ST. PAUL, Minn. – It has long been established that manure is one of the best fertilizers farmers can use on their fields, but it is also one of the more challenging fertilizers to manage. Application rates with manure are extremely important and regulated. Going into this fall, the University of Minnesota has adjusted their manure application rate guidelines to better reflect the needs of the crop.
“We had some new nutrient recommendations that came up this past year with our soil fertility group,” said Melissa Wilson, University of Minnesota Extension manure management specialist. “We wanted to reflect that with our manure recommendations as well, so what's actually increased is the non-irrigated corn in particular.”
All across the state of Minnesota, the university has research sites looking at fertilizer, nitrogen and phosphorus. The goal is to determine the best application rates based on the needs of the crop.
The data collected at these sites is compiled into a database and reviewed.
“We added the more recent years’ data, 2017 and 2018, and re-evaluated the nutrient needs. We found that they were higher than they've been in the past,” said Wilson. “For continuous corn, the suggested maximum rate of nitrogen was 180 pounds and we moved it up to 195 to reflect the increasing nitrogen needs that we've seen in the corn crop.”
For corn following soybeans, the suggested maximum nitrogen application was 140 pounds. That has been adjusted up to 150 pounds.
With manure, being that it is regulated by the Minnesota Pollution Control Agency and other government organizations, and the fact that it contains phosphorus as well as nitrogen, application rates are not as simple as applying the maximum recommended nitrogen rate.
“You can apply based on the maximum nitrogen needs of a crop, unless you have high soil testing phosphorus levels, especially if you're within a certain number of feet of a water body,” she said.
In that case, manure application rates need to be based on the amount of phosphorus in the manure, not the nitrogen. Over the course of six years, whatever manure applications the farmer plans to do on that land, the phosphorus applied cannot exceed what the total crop demand would be over that time frame.
“You're allowed to still apply these higher rates, but you just have to keep an eye on your soil phosphorus test level,” she said. “I've been getting more and more questions about how we apply at a phosphorus-based rate, so we did add those to our guidelines as well.”
The university’s manure guidelines, recommendations and calculations to help determine how much manure to apply, can be found online at z.umn.edu/ManureRates.
For those applying manure based on the phosphorus value, they still need to pay attention to the nitrogen value because they will likely need to supplement nitrogen with a commercial fertilizer.
“Usually, there'll be plenty of phosphorus, but not as much nitrogen as we need if we apply at that phosphorus-based rate,” said Wilson. “There's still that maximum amount of nitrogen you're allowed to apply, so that's always kind of the guideline.”
That maximum nitrogen rate value is based on crop needs. Shorting that rate could result in shorting needed nutrients and costing yield, unless then nitrogen is made up in other ways, either through application or management.
The other difficult part to consider with manure is that the nutrients are not all plant-available that first year.
“Some of that nitrogen will still slowly release over the next year or maybe even two,” she said. “With nitrogen and phosphorus, it's really important to think about what’s plant-available and what will be released later on. Sometimes that makes it even more complicated on top of everything.”
A lab test for manure content will always test for total nitrogen. Some labs will provide information about what is available the first year, but it is important to ask what the lab is basing that information off of.
“Sometimes our guidelines differ. What works in North Dakota may or may not work here in Minnesota for estimating those availability percentages,” she said.
For help determining proper manure application rates, farmers can consult the University of Minnesota’s website. They can also speak to their county feedlot officer, county Extension educator, or NRCS office.
“Manure is more complicated than commercial fertilizers, but you do have the added benefits of the micronutrients, secondary nutrients, as well as added organic matter to help build soil health,” said Wilson. “It’s more complicated, but it does come with some perks.”