Variation in state-based manure nitrogen availability approaches

The phosphorus (P) index is the primary strategy used in nutrient management planning to identify field management strategies and/or manure application strategies likely to lead to excessive risk of P loss.  Current P Indices were developed primarily as strategic planning tools guiding the development of a nutrient management plan spanning one to five years.  In reality, a nutrient management plan should be viewed more as a process than a result.  After completing the initial strategic plan there are tactical adjustments for new information such as new soil and manure tests and changes in crop selection.  Additional assessments are needed when implementing the plan, determining if current weather and soil conditions are appropriate for application.  Many current P Indices require using the soil erosion program RUSLE2 which is then a barrier to the use of these P Indices by anyone except planners with specialized planning.  Such expertise is never available on some farms and unlikely to be available on most farms during tactical and implement phases of the plan.  There has also been suggestions that more complex strategies such as models should replace existing P Indices; this will lead to more complex P loss assessment tools.  Next generation P Indices will be more effective if we consider the capabilities and training of those likely to be making decisions at each critical juncture.  Instead of “the” P Index we need to design a suite of tools that target key decision points.  In each instance a first step of the development process must be defining who the likely decision maker is and what are their skills and training.  We can only succeed if our tools are accessible to those that need to use them.


Extensive research has documented fertilizer value of manure nutrients for crops. It has been long recognized that manure nitrogen (N) excreted by animals is not 100% available to crops. Surveys indicate failure to credit or under crediting manure nutrient value to a crop by farmers continues to be an issue. Our goal was to assess the current state of manure nutrient availability recommendations and requirements in the US.

What did we do?

We surveyed state recommendations for state nutrient availability calculations for four sources of manure: finish hog slurry, dairy cow slurry, solid cattle manure and broiler litter. The top 12 states for production of each associated commodity were determined using inventory data from the 2012 Agricultural Census; the top 12 states for production were states we surveyed for each manure type. For each state and each manure type surveyed we attempted to identify nitrogen availability calculation recommendations from three sources: the State Land Grant University, the state USDA Natural Resource Conservation Service (NRCS) standards and supporting documents, and the state regulatory documentation for operations with a National Pollution Discharge and Elimination System (NPDES) permit.

What have we learned?

We were able to identify a primary publication or publications published by the State Land Grant University in all but four of the 30 surveyed states for the manure types of interest. Median date of publication for the 22 dated publications was 2006 (range 1991-2014). The NRCS documentation referenced the state Land Grant publication (10 states), a state-specific NRCS worksheet or reported numbers in the standard (7 states) or referred to regional or national reference (3 states). The USEPA NPDES regulatory documentation did not specify availability coefficients in 11 of 30 states. In nine states the regulatory documentation cited the USDA-NRCS 590 standard but in three of those states the NRCS standard did not provide nutrient availability coefficients. Consequently it was not possible to determine regulatory nutrient availability coefficients in nearly half of the surveyed states (14 of 30). Availability calculation approaches fell into two main categories, states that calculate availability based on manure total nitrogen content and states that account separately for availability of organic and ammonium nitrogen. Availability estimates among states were more variable for strategies known to be more variable (e.g. surface application of liquid manure).

Future Plans

Our work emphasizes the varied approach to N availability calculations as we cross state borders. We hope this publication will encourage regional discussions among states with similar climate to work towards more consistent recommendations. More consistent recommendations may help farmers have more confidence in those recommendations,

Our work also demonstrates how difficult it can be to identify the appropriate calculations within a given state. We encourage that state recommendations from all three organizations (Land Grant, NRCS, regulatory) be documented in a standard place in the state NRCS Nutrient Management Standard so planners, farmers, and people developing and managing nutrient management tools can easily and with confidence access the most current information on N availability information for manure nutrients.


Dr. John A. Lory, Associate Professor of Extension, University of Missouri, Columbia, MO

Ms. Caitlin Conover, USEPA and Visiting Scholar, University of Missouri, Columbia, MO

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