Cover crops keep nutrients on farmland and out of waterways. That’s a proven fact, according to several experts who made presentations at the recent Iowa Soybean Association’s research conference. Water quality and soil health improve by sowing cereal rye, oats, radishes and other plants just before or after harvest of corn and soybeans, studies show. Cover crops extend biological activity in the soil until spring planting of the row crops.

But there are still plenty of unknowns about this conservation practice. “There’s still a lot to learn,” says Scott Wohltman, agronomist with La Crosse Seed based at La Crosse, Wis. The company is conducting cover crop research in cooperation with ISA. Wohltman’s presentation focused on lessons learned about cover crops in 2016, how to integrate the practice into different cropping systems and future challenges.

Cover crop acres increasing
Cover crop acres are on the rise in Iowa. Nearly 475,000 acres were planted last year, according to Practical Farmers of Iowa estimates. That’s up from less than 10,000 in 2009. Acres planted to cover crops from seed purchased through La Crosse Seed increased by 33%, and pounds of seed sold almost doubled.

“The trend continues to increase. There were some issues with seed quality in 2016,” he says, referring to seed from several suppliers that contained Palmer amaranth, and many other nasty weeds. To make sure enough quality, clean cover crop seed is grown locally, Wohltman says a better reporting system of acres is needed to gauge how much seed the industry needs to produce.

He emphasizes that farmers need to buy cover crop seed from a trusted source.

“That’s a big thing in Iowa,” he notes. If a legume cover crop is planted, like winter peas or hairy vetch, he recommends that a fresh inoculant is added to the seed at planting for healthier roots and better nitrogen fixing.

Lessons learned by farmers in 2016
Planting brassica cover crops — forage radish, rapeseed, turnips, mustards, etc. — last year offered learning opportunities. Here are some key lessons learned:

• Seed didn’t germinate as well from aerial application.

• Radishes that get too big from planting too early can cause planting issues because holes in the soil could be several inches wide, hampering germination of the cash crop corn or soybeans you plant.

• Planting brassicas in mixtures with other types of cover crop seed too early increases competition, making it hard for the other cover crops to capture light and water.

Grazing cover crops is the easiest way to make the practice of planting cover crops pay the first year. When brassicas are part of the mix, Wohltman says farmers need to supplement cattle diets with hay or other high-fiber feeds to keep animals healthy.

ISA member Larry Marek of Riverside is a big cover crop proponent. He and his son planted 400 acres of cereal rye last year and use it for grazing. “The cows aren’t eating the hay and prefer the rye,” he says. “We may earn more money grazing cover crops than we earn on producing the corn and soybeans. If you are going to aerial-apply the cover crop, stick with rye.”

What ISA cover crop trials show
There’s a huge push to improve water quality statewide. The Iowa Nutrient Reduction Strategy — a science-based initiative to reduce nitrate and phosphorous loads in Iowa waterways by 45% from point and nonpoint sources — was started statewide several years ago to do just that.

“Cover crops are a big part of the strategy,” notes Theo Gunther, ISA Environmental Programs and Services resource management specialist. Based on Iowa State University research, NRS documents show a rye cover crop reduces nitrate loss, on average, by 31%. But corn yields may drop by 6% following rye. While final yield is a concern, other agronomic and environmental factors should be considered as well.

ISA resource management specialists are working with ISU researchers to use data collected from six fields using rye cover crops to model the cropping systems with and without cover crops. Tile water is monitored along with a host of other data such as weather, soil moisture, soil temperature, biomass, cover crop seeding rate, fertilization, cover crop termination date and soil organic matter.

One site had more than 80 pounds of nitrogen per acre taken up by a thriving rye cover crop last spring. At another, with patchy cover, it was 46 pounds on average. At one site no difference in nitrate concentration in the top 12 inches of soil was observed between the control and cover crop plot, but the others had large differences.

Weather big factor in nitrate loss
Weather is the most important driver in nitrate loss in a corn-soybean rotation. The effect on the rye growth and soil nitrogen is dependent on weather as well. “Results can vary site to site and year to year. The take-home message is the cover crop is doing its job taking up nitrogen, but the effect on soil and water nitrate can differ a lot between locations and over time,” says Gunther.

Modern modeling tools can be used to quantify and compare different crop production scenarios when assessing the impact of cover crops on the soil nitrogen budget, corn and soybean crop yield, and water quality.

Peter Kyveryga, ISA research director of analytics, and Gunther presented preliminary findings of the crop systems modeling project at the conference. “The key to this case study is we can calibrate a cropping systems model with data from farmers’ fields and develop different what-if scenarios,” Kyveryga says. “These will help quantify the impact of different scenarios, such as timing of planting and termination, timing and rate of commercial fertilizer, and/or manure applications on crop yield, soil nitrate and nitrogen loss through tile drainage.”

Based on modeling results, the earlier a cover crop is planted and the later it is terminated, less nitrate leaves via water through tile lines. “When swine manure is part of the equation, cover crops can reduce the chance of nitrate loss,” notes Kyveryga.

Source: Iowa Soybean Association