March 08, 2021

Research backs cover crops’ effectiveness

MACOMB, Ill. — The extensive research is there for real data and the technology is available to do effective and efficient fieldwork, yet there’s been no widespread use of cover crops across the Prairie State.

“There is still some resistance to planting cover crops. Cover crops are not a dominant practice in Illinois,” Joel Gruver, Western Illinois University associate professor of soil science and sustainable agriculture, said in a recent Illinois Soybean Association webinar.

Despite the slow acceptance of cover crops, Gruver said there are multiple reasons to implement the system. He began with the Illinois Nutrient Loss Reduction Strategy, a voluntary initiative with goals of reducing total nitrate and phosphorous losses into waterways by 45% each by 2035.

The strategy also includes and interim goal of a 15% reduction in nitrate and 25% reduction in phosphorous loss in the next four years.

“Actually we’re not doing that well. Obviously we’ve made all sorts of progress in terms of technology in agriculture, but when you take a look at the measurements of nitrate and phosphorous loads, we actually have increased about 7% for nitrate loads and 26% for phosphorous loads statewide,” Gruver said.

The data, released last year, compares recent load levels to the baseline of nitrates and phosphorous released into the Mississippi Watershed from 1980 to 1996.

Point sources such as sewage treatment systems have fared much better. Since 2011, there’s been a 14% reduction in nitrogen losses from point sources and a 22% reduction in phosphorous losses.

From a soil health and erosion standpoint, the Illinois Conservation Transect Survey that identifies soil loss, also reports the state is losing soil in most areas.

The Science

Gruver referred to the work of Shalamar Armstrong, one of the leading cover crop researchers while at Illinois State University and now at Purdue University.

While at ISU, he conducted field trials to determine if cover crops can improve the efficiency of fall-applied nitrogen within conventional Midwestern cropping systems.

There was a 49% reduction in the nitrate load when spring anhydrous was applied with cover crops and a 46% loss reduction for fall-applied anhydrous into cover crops.

Armstrong also led in a landmark study on the impact of widespread cover crop adoption throughout a watershed. The research compared a watershed with a high level of cover crops to a similar watershed with low levels of cover crops.

“What they were able to show really for the first time was that there is a clear effect of cover crops if you have the great majority of fields in a watershed planted to cover crops. Where you have cover crops you have substantially less nitrate being lost through the tiles,” Gruver noted.

Armstrong’s most recent project involves precision planting cover crops in 7.5-inch rows that drops the amount of cover crop seed for a field in half. The standard seeding rate is used in the drill and half of the rows in the planter are not used. The cash crop is planted between the rows of cover crops.

“They found it reduces nitrogen immobilization, has non-intersecting growing zones for cover and cash crops, has the potential to reduced seeding rates, gives flexibility with spring termination and could increase spring warm-up in the cash crop growing zone,” Gruver said.

“The other key thing, maybe the most interesting, is the cover crops are very responsive to having that extra space and so with half as much seed you can actually grow an equivalent amount of biomass to drilling every row, and you can accumulate an equivalent amount of nitrogen in the rye with only growing half as many rows.”

Research in Iowa focused on the impact of planting corn into cereal rye and the potential for a negative impact on the crop.

The Iowa research confirmed the potential of rye cover crops to elevate disease pressure on corn seedlings when conditions are cold and wet in the spring.

Among several potential corn seedling pathogens, organisms belonging to the genus Pythium appear to be the most important in causing corn seedling disease after rye cover crops. The research also found that seed fungicides that contained the active ingredient metalaxyl were the most effective.

Crop Health

Gruver was involved with a project about 10 years ago looking at the impact of cover crops on cash crop health, focusing on soybeans.

Findings included that cereal rye and rapeseed improved soybean stands in plots inoculated with rhizoctonia solani and decreased levels of soybean cyst nematode in the soil.

Cereal rye increased soil suppression of rhizoctonia and fusarium as measured in greenhouse bioassays. Cereal rye significantly improved yield when rhizoctonia root rot was a problem.

Using cover crops repeatedly, in the same field, may achieve more distinct effects on suppressing soybean diseases and build-up beneficial properties in the soil.

“This isn’t an effect that we see all the time, but it is one of those finer details that when we drill into how cover crops work in our soybean systems we see that we can have some beneficial effects in terms of disease management,” Gruver said.

A University of Maryland study investigated the impact of radish and cereal rye cover crops ahead of corn and soybeans. Consistently where radish was grown preceding corn, there was a higher number of corn roots at depth and there was a smaller effect on soybeans. There was an even smaller impact where rye was planted and consistently less root density where there was no cover crop.

“They also measured soil moisture and found consistently that when the preceding cover crop was radish they found more moisture being extracted from the subsoil in both corn and soybeans. So, that was an indication that more roots were getting to depth and also more effective at removing soil moisture,” Gruver said.

He added that radish is a cover crop that has been misunderstood over the last decade.

Giant radishes as a result of early planting are not needed to be effective in fields. Fine radishes have roots 3 to 4 feet deep that create pathways that the following cash crop’s roots can follow.

“If you plant radishes that are able to get even one-half inch in size, you typically have roots that have grown several feet deep and that can beneficially impact the following cash crop roots,” Gruver said.

“Radishes are also really good at extracting phosphorous. We’re not talking about new phosphorous being applied. We’re talking about phosphorous already in he soil being made more plant available by growing the radish cover crop.”

Multiple Species

New research at Penn State shows that multiple species can be used together to change the level of nutrient access.

“We can increase nutrient uptake by having more than one cover crop species. We can increase water uptake by having more than one cover crop species,” Gruver explained.

Cover crop objectives must be aligned with realistic establishment options.

“The most straightforward timeframe for planting cover crops is after harvest, but that also is the time that has the least opportunity for growth. So, we need to think through our possibilities and then come up with realistic establishment options,” Gruver said.

“We can choose earlier-maturing cash crops. We can think about where on our farm we are likely to harvest the earliest. Basically pick out the situations that are most likely to make our cover crops success.

“One of the things cover crops do that is maybe under appreciated is they can help us reconnect our topsoil and subsoil. That disconnection really interferes with access to subsoil moisture and nutrients, particularly subsoil moisture. When we grow cover crops effectively, we are reconnecting our topsoil and subsoil and we’re also helping repair hydrological function.

“We’ve been having more extreme rainfall events in the last decade and if we can improve our connectivity between to topsoil and the subsoil we will be repairing our hydrological function — basically allowing water to move more quickly into our soils.”

Information Resources

There are numerous sources available for further information on cover crops, including the Precision Conservation Management program through the Illinois Corn Growers Association and Illinois Soybean Association. The program focuses at both conservation and profitability.

Other sources include the Midwest Cover Crop Council, the Practical Farmers of Iowa Cooperators Program, the Iowa Soybean Association’s On-farm Network and University of Illinois Extension.

Tom Doran

Field Editor