Cover crop benefits add up
Cover crops are starting to catch on in Iowa to protect the soil and recycle nutrients like nitrogen that would otherwise be lost to leaching during winter and spring. More farmers are going to have to start using a combination of cover crops and no-till to reach the erosion control and water quality improvement goals the public is demanding.
At a mid-November field day on Gordon Wassenaar’s farm near Prairie City in central Iowa, farmers attending had a chance to look at cover crop test plots established with the help of the Jasper County Soil and Water Conservation District. The Jasper SWCD also has plots on a farm near Newton, the other stop on the tour.
Wassenaar and his farming partner, Will Cannon, have nearly 1,500 acres of cover crops, mainly cereal rye, seeded this winter on land they’ll plant to corn and soybeans in the spring.
The field day was sponsored by Iowa Learning Farms, Iowa State University Extension, Jasper County Soil and Water Conservation District and USDA’s Natural Resources Conservation Service.
Curtis Donohue, NRCS district conservationist, and Holly Giombi, soil conservationist, say the first thing to think about when planning a cover crop is: What do you want the cover crop to do?
“Some of the goals we hear when talking to farmers about seeding cover crops are reducing erosion, increasing soil organic matter, scavenging nutrients and relieving soil compaction. Some want to graze a cover crop for forage,” says Donohue.
If you draw a circle around Prairie City, at least a dozen farmers with good-sized farming operations are starting to seed some kind of cover crop.
Effective erosion control
The foremost goal is to reduce erosion. Anytime you remove crop residue or create a disturbance such as tillage, you want to put something back to help hold the soil in place, says Donohue. Soybean stubble is a good place to start seeding a cover crop. Likewise, if you chop corn for silage or bale cornstalks, think about a cover crop. And the more time it has to grow, the better. Plan to get it drilled as soon as you can after the field is harvested. Or hire someone to aerially seed it in late summer or early fall into standing corn or soybeans, he says.
Cover crops also increase soil organic matter. “Historically, organic matter in crop fields has decreased over the last 100 years of farming. We see lots of soil test results in the 2% to 3% organic matter range,” says Donohue.
After 20 or 30 years of no-till corn and soybeans in combination with cover crops, you can build up organic matter to perhaps 8% to 10%. Organic matter is important for soil biology, tilth and structure, and the amount of water soaking in instead of running off the field.
Building organic matter is a key reason to plant covers. “If this is one of your main goals, seeding a cover following every corn and soybean crop in your rotation is very important,” he says. “Not just after soybeans, not just after corn, but both crops.”
Improving soil health
Research shows mixtures of brassicas, legumes and grasses can generate more organic matter over time than a single species. Mixes are important to build organic matter. But even growing a single cover crop like winter rye can increase soil organic matter and relieve compaction, in addition to reducing erosion.
You want to establish cover crops with the least amount of soil disturbance, says Giombi. You can see in the cornstalk trials the benefits of an extra month that aerial application provides versus seeding after the corn is harvested. “There are cost considerations when deciding whether to have cover crop seed aerially applied in standing corn or soybeans in late August, or wait and drill it after harvest,” she notes. “In our trials, we can see the difference. We seeded some fields and plots at the end of August and others at the beginning of October. There is a significant difference in the amount of growth.”
Adding a cover crop to your rotation puts a live root system in the soil for more months, beyond the roots of your commodity crop. This brings more earthworms and soil organisms, helping increase soil organic matter, says Donohue. Biological activity in the soil breaks down crop residue. It creates pore space for water to infiltrate and for air to be down there; root systems need not only water, but also air to thrive.
Preventing nutrient loss
Nutrient scavenging is another reason for cover crops. Scavenging and nitrogen fixation work together. The cover crop needs to be near maturity for either of these two benefits to occur in a field at a meaningful rate. If a cover crop is to scavenge nutrients, it needs to have a good amount of growth so it can do the job. Also, if you want a cover crop to fix nitrogen, plant a legume. Likewise, legumes need to be near maturity before they will fix N at a rate to be of benefit.
Cover crops break up soil compaction. Tillage radish is a champion at this. If you can get brassicas, radish or turnips (deep-rooted cover crops) established early, you have potential to develop a large tuber and deep taproot.
“In our plots, we see a significant difference in maturity of the plants and size of tubers based on a one-month change or delay in cover crop planting dates,” says Giombi. Also, keep in mind that radish is killed by freezing temperatures in winter, so you don’t have to terminate it in spring. And while radishes do a great job of breaking up compaction, “you’ll be amazed at how good of a job grasses such as rye will do, too,” she says.
When considering a cover crop, don’t overlook its use for forage. “There’s great potential for grazing cover crops,” Wassenaar says. “For grazing, you likely want to seed the cover at a higher rate. Some farmers in our area are doing this, and it’s almost like a pasture in late November. In a cow-calf operation, cover crops have a definite added advantage.”
This article published in the December, 2015 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2015.