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Calculating manure application rates

Before planning for manure applications, consider how the nutrients will be managed for the upcoming crop. Will all the nitrogen (N) be supplied from manure? This will require a higher application rate and will overapply phosphorus (P) and potassium (K) for the crop year. If done too often, there could be environmental consequences when excess P enters waterways.  A lower rate can be used to supply all the P the crop needs. This will allow the manure to be spread over more acres but will require additional N and possibly K to be applied as a commercial fertilizer.

Once you choose a strategy, use the following steps to determine application rates:

  • Step 1: Find the nutrient needs of the crop

  • Step 2: Calculate the actual nutrient availability in the manure

  • Step 3: Divide the answer from Step 1 by Step 2

While this sounds simple, there are slight differences in calculations depending on whether you are basing your applications on N or P needs of the crop or are using liquid or solid manure. The following sections will walk through the calculations for N- and P-based application rates, respectively.

Nitrogen-based manure application rate calculations

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Manure application rate calculations - phosphorus-based

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Table 1: Nitrogen availability (%) as affected by method of manure application and animal species.

Year Broadcast + incorporate
>96 hours
Broadcast + incorporate
12-96 hours
Broadcast + incorporate
0-12 hours
(including double disks)
Injection with sweeps Injection with knife or coulter
N available/ year N available/ year N available/ year N available/ year N available/ year
Beef -- -- -- -- --
1 25% 45% 60% 60% 50%
2 25% 25% 25% 25% 25%
Dairy -- -- -- -- --
1 20% 40% 55% 55% 50%
2 25% 25% 25% 25% 25%
Swine -- -- -- -- --
1 35% 55% 75% 80% 70%
2 15% 15% 15% 15% 15%
Poultry -- -- -- -- --
1 45% 55% 70% n/a n/a
2 25% 25% 25% n/a n/a

Melissa Wilson, Extension manure management specialist

Reviewed in 2019

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