Cover crops are equally effective as restored prairie in limiting ex­cess nutrients from reaching Iowa’s streams, according to a research study reported by Iowa State University.

Cover crops are very effective in reducing nutrient concentrations from tile-drained water that could eventually flow into waterways, said Marshall McDaniel, associate professor of agronomy at ISU, who led the study.

“(This) means cover crops have a big impact on water quality,” McDaniel said. He encourages farmers to look for programs and initiatives that can help offset cover crop costs.

“Talk to university extension, co-op agronomists and other farmers already using cover crops about using them successfully on your farm,” McDaniel said.

Sensitive to change
The research study analyzed how stream history (low, medium or highly impacted streams) and land management (conventional cropping, cover crops or restored prairie) affect water quality measured for eutrophication (excessive nutrients), as it relates to algal growth and oxygen depletion.

Subsurface tile drainage is used in more than 40% of Midwest farm fields to improve yields, but tile drainage sometimes bypasses natural buffers as water is transported into nearby streams, the study says.

Streams also have unique characteristics, including historical, prior nutrient loadings, which can affect how they respond to any land-use changes in a watershed or excess nutrients, McDaniel explained.  

“These low-impacted streams, or ‘pristine’ for Iowa standards, are more sensitive to changes in management and had greater increases in algae growth when tile water from conventional cropping was added,” McDaniel said.

For the study, water was collected from Iowa streams based on historically high, medium or low impacts from nutrients.  
Researchers sampled water from subsurface tile under test plots managed with three different treatments (a corn-soybean rotation, cover crops and restored prairie).

Less Algal Growth
In a laboratory setting, the water from tiles and streams were mixed and incubated for two weeks before analysis, and changes in algal growth were monitored.

When the results came in, the tile-drained water from cover crops and restored prairie caused less algal growth. This is good news for farmers and the state investing in these practices, researchers said.

“What was not really anticipated was that the cover crop performed almost as well as the prairie overall,” McDaniel said.

Test plots with a winter rye crop showed reduced algal growth by 43% in water drainage compared to conventional corn-soybean rotation plots. Restored prairie area drainage saw a 46% reduction in growth compared to a conventional system, the research study showed.

Matt Helmers, director of the Iowa Nutrient Research Center, said while there have been many studies documenting nitrate re­ductions with cover crops at their drainage water quality plots, this study adds additional information on the benefits of cover crops relative to water quality.

“Cover crops are an important in-field practice for protecting the soil and improving water quality that we hope to see in­creased use of in the future,” Helmers said.