Do the “4Rs” apply to manure? You betcha!
September 20, 2022
Writer Carol Bishop Hipps spoke of fall as “the mellow, messy, leaf-kicking, perfect pause between the opposing miseries of summer and winter.” How could she forget to include “manure-slinging” in her list of fall descriptors? Since the manure application season is upon us, it’s time to consider best management practices. When it comes to fertilizer, everyone promotes the “4Rs” for nutrient stewardship - the Right Source, Right Rate, Right Time, and Right Place. These also apply to manure! Proper manure management will help you get the most fertilizer value for next year’s crops and avoid runoff and leaching that could pollute waters.
In this case, the right source is manure! But don’t forget that you have to credit other nutrient sources as well - especially when it comes to nitrogen and phosphorus. Will you use a commercial starter fertilizer that contains nitrogen or phosphorus at planting? Was last year’s crop a legume? Was manure applied last year? Does your irrigation water contain nitrogen? If you answered “yes” to any of those, make sure to take credit!
Nutrient management is a balancing act. You want to apply enough nutrients for your crops, but avoiding overapplication will save money and prevent nutrient losses to water. So how much does your crop need? Current guidelines for manure application rates in Minnesota are determined by the nitrogen or phosphorus needs of the crop. But which rate should you choose? Remember, manure is like a multivitamin since it contains multiple nutrients. The right application rate is the one that minimizes overapplication of nitrogen and phosphorus. Then, using the Right Source principle, you can balance out the rest of the underapplied nutrients with commercial fertilizer. To make sure you hit the rate you want, pair a recent soil test with a manure nutrient analysis; don’t simply rely on manure nutrient book values. And don’t forget to calibrate your spreader to make sure the rate you planned for is what you’re actually applying.
Theoretically, the best time to apply manure is when the crop starts taking up nutrients in large quantities. But we know it is not always that simple as there are other considerations, like how much manure storage you have remaining and whether the fields can handle heavy manure application equipment with minimal compaction. In Minnesota, the majority of manure is applied in the fall after harvest. This means there is a long time until spring, and, unfortunately, nutrients have a knack for escaping into the environment.
The key for keeping nitrogen in place is cool soil temperatures. Nitrification -- the process by which nitrogen is converted into water-soluble nitrate -- happens rapidly at high temperatures, but slows with cooler temps. Therefore, we recommend waiting until soils are 50°F or cooler to apply manure. This is especially important for liquid or slurry manures that have a higher proportion of inorganic nitrogen that is more easily nitrified. Note that nitrification is not halted at cool temperature, just slowed. Even around freezing, the process continues very slowly.
We also recommend incorporating a cover crop into the rotation when manure will be applied. The living roots help trap nutrients that otherwise might be lost. We suggest getting cover crops planted as early as possible since Minnesota has a shorter growing season than other parts of the country. Researchers and farmers around the state have had success drilling cover crops after harvest of a summer crop or broadcasting the cover crops into standing corn or soybeans. Liquid or slurry manure can be injected into the cover crop later in the fall with low- or minimal-disturbance injectors. Solid manure can be applied over the cover crop at rates low enough to avoid smothering the cover crop once soil temperatures have cooled. If you need to incorporate solid manure, it may be best to do so prior to planting the cover crop to avoid ripping it up entirely.
A critical aspect of good manure management is knowing where to apply manure to get the best bang for your buck. Remember, you already paid for those nutrients when you purchased the feed for your livestock. It doesn’t make sense to overapply them and then have to purchase more fertilizer for other fields. Spread the wealth! You will want to prioritize fields that can benefit from all of the nutrients in manure, not just the nitrogen. Having a manure management plan in place for your entire operation can be beneficial to help you get the nutrients to the fields where they are needed. Then you can plan for additional commercial fertilizers as needed to balance everything out.
The other key component of a good manure management plan is identification of and planning for environmentally-sensitive features in your fields. According to the Minnesota Pollution Control Agency (MPCA), sensitive features include bodies of water or landscape features that lead directly to bodies of water. This includes lakes, rivers, intermittent and perennial streams, protected wetlands, open tile inlets, sinkholes, and drainage ditches with open side inlets or no berm. When applying manure within 300 feet of a sensitive feature, manure must be incorporated within 24 hours and before it rains. You cannot apply manure within 25 feet of any sensitive features. More details can be found in this MPCA factsheet.
When it comes to actually applying manure, we know that the weather can be unpredictable. Because of this, it can be helpful to plan ahead. When the weather and field conditions are favorable, apply manure first to fields that have the greatest risk of runoff (i.e., steeper slopes, closer to sensitive features, etc). Save the flattest fields that are furthest away from sensitive features for last in case any adverse weather conditions pop up. The Minnesota Department of Agricultural has a runoff risk advisory tool that predicts the likelihood of runoff due to soil moisture content, forecast precipitation and temperatures, and snow accumulation and melt to predict the likelihood of daily, next day, and 72 hour runoff events. This could be useful to help you plan your manure applications. It also has predicted soil temperatures at 2- and 6-inch depths to help you time your fall manure applications!
Don’t forget safety!
What about the “s” in 4Rs? When it comes to manure, the “s” should represent Safety. Manure spills and exposure to toxic gases are just a few of the dangers of working with manure. Accidents happen, so you should have a plan in place. The Upper Midwest Agricultural Safety and Health Center has a manure safety toolkit to help.
When it comes to manure spills:
- Be sure all personnel are safe.
Stop the spill from spreading:
- Close a valve, drive a vehicle onto a dragline hose, or turn off a pump.
- Use tillage to slow spill movement toward sensitive features in fields, build dirt berms, or use hay, straw or corn stalk bales to absorb the spill.
- Plug culverts and open tile intakes.
- When manure is discharged or spilled, you are responsible for an adequate clean-up and recovery of the manure.
- Remember, you are required to contact the Minnesota Duty Officer (1-800-422-0798) as soon as possible.
Additional things to consider
The rules and regulations for manure application vary depending on the size of a livestock facility and whether it has a state or NPDES permit. Details for minimum state requirements for land application of manure can be found in this MPCA factsheet. For permitted facilities, you can find more details in this factsheet.
- More information about the 4Rs
- Tips for fall manure application and how to avoid nutrient loss
- Online course for manure management planning in Minnesota
- MPCA: Maximize your soil value with fall best practices