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Guidelines for manure application rates

Quick facts

  • Manure nutrient management planning is important for maximizing crop productivity while protecting water quality
  • Guidelines for manure application rates vary depending on crop and cropping history
  • Manure application rates should consider all nutrient sources that will be or have been applied to a field. For example, if commercial, inorganic fertilizers will be applied or if manure was applied in the previous two years, take credit!
Manure_MPCA
Credit: MPCA

Animal manure is a good source of nutrients for crops, including nitrogen (N), phosphorus (P), and potassium (K). The proportion of the nutrients in manure are typically not the same as needed by the crops, however. Manure application based on one nutrient may over- or under apply other required crop nutrients. Nitrogen is required in the largest quantities by non-legume crops. Applying manure to meet crop N needs will likely overapply P, and possibly K, for a crop such as corn. On the other hand, using manure to meet P needs of the crop will likely result in a lower application rate and will underapply N and possibly K. Commercial fertilizers will then be needed to balance out N and K needs. Consider the pros and cons of these two options when choosing a manure application rate.

Nutrients in manure are not 100% available in the first year. First-year plant-available N (PAN) will depend on animal species and how the manure is applied. Plant-available P (PAP) is assumed to be 80% of the total P applied in the first year. You can learn more about calculating PAN and PAP, including first-year PAN and PAP, from our “calculating manure application rates” recommendations. The guidelines for manure application rates below are based on PAN or PAP, not total N and P.

Nitrogen guidelines for manure

The rates below are the suggested maximum amounts of N that should be applied when manure is used, whether it is all manure or a combination of manure and inorganic commercial fertilizers. Lower rates may be considered based on the productivity of the soils in your fields, economics, or environmental concerns. In all cases, all sources of N should be taken into consideration when estimating how much N to apply, including:

  • N from irrigation water,
  • Credits from manure or other organic N sources that were applied in the past 2 years, and
  • Credits from legumes like edible beans, red clover, etc.

Why is that? Because research across the US Midwest has shown that applications of N above the economically optimum N rate (EONR) for a crop significantly increase N leaching. Once N leaches past the plant root zone into the groundwater, it becomes a concern for drinking water and will eventually end up in lakes, rivers, and streams. When manure N becomes plant-available, it behaves exactly the same in the environment as N from commercial inorganic fertilizer, so it is important that all forms of N applied to the soil are taken into consideration. Don’t waste your manure!

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Phosphorus guidelines for manure

In cases where manure is readily available on a frequent basis, using a P-based manure application rate may make the most long-term, economic sense because the nutrients will be used more efficiently by the crops. For manure, it is recommended to apply as much plant-available phosphorus (PAP) as the crop will use. See the table below for crop P removal rates in the harvested portion (the units are in P2O5 to make it comparable to what you would apply with commercial fertilizer). Multiply this number by the amount of yield you expect from that field to get your PAP application rate.

Note that soil testing should be conducted regularly in your fields where manure will be applied. If the soil P test level is very high (over 21 ppm Bray-P or over 16 ppm Olsen-P), you are not likely to see a yield benefit from the additional P in the manure. Instead, consider using the manure in a field where P applications will help improve yields. If soil test P levels are excessively high (over 50 ppm Bray-P or over 30 ppm Olsen P), application of manure is not recommended, especially near water bodies, because building soil test P levels further will increase the risk for soluble P losses from the soil.

Once you determine your P-based application rate, remember to credit the manure for the nitrogen that will be applied! Balance the remaining N needs of the crop with commercial fertilizers.

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Where do our guidelines come from?

Inorganic commercial fertilizers are often used to figure out crop nutrient needs in experiments across Minnesota. These fertilizers are designed to release 100% of the N and P in the first year, so it makes it easier to determine how much to apply to get the optimized yields. As an example, N guidelines for corn are based on 170+ experiments across the state, most of which occurred in the past five years. As new experiments are completed, the data on optimal N needed are added to the overall database, and N guidelines are adjusted accordingly.

With manure, we can calculate the estimated plant-available nutrients that will be available in a given year. Once nutrients from manure are plant-available, they behave in the environment exactly the same as a nutrient from a commercial fertilizer. Thus, our guidelines for manure application are based on optimal nutrient rates needed - which is known from fertilizer experiments - and how much plant-available nutrient will be available in the first year after application.

Melissa Wilson, Extension manure management specialist

Reviewed in 2019

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