With nitrogen (N) prices on the rise, farmers are investigating alternative ways to ensure their crops get the nutrients they need. One solution is applying manure to cover crops.

“This year, when we’re seeing higher nitrogen prices, there’s going to be more manure [applied to more fields],” says Melissa Wilson, a manure management specialist with the University of Minnesota Extension.

Wilson says manure does great things for cover crops, particularly when injected beneath the surface instead of spread on top.

“One of the biggest reasons we inject it is for nitrogen,” she says.

N appears in two forms in manure — organic N and ammonium. According to Wilson, the latter easily converts to ammonia gas and gets lost when manure is applied on the surface.

“If we inject [the manure], the soil will keep it in the ammonium form, instead of the ammonia gas form,” she says. “It’s one of the pros of getting manure incorporated into the soil.”

But it’s important that growers consider their soil type when deciding on the best system for their farm.

“There are areas in Ontario where we’ve got heavy clay soils,” says Christine Brown, a field crop sustainability specialist with the Ontario Ministry of Agriculture. “In those soils, injection isn’t always an option because you’re doing more damage and potentially causing compaction and other issues.”

Appropriate Equipment

Wilson recommends an injection depth of at least 3-4 inches in wet soil conditions, or up to 6-8 inches if the soil is drier. 

In addition to achieving proper depth, it’s important that farmers choose equipment that allows them to apply the manure evenly across the whole soil profile, according to Brown.

One example Brown notes is a dribble bar with hoses every 8-10 inches and a small shoe attached to the bottom of each hose for making the slot.

“It’s more common in Europe, but it’s becoming popular here,” she says. “It’s not perfect injection, but it’s increasing the soil-to-manure contact.”

Wilson advises farmers to avoid equipment with overly aggressive tillage.

“If you’re disturbing the soil, you’re also disturbing the seed bed for the cover crops,” she says.

Her team had success with an opening coulter that created a slot in the soil into which they could directly inject manure. 

“[The coulter] was followed by a small sweep,” she says. “It lifts the soil up a bit and opens up that pocket.”

Test for Success

Both Wilson and Brown stress the importance of testing manure.

“Do it annually if you can,” Wilson says. “Whether you’re applying it on a phosphorus-based rate or a nitrogen-based rate, you’ll base your decisions on what you actually have in your manure.”

Brown advises farmers to test their manure as they’re taking it to the fields so that samples are properly agitated. She says to look for total N, ammonia N, phosphorus (P), potassium (K) and sulfur.

Additionally, farmers using solid manure should test the carbon-to-N rate, Brown says, and growers using liquid manure should test pH levels. 

“If the pH is high, we have higher potential for ammonium volatilization,” Brown says. “You might lose up to 80% of the ammonium portion of the nitrogen when surface applied in good growing conditions. That can be significant if you’re planning on that nitrogen being available.”

Nutrients Inform Application Rate

Once farmers know what nutrients are in their manure, they can determine the rate of application, Wilson says. 

“Swine manure usually has a higher amount of nitrogen, so we expect to see a lower application rate,” she says. “In Minnesota I’ve seen anywhere from 3,000-5,000 gallons per acre for swine manure. For something like dairy, with a lower nitrogen concentration, you might have to put higher rates on. Depending on how dilute it is, I’ve seen anywhere from 8,000 gallons per acre to 17,000 gallons per acre. It really depends on the manure type.” 

Farmers also must consider the frequency of manure application. Multiple factors will impact that decision, Brown says, including labor, equipment, soil conditions and manure composition.

“In Ontario a large percentage of dairy manure gets applied immediately after a forage harvest (sometime just after 1st or 2nd cut and sometimes after all cuts, weather permitting),” she says.

“It’s important to plan ahead,” she says. “See where there are opportunities to apply, instead of looking at manure as a waste product to deal with once a year.” 

Wilson notes that farmers may also base their decision on storage issues.

“Sometimes it can be logistics, too,” she says. “If it’s really wet and they don’t want to put that much more water on their field with the manure, they might want to put a lower rate and then wait until the spring to put the rest on. 

“There are different management decisions to think about.”