Iowa farmer Steve McGrewCover crops are one of the fastest-growing conservation practices in Iowa as farmers pursue the environmental, and hopefully economic, gains of having plants growing in their farm fields year round.

About one in four Iowa farmers are using cover crops on at least a portion of their farms, and the number of cover crop acres in Iowa has skyrocketed from fewer than 10,000 acres in 2009 to about 300,000 acres in 2013, says Sarah Carlson, Practical Farmers of Iowa (PFI) research coordinator.

“We shouldn’t leave soils uncovered during the winter,” she says. “The more we can do to cover them up with green plants, the more impact we can have on water quality.”

Cover crops are among the practices that reduce both nitrogen and phosphorus losses to surface waters. On average, cover crops reduce nitrogen losses by 30 percent, and some research sites have shown an 80 percent reduction in nitrogen losses, according to Carlson.

Many benefits from cover crops
While water quality improvement has been the main reason behind the recent surge in interest, cover crops also bring many other benefits, Carlson says. In addition to water quality gains, studies have shown cover crops can help improve soil quality, reduce chemical input costs, improve farm resiliency, boost yields, increase forage availability and improve wildlife habitat.

“It’s not just to improve water quality. It’s to keep nutrients and soil in place for crop production,” says Carlson. “The more we can have something green growing, the more microbes are holding the nutrients.”

However, cover crops aren’t an overnight success story, she cautions. It can take five to 10 years to show measurable benefits in soil organic matter or consistent yield gains, and there are a number of pitfalls that can occur as a result of poor management.

“It takes some time,” Carlson says. “In year four or five, we start hearing anecdotes that (the ground) is working easier.” Iowa Soybean Association research trials in 2013 and 2014 showed yield gains and losses were about equal in corn following a cover crop. Fifteen trials resulted in a yield loss, while 13 showed yield gains and one had no difference.

 Veteran cover crop users say they’ve noticed about a 1 percent increase in organic matter after planting cover crops for a decade in conjunction with conservation tillage. A 1 percent increase in organic matter will increase yields by 12 percent, studies show.

Carlson recommends first-time cover crop users stick to a simple cover crop like cereal rye, which grows well in most of Iowa and is relatively easy to manage. As farmers gain experience, they might experiment with seed mixes incorporating tillage radish or legumes.

Steve McGrew started using cover crops in 1993 primarily for erosion control on the hilly terrain of his southwest Iowa farm. He’s had success with cereal rye before soybeans and prefers mixtures like hairy vetch, rapeseed and wheat before corn.

In a four-year cover crop trial examining yields after cereal rye, McGrew found a yield boost in soybeans most of the time, while corn yields varied. “As a rule, you will worry about yield drag if you get your grass very tall before corn,” McGrew said. “Corn loved being planted in hairy vetch, and I saw a yield increase.”

There are a number of seeding methods for cover crops, but the general rule is the earlier, the better, Carlson says. Costs typically range between $15 and $40 per acre, depending on seed and application method.

Drilling the seed after corn or soybean harvest provides the most consistent stands, but planting may be too late to get much growth before cold weather settles in, especially in northern areas. Aerial seeding into standing corn or soybeans gives more time for cover crops to get established, but germination depends greatly on rainfall.

 Perennial cover crops will green up again before planting in the spring, helping hold soil in place during heavy April rainfalls.

 “Even if we don’t get growth in the fall, we still get benefits in the spring when we really need it,” Carlson points out.

Long-term commitment
Carlson encourages farmers to make a long-term commitment to cover crops, even if they don’t see immediate benefits. She worries farmers who receive one-time incentives to try cover crops will abandon the practice too quickly, especially in times of tighter crop margins.

 “What would be great is if we had something to get people to try it for three to five years,” she said. “By year five, I guarantee you’ll see a yield benefit in soybeans. Over time, it should be paying you back, but it’s like the 10-year mark.”