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:
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Step 1: Find the nutrient needs of the crop
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Step 2: Calculate the actual nutrient availability in the manure
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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
Tips:
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For Total N needed:
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Use the University of Minnesota’s guidelines for N-based application rates
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N credits include N from previous legumes or manure application
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Other N sources include starter fertilizer, irrigation water, etc.
Tips:
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Total N Content (from manure analysis): Make sure to use the reported test value, not the “first year available” numbers
Tips:
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Liquid vs. solid manure:
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Since liquid manure nutrients are described on a “per 1000 gallon” basis, multiply the Manure Application Rate by 1000.
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If using solid manure, you do not need to do anything further.
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Manure application rate calculations - phosphorus-based
Tips:
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For Total P needs:
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Use the University of Minnesota’s guidelines for P-based application rates
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Other P sources include starter fertilizer, etc.
Tips:
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Total P Content (from manure analysis):
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Make sure to use the reported test value, not the “first year available” numbers
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Make sure the Total P is reported as Total P2O5 content on your manure analysis. If not, multiply Total P by 2.29 to get Total P2O5.
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P Availability Factor: 80% of manure P is available the first year, so multiply the total P content by 0.8.
Tips:
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Liquid vs solid manure:
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Since liquid manure nutrients are described on a “per 1000 gallon” basis, multiply the Manure Application Rate by 1000.
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If using solid manure, you do not need to do anything further.
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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 |
Reviewed in 2019