The Economics of Manure Application Methods

 By Daniel Hudson, UVM Extension Agronomist

A very interesting question was posed by a farmer yesterday in response to the article Points to Ponder as the Manure is Being Applied.  The question was: How much is injecting liquid dairy manure worth in this weather?   The short answer for farmers applying around 12,000 gallons per acre is: the approximate value of the nitrogen saved is between $25 and $100/acre, depending on the alternative application method chosen.

image from extension.org

Background

For those less familiar with the science behind the question, manure contains ammonium (NH4+).  Once applied, much/most of this ammonium is converted to ammonia (NH3), which evaporates/volatilizes very quickly, especially if it is warm, sunny, and/or windy.  The nitrogen lost to the atmosphere could have otherwise been used for plant growth and usually needs to be replaced by another [costly] source of nitrogen.  Injecting the manure below the soil surface reduces the exposure to the atmosphere and reduces ammonia loss.

Here we will explore the economic value of the nitrogen retained when manure is injected rather than surface broadcasting the manure:

  • without incorporation
  • with immediate incorporation; and
  • with delayed incorporation

Manure Math

The value of injection depends on the analysis of the manure being applied and the alternative way the manure would have been incorporated.  Nutrient content of manure varies widely among farms and even on a particular farm.  Farmers should test manure from their own farms rather than relying on book values.

Retention of ammonium-N in an injection system will also depend on how the injection is done.  For the sake of simplicity, we will assume that the injection is done under reasonable conditions and results in basically no loss of ammonium-N.

The example manure report used in this discussion is provided on the second to last page of the UVM publication “Nutrient Recommendations for Field Crops in Vermont.” The manure analysis report showed that every 1,000 gallons of that liquid dairy manure contains:

  • 10.5 pounds of nitrogen in the form of ammonium (NH­4+)
  • 24.8 pounds of nitrogen in organic forms (about 35% of this will be available this year, 12% next year, and 5% in the third year)
  • 8.7 pounds of phosphate
  • 23.2 pounds of potash

The farmer who asked the question was in an area where he should expect a 25 ton/ac or higher corn silage yield.  Twenty five tons of corn silage (as harvested) will remove about 225 pounds of N, 125 pounds of phosphate (P), and 275 pounds of potash (K).  These values are calculated from Table 19 of Nutrient Recommendations for Field Crops in Vermont.

Let us assume that the soil test report on this farm is such that he decides to aim for a

The less manure that is visible on the soil surface after injection, the lower the ammonia-N losses will be. Photo from Virginia Tech

manure application rate that will satisfy the potash requirement for his crop.  This means that he would need to divide the 275 pound K­2O needed by the 23.2 pounds of K2O provided per 1,000 gallons of manure. This yields a target application rate of 11,800 gallons of liquid dairy manure per acre.  Keeping in mind that this manure also has 10.5 pounds of N (in the form of ammonium) per 1,000 gallons, this would also provide 124 pounds of plant-available nitrogen.  Because this producer has great equipment and process, we will assume that he is not losing significant ammonia to the atmosphere.

Finally, we must not forget that the manure analysis is also showing that he is applying

24.8 pounds of organic N per thousand gallons.  Multiplying 24.8 by 11.8 (thousand gallons) = 295 pounds of N per acre is being applied in various organic forms.  Of this 295 pounds, 35% (which amounts to 103 pounds) is expected to be available to the crop this year.  Assuming last year’s manure application was the same, you will get 12% of the nitrogen released from last year’s nitrogen application: 103 X 0.12 = ~12 pounds per acre.  This brings your total nitrogen availability up to 124 + 103 + 12 = 239 pounds of plant available nitrogen from manure alone!    Remember in school when everyone was wondering when in the world they would need to do story problems?

That is a lot of nitrogen!  How much might I lose if I do not inject or quickly incorporate, and what is that worth?

This is a more relevant question for most farmers because few use injection systems. In the Cornell factsheet called Nitrogen Credits from Manure, the authors suggest that loss of 35% of the ammonium nitrogen in liquid dairy manure can be lost in a SINGLE DAY if it is not incorporated; 47% in 2 days; and 71% in four days.  Immediate incorporation can be nearly as effective for retaining N as injection if you have the equipment and labor handy.  If manure is topdressed without incorporation, you should expect to lose at least 80% of the ammonium N.  Using the example above, this would be equivalent to losing 99 of the 124 pounds of ammonium-N.  Last year sidedress nitrogen costs about $1/lb (applied), this scenario would basically cost the farmer $100/acre.

Emerging from these numbers is the impressive amount of N that can come from manure, as well as the importance of recognizing it, keeping it, and accounting for it during the cropping season.

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About Daniel Hudson

Daniel is an agronomist for University of Vermont Extension in the areas of agriculture and nutrient management.
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