As I’ve pointed out previously, there’s a lot of conflicting research when it comes to the effects of cover crops in farm regions where annual moisture is limited, especially around the issue of water use by covers.
I came across another piece of research organized this year by Kansas State University as part of an ongoing investigation of potential cover-crop applications across the state. In this case, Kansas State agronomists Kraig Roozeboom and Gretchen Sassenrath began tackling the issue of water use with single species vs. complex mixes of covers.
In central and western Kansas, some no-tillers are concerned that covers seeded after harvest will deplete too much moisture from the soil profile ahead of the next crop, potentially hurting plant development and yield.
“There have been a lot of claims out in the countryside that complex mixes somehow produce all these benefits but use less water than single species, for some reason. So we set up an experiment to look at that,” Roozeboom said in a recent podcast.
Researchers compared the results of seeding single and mixed species of cover crops after wheat harvest, terminating the covers in fall and planting the fields to corn the following spring. A control field was left in chemical fallow.
Researchers seeded single species of grass (sorghum- sudan and pearl millet), legume (clover and sunn hemp) and brassica (rape and radish) covers, as well as component mixes of the three species, and a more complex non-component mix that had all those cover crops and more.
Here’s what they found:
• Researchers measured soil water depletion from all cover crops, whether mixed or single species, compared to fallow. While there was some moisture recharge during the winter, the water removal was still detectable in spring.
• Brassicas extracted water from very deep in the soil profile, more than with other covers. Corn yields were more negatively affected after brassica and grass covers, but corn following legumes was equivalent to chemical-fallow plots.
• Citing water-use data, Roozeboom says he found no evidence that cover-crop mixes use less water than single species, although he added it’s possible that mixes bring characteristics of various species and that keeps some components from dominating.
The situation is a little different in southeastern Kansas, where typically plentiful spring rainfall and high amounts of no-till residue potentially make it harder for fields to dry out ahead of planting. Sassenrath says there’s potential to use covers to extract water from the soil profile at critical times of the year so growers can keep the benefits of no-till and still get planting done on time.
“We have shallow soils with high clay content that holds water, but it’s not available to the plant,” Sassenrath says. “If we change the soil structure with different covers, we can make more water available and crops won’t be as sensitive to droughts or high rainfall.”