Cover crops on prevented planting fields

Submitted by Sara Berges, Allamakee County Soil and Water Conservation District

The rain this spring has made planting difficult and, in many cases, impossible. The weather outlook isn’t looking good for the next 10 days.  More producers are starting to talk about prevent plant.  If you are unable to plant a crop this spring, a cover crop can provide many benefits to help reduce weed pressure, reduce soil erosion, and reduce the risk of fallow syndrome.  The cover crops can also take up nitrogen, build organic matter, and hold potassium and phosphorus in place. There are many things to consider when choosing cover crops.  If an herbicide has already been applied, look at the restrictions. You don’t want to spend the money on cover crops only to have them fail due to your herbicide, so do the research and find a cover crop that will work with your herbicides.  

If your next planned crop is corn, consider planting a mix heavy in legumes to fix nitrogen.  Also consider planting something with a lower C:N ratio. If your next crop will be soybeans, it would be better to have a mix with a higher C:N ratio, so heavy in grasses with some forbs like sunflowers and buckwheat. Determine what your goals are for the cover crop to pick what species will best meet those goals. Order your seed sooner rather than later.  Seed supplies are likely to get tight due to prevent plant and increased interest in cover crops. Also, mixtures tend to do better than monocultures and provide more benefits for the microbes living in the soil.

Keith Berns from Green Cover Seed has many recommendations for cover crops to meet your specific goals. If you want to suppress weeds, plant a crop that forms a canopy quickly to shade out weeds. Some suggestions are sorghum-sudan, forage sorghum, pearl millet, sunflowers, buckwheat and cowpeas. If you wait until late July or early August, you can add in cool-season plants like cereal rye, oats, barley, turnips, radishes, mustard collards and hairy vetch. Cereal rye and hairy vetch both contain chemicals that stunt weed growth.

If your goal is fixing nitrogen, use a mix dominated by legumes. The best summer annual species are sunn hemp, cowpeas, forages soybeans, mung beans, and guar. If planting later, you can plant cool-season legumes that will winterkill like spring peas, faba beans, spring lentils, and common vetch.  Options that could overwinter include balansa clover, winter peas, hairy vetch, winter lentils, and crimson clover.

If your goal is building soil organic matter, diversity is key. Some plants that produce a lot of summer organic matter are sorghum-sudangrass, sunn hemp, and sunflowers. Rye, triticale, and annual ryegrass are some of the better cool-season options. In order to build organic matter, it is important to maximize photosynthetic days. While sorghum-sudangrass will produce a lot of biomass early, it will kill with frost. Berns suggests mowing the sorghum-sudangrass in late August but not terminating it and drilling a cool-season blend with a high percentage of a winter-hardy species like rye.

If you want a grazing option, keep in mind that the cover crop cannot be grazed or harvested for forage until after November 1 if you receive a prevented planting payment. Be aware of prussic acid issues with sudangrass, sorghum-sudan hybrids, and forage sorghum. There may also be a potential for high nitrates if the summer turns dry. Berns suggests planting a sorghum that does not head out.

Another option is browntop millet because the millets don’t have prussic acid issues. Mung beans, guar, and sunflower can add protein to a mix. If planting in August, most cool-season cover crops like spring oats, spring barley, spring forage peas, collards, turnips, rapeseed (canola), and radishes provide excellent November grazing. Overwintering cereals like rye and triticale as well as legumes like crimson clover and hairy vetch provide less fall grazing but can provide spring grazing opportunities.

You can come up with a mix that meets your goals, but the most important thing is to get the ground covered.  There are tools on the Green Cover Seed and the Midwest Cover Crop Council websites to develop your own cover crop mixes. You can also call or stop by the Waukon NRCS/SWCD office to get additional help.

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