Nutrients in manure are valuable for crops and should be handled appropriately and responsibly to make the most of them. While there are generally accepted “book values” for various kinds of manure — based on the kind of livestock and how it has been stored — it’s a much better practice to test manure and know exactly what nutrients are in it.
Jim Friedericks, director of outreach and education for AgSource Laboratories, notes that how animals are raised, fed and handled will affect their manure. Sows in a farrowing house produce manure that’s different from hogs in finishing houses. Dairy cows in freestall barns have different manure than heifers in a loafing shed.
“The best practice is to take a sample of the manure and have it tested by a certified laboratory,” he says. “How manure is stored will also have an impact and should be accounted for when pulling samples for testing.”
Sampling most important step
“Taking a sample lets you find out what’s in it, but the samples have to be taken in a way that they truly represent what’s in the manure,” Friedericks says.
If manure is stacked, he recommends taking samples 3 feet into the stack rather than taking dry material on the outside of the pile. Taking five to 10 samples from various locations and mixing them together in a bucket is the best way to ensure it’s a good, representative sample.
Lagoons or manure pits are a bit trickier since that manure will stratify the longer it is stored. Manure in pits is generally stable, but it’s somewhat tougher to get a representative sample. The best samples would be taken once manure is agitated. Some farmers or custom manure haulers take samples as they pull manure out of the pit. While that’s not as ideal as having tested the manure before spreading it, it is a way to get information on the nutrients that have been spread and can be credited in the manure management plan.
Properly submit samples
Once manure samples are collected, they should be cooled or preferably frozen right away. If frozen, they are completely stable. AgSource Laboratories in Ellsworth, Iowa, and Bonduel, Wis., are already starting to receive manure samples from farmers in anticipation of crop harvest and manure spreading.
What the lab technicians don’t like is manure that is not submitted in a regulation container. “Every lab will be more than happy to supply the proper container for manure samples,” Friedericks says. The proper containers are never glass, are roughly quart-sized, and have a wide mouth into which the mixed manure sample can be loaded and safely shipped to the lab.
What do results tell you?
What farmers and manure applicators should be most concerned with when reviewing the results are total nitrogen, total phosphorus, total potassium and sulfur. Generally, the nitrogen number is the one that determines application rates, unless regular soil testing shows high levels of phosphorus, which then becomes the most important criteria and limiting factor for manure application. Additionally, a manure report will typically include percentage solids or dry matter.
For most Midwest farmers, the nitrogen information is the most important criteria when it comes to deciding how to use manure on the land. On-farm manure management plans are generally designed to provide enough manure-based nitrogen for the crop that will grow in that soil the coming year.
Many farmers have official, registered manure management plans, which are not quite as stringent as overall nutrient management plans. These manure plans require regular soil testing and annually updated records of manure applications on fields.
Application affects nutrient availability
How manure is applied is critical to availability of nutrients for the following crop. For example, if liquid manure is injected, 98% to 100% of the nitrogen is considered available to the crop in the first year. If manure is surface-applied and incorporated within four to six hours, 95% is considered available to the crop in that first year.
The least desirable option, which is land spreading without incorporating, means only 60% of the nitrogen will be available for the crop in the first year because ammonia in the manure will volatilize and be lost from the soil.
“Manure that has a high proportion of ammonia, like liquid swine manure, should be injected or incorporated right away to capture as much of the nitrogen as possible,” Friedericks says.
The lab report will show how much of each nutrient a farmer can expect to carry over for the second year’s crop and the third year’s crop based on the manure analysis and type of manure. Manure reports also inform the farmer about how much of each nutrient is there in various measurements: pounds per thousand gallons, or pounds per ton of manure.
While applying manure closer to the time when the crop will need it would be ideal, which is to say in the spring, that’s often not possible given the workload on a farm and time constraints in spring. As a result, fall spreading and emptying of manure pits is a time-honored tradition. Testing manure, coupled with regular soil testing, will make that tradition more productive and allow manure nutrients to best be used. For more information, visit agsourcelaboratories.com.