If you're just getting into cover corps and you planted turnips or radishes early enough in the fall, you probably saw lots of growth. That is an impressive sight. But Jamie Scott, Pierceton, who has no-tilled and done pioneering work with cover corps, says brassicas, although they have a fit, may have been oversold as cover crops compared to other choices that are available.
His comments apply at least to those areas of Indiana where fall crops like radishes or turnips usually winterkill. Once that happens, the top growth dies, releasing gases that often resemble a propane gas leak. That may be an annoyance if your neighbors call the fire department, thinking there is a real emergency, but it's not the real reason Scott advises being careful with how you manage brassica cover crops.
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"If the plants winterkill relatively early, you can get a situation where the nutrients that they have scavenged form the soil and tied up are released more quickly than you like," he says. "Unless you have annual ryegrass or something else in the mix that stays alive and can recapture and hold those nutrients upon release, you may lose at least part of them. The whole idea of cover crops is to capture nutrients, both to help future crops at some point, and also to help nutrients from moving off your farm and becoming an environmental hazard somewhere else."
Nitrogen is the main nutrient that could be released too quickly if there's no growing cover left to recapture it after the brassica crop dies, Scott says. Nitrates heading downstream all the way to the Gulf of Mexico are blamed for hypoxia issues in the Gulf. That refers to a lack of oxygen in the water that affects the type and amount of fish populations that can flourish there.
When Scott uses a brassica, it's usually rapeseed. It has a better chance of overwintering, he says. Winterkill of some cover crops is why he uses blends, to make sure a green crop will still be there by spring.