Cover Crop Basics: A Q&A with Gary Zimmer

Midwestern BioAg President Gary Zimmer has planted cover crops on his farm for years. In this Q&A, we sit down with him to learn the basics of cover crop management and the resulting benefits they bring to the farm.

How do I select a cover crop for my farm?

Zimmer: First, identify the purpose for your cover crop. Will it provide nitrogen or scavenge it? Do you want to use it for grazing? How about to build organic matter? Then, identify the cover crops that will best serve that purpose within your soil type and region. Choices can be limited. In this part of the Midwest, cereal rye is sometimes all we can plant following corn and soybean harvest. In cooler regions, varieties with winter hardiness and tolerance for cold temperatures and frost are key. In the south, drought- and heat-tolerant varieties with lower water requirements are a necessity.

Should I plant a cover crop seed blend, or a single variety?

Zimmer: It can depend on the time of year and, again, your purpose for the crop. It took me 30 years to learn this, and I’ll give it to you in one sentence: Plants determine soil life. The more plant diversity, the more soil life diversity there is. Plants extract minerals from the soil differently. So, the bigger the variety of plants, the bigger the array of minerals I’ve extracted, and the more diverse biology I have. Which, if you add that all together, leads to healthy, mineralized soils.

What is the best way to establish a cover crop?

Zimmer: Treat it like a cash crop. Drilling is a common method and helps improve seed-to-soil contact. If soil is really wet, drilling isn’t always an option. We’ve used a bulk spreader in those situations. On our farm, it works best to prepare the surface to ensure seed establishment.

Which variety should I try first?

Zimmer: Oats are great at scavenging nutrients, and they are easy to manage and terminate. Red clover and rye grass are two easy-to-manage, shade-tolerant cover crops that work well interseeded into corn.

If I want to scavenge nitrogen following corn, which cover crop should I plant?

Zimmer: Cereal rye is a go-to option to scavenge nitrogen following corn. If it’s too late in the fall to establish a cover crop, I would no-till in oats in early spring. Another option is to plant shorter-day corn. With advances in technology, we can grow a high-yielding crop and have a better chance of establishing a cover crop after harvest.

Do cover crops generate nutrient credits for next season’s crop? How does that factor into my fertility plan?

Zimmer: It depends. The stage at which cover crops are harvested is critical. They will trap nutrients, but the question is: How fast are they released? If the cover crop contains nutrients and it gets too mature, it turns into fungal food. At that stage, nutrient release is slow and plant residues are hard to digest. Fungal food has a high carbon-to-nitrogen ratio, meaning nitrogen will tie-up in the soil as residues break down. This is ideal before planting soybeans, because soybeans don’t require large amounts of soluble nitrogen, and the cover crop effectively scavenges nutrients away from weeds prior to planting.

If it’s a young, succulent cover crop, it becomes bacterial food and the resulting nutrient release will be much faster and more available that season. This management approach is best before planting corn, which needs more soluble nitrogen and other nutrients. You won’t get as many credits the first year of planting cover crops as you will on a long-term basis. If you plan to add cover crops into your rotation, you have to commit to them for about three years to gain the full benefits. Cover crops have a learning curve — expect to make mistakes while learning how to manage them.

Do I need to fertilize a cover crop?

Zimmer: Yes, whether it’s manure or commercial fertilizer, fertilize right in front of your cover crop. We want maximum plant growth and sequestration of minerals in the carbon-biological cycle. When the cover crop is terminated, incorporated into the soil, and combined with nutrients from manure or fertilizer, it provides fertility for the next crop.

I’ve heard that some cover crops can tie-up nitrogen in the soil if not properly managed. Can I prevent this from happening?

Zimmer: Many people think that the bigger the cover crops get, the more nitrogen and benefits they provide. However, as the cover crop grows, the carbon-to-nitrogen ratio increases and the digestibility of the plant material decreases. This can reduce seed-to-soil contact at planting and affect germination rates. It’s important to either harvest cover crops or put them back into the soil at the right stage to control the areas of soil biology that you want to improve. Feed your soil like you would feed a cow. You want it to produce? Feed it highly digestible, highly soluble feeds harvested at the right time.

What cover crop(s) should I plant as supplemental forage?

Zimmer: Summer grasses, small grains, spring triticale and rye grass are options for late-summer planting in fields rotating out of corn silage or wheat. Last year, we planted an 85-day silage corn. Our workload was done earlier, and we had time after harvest to do some tillage and establish a cover crop. In the beginning of August, we fertilized and planted our grazing cover crop. It was grazed from mid-October until Thanksgiving. Harvesting was an option, but I worked it into the soil because I wanted to capture nutrients from the livestock manure to feed the soil.

We had a lot of rain early this summer, and we weren’t able to establish corn in some fields. Should I plant cover crops in these fields?

Zimmer: Yes, keeping soil covered helps build soil health. So, for producers with goals of driving yields, establishing cover crops in those fields is the natural decision. It will save money on pesticides and insecticides, and can reduce nitrogen needs for the next crop. Overall, I would expect better crops during the next rotation in that field.

What is the best way to terminate cover crops?

Zimmer: Cover crops feed soil life. If I terminate cover crops with spray, we’ll still get some plant nutrients back into the ground, but everything above ground can no longer feed the soil. The ideal situation is discing-in the cover crops, or shallowly incorporating them into the top soil. o

The USDA’s Environmental Quality Incentives Program (EQIP) offers financial assistance to growers meeting NRCS cover crop seeding requirements. Growers can receive up to three annual payments, with rates based upon the seed mixture used. Learn more at