 15th of April, you’re not getting the nitrogen fixation. Why are you going to spend the money on expensive seed if you’re not going to get the full benefit out of it?”

Because of this, Cooper favors a simple, winter-hardy grass and brassica mix like Glazik for fall planting: “Adding the rapeseed to the cereal rye has a lot of benefits. It will help cyst nematode in soybeans and it gives you another kind of rooting. I only use about ¾ pound per acre so it costs me almost nothing. If it winter-kills, I’m not out much, but if it makes it through winter then I get the benefits.”

Selecting cover crop mixes for grazing
If livestock are added to the system, a farmer has slightly different considerations when choosing cover crops. Nutritional quality, compatibility with herbicide residue requirements and even palatability to animals’ tastes become relevant to designing the perfect mix.

Iowa farmer Nathan Anderson, near Aurelia, runs cattle on his cover-cropped corn and soybean fields in the fall and spring, adjusting his mix to the time the field will be grazed: “We like winter rye and spring forage barley, varying the rates of these two species in the mix depending on when we’ll graze it. Fall grazing fields [corn stover] gets more barley than rye, and spring fields [soybean stubble] have more rye and less barley. Then we mix in a brassica, primarily rapeseed because it’s really cold-tolerant, and sometimes we mix in brown mustard. If you’re grazing it in fall, you get a really high forage quality because the brassica balances the diet against the cornstalks.”

Like Glazik and Cooper, Anderson avoids legumes in his fall mix. “A lot of herbicide labels list canola or rapeseed, but they don’t list legumes,” he says. “They fall into ‘all other crops,’ which require a year or 18 months before you can graze.”

When Anderson plants cover crops in summer after his small grains, he takes full advantage of mega-diverse mixes. He planted a 16-variety blend last year that ran the gambit of grasses, brassicas and legumes. Again, he had to choose the varieties with herbicide labels in mind and he also considered nutritional value for cattle. “My intent with our mix was to graze it and leave a lot of residue on the surface, making sure I had enough carbon so the residue didn’t just disappear. Next year I think I’ll add some more fiber, maybe millet. This would have balanced the diet more and kept that residue on the field,” he says.

In this case, Penn State and farmers concur: If you’re adding other species to a winter cereal or grass planted in September or October, it’s got to be cheap or winter-hardy. In most cases, this means opting for a brassica instead of a legume, saving the super diverse mixes with big nitrogen fixing power for summer planting, or underseeding legumes like red clover with a small grain in the spring during planting when they can really make a dent in synthetic nitrogen applications.

For more information on cover crop variety selection, visit practicalfarmers.org/cover-crops and the Midwest Cover Crop Council mccc.msu.edu.

Bower is Midwest cover crops associate for Practical Farmers of Iowa in Ames. Contact her at [email protected].