Manure Nitrogen Credits Simplified
Ken Janovicek, University of Guelph and Greg Stewart, OMAFRA Corn Specialist

Calculation of plant available nitrogen (N) from manure is an important step in a cropping system budget. However, there is some difficulty associated with sorting through the numerous tables of coefficients in order to calculate plant available N from manure. This complexity has resulted in many producers not getting accurate manure N credit estimates for their manure applications. For some Ontario producers, the tables in OMAFRA Publication 811 (Field Crop Agronomy Guide) can be used to calculate manure credits, for others the NMAN2 software is used to arrive at these estimates. Our aim was to provide a simplified calculator (Excel spreadsheet) which would assist more producers in arriving at the most appropriate estimates of plant available nutrients in their manure applications.

The framework for calculation of plant available nutrients from manure is based on current assumptions and methods utilized by NMAN and official OMAFRA
recommendations. The manure information input section of the calculator was designed to enable producers to provide answers to easily understandable questions regarding their manure analysis, application rate, application method, application timing and incorporation timing. Based on answers to these questions, the most appropriate information for plant available nutrient calculations is automatically selected by the Excel calculator and estimates of plant available nutrients are calculated.

Often, a manure analysis is not available; which makes calculation of plant available nutrients impossible without making assumptions regarding the nutrient analysis of the manure. Whenever this occurs, producers should use historic analysis, if available, provided that the manure analysis was obtained when the livestock enterprise was managed in a similar manner. Otherwise, the calculator can provide an estimate of nutrient analysis based on an OMAFRA database of
average nutrient analysis for manure categorized by:

1. livestock type (i.e. hogs, dairy cattle, beef cattle, broilers, etc.) and
2. the dry matter content of the manure.

Figure 1. Display from the Ontario Manure Nutrients Calculator

Fig 1

Link to the Ontario Available Nutrients from Manure Calculator

The assumed average nutrient analysis for the various manure types are based on actual nutrient analyses of manure conducted by Ontario labs over the past 10 years. If there is uncertainty surrounding the dry matter content, then the most appropriate average manure analysis can be selected based on:

1. type of livestock enterprise (i.e. hogs farrowing, hogs finishing, etc.) and
2. type of manure storage (i.e. liquid covered, liquid open, etc.).

The selection of the most appropriate average manure analysis is based on dry matter estimates derived from the M-Store assumptions.

The result of this project is the creation of the Ontario Manure Nutrients Calculator. It is a relatively simple one-page spreadsheet that can be obtained at the website To demonstrate the operation of this calculator, we will use an example of liquid hog manure application. Let’s assume:

1. liquid hog manure with an analysis of 3.9% dm; 2000 ppm Ammonium (fresh weight basis); 30 lb-Total N /1000 gal; 10 lb-Total P/1000 gal; and 13 lb-Total K/1000 gal.

The manure is to be injected pre-plant in late April 2008 at 2500 U.S. gal/ac on a field following corn with residue cover between 30-45%. The data entry and results from the Calculator are illustrated in Figure 1. The amount of plant available nutrients for the 2008 crop is 46 lb-N/ac, 8 lb-P2O5/ac and 27 lb-K2O/ac. If this same manure was injected in late November of 2007, plant available N is estimated to be 30 lb-N/ac.

This Ontario Manure Nutrients Calculator provides an easy and quick method for producers to calculate estimates of plant available nutrients for the type and conditions under which they apply manure. The nitrogen credit derived for 2008 in the above scenario (46 lbs N/acre) can now be plugged into the N Calculator to arrive at the most economic fertilizer N recommendation for that field.

It should be noted that while the estimates of available nutrients will be the same as those generated by OMAFRA tables or NMAN2 software, there are many uses and functions that NMAN software provides that are not captured in this spreadsheet. To obtain more information or order the NMAN software please connect to the following website:

Funding for this project was obtained from the Agricultural Environmental Stewardship Initiative (AESI Project #151).

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