Cover crops, manure go together
For crop and livestock producers, manure can be a huge asset or a big headache, depending on your cropping system, management and the weather.
The challenges can include uncertain nutrient composition; losses due to volatilization during storage, handling and application; and logistical difficulties in application. Getting the most value out of manure can be tricky, but pairing manure application with cover crops may be one way to solve some of these problems.
Greg Brenneman, ag engineering specialist for Iowa State University Extension, says there are benefits to growing cover crops on manured ground. First, manure is good for the cover crop. “We’ve seen the cover crop really green up and get some extra growth,” he says. And just as with corn and soybeans, cover crops help reduce leaching loss. “Between the manure and cover crops, they do a good job of improving soil health.”
Farmers may have heard of “slurry seeding,” which is mixing a cover crop seed into liquid dairy or swine manure. The process, developed at Michigan State University, encourages producers to limit field passes by combining low-disturbance tillage, cover crop seeding and manure application in one pass.
“In the past 10 years, the best cover crops I’ve had have been the ones I’ve established with slurry seeding,” says Tim Harrigan, assistant professor of ag and biosystems engineering at MSU. He says there are a number of ways to improve soil health in a cropping system: disturbing tillage less, limiting field operations and adding organic matter.
Harrigan says success with slurry seeding depends on having the right equipment and right climactic conditions. In his research, they used an AerWay, which is a rolling tine, low-disturbance aerator. He says by fracturing and loosening the soil, manure and cover crop seed can quickly infiltrate.
For corn and soybean producers, waiting until after harvest to establish cover crops can be problematic. “Once you get later, you start running out of growing degree days,” says Harrigan. “A lot of this comes down to a question of what your windows of opportunities are.”
Brenneman remains cautious when advising the practice in Iowa on corn and soybean ground. “My biggest concern is the timing for good cover crop establishment and timing for good manure application don’t line up well.” The ideal time for cover crop seeding is as soon as possible after corn or soybean harvest, meaning late September or early October. “On the other hand, we want to apply manure to cooler soils,” he says. The ideal time for manure application is much later, shortly before ground freezes in early November. Applying manure to cooler soils slows the process that converts ammonium to nitrate.
Eastern Iowa farmer and certified crop adviser Tyler Franzenburg says the slurry seeding method probably won’t work well in Iowa with corn and soybeans. He also runs Franzenburg Family Consulting, to assist with manure management, soil and manure sampling, crop scouting, and organic seed. As part of the custom manure application side of his business, he applies a lot of manure: 8 million to 9 million gallons a year.
Franzenburg has worked some with slurry-seeding cover crops and thinks there are other methods that work better. “When it’s time to inject hog manure, it’s time to inject hog manure,” he says, “not fumble around trying to get rye mixed with manure.” He adds it’s hard to get rye to stay in suspension, unless you have a very aggressive in-tank agitation system.
Harrigan agrees, saying best results come from slurry tanks that restrict flow to the manure injectors and return the excess flow to the tank, similar to the bypass flow in a field sprayer. In their research, a bypass flow rate of about 800 gallons per minute provided good seed distribution in a 3,000-gallon spreader tank.
Many of Franzenburg’s customers have seeded cover crops on manure-applied ground, and based on what has worked, he has a couple of recommendations. “Aerially apply the rye; then inject the manure into the growing cover crop using low disturbance injectors, not disk coverers.” He says most of the rye will survive the manure injection. Other customers have had good success seeding the rye after manure injection, either by broadcast or drilling, depending on the weather and the year.
Brenneman’s recommendation is similar: “Try and get the cover crop seeded; then later in the fall, knife in the manure, recognizing that you’ll tear up some of the cover crop.” He says you want to use a low-disturbance knife, but that the cover crop should do fine.
Ohde is a program assistant with Practical Farmers of Iowa.
This article published in the July, 2015 edition of WALLACES FARMER.
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