Lessons Learned: Implementation of Nutrient Management Plans On Whole Farm Nutrient Balance

Click here for introduction to WFNB

Implementation of a Nutrient Management Plan (NMP) is a current focus of animal manure management regulations. The intent of an NMP is to encourage more efficient utilization of manure nutrients as an offset of purchased commercial fertilizers for meeting crop nutrient requirements. The primary environmental benefit of a NMP¹ is limited by the amount of commercial fertilizer that can be displaced. The purpose of this summary is to identify the value of NMP implementation on the thirteen farms involved in this summary.

Current Fertilizer Contribution

At the start of this project, all farms participating in this study suggested that they had a NMP in place and had implemented part, if not all, of the plan. Fertilizer represents a small portion of the nutrients entering the 13 case study farms. On average only 11% of the nitrogen and phosphorus arriving on farm was as fertilizer. Fertilizer also represented 31% and 32% of the whole farm’s N and P imbalance² respectively. The whole farm nutrient imbalance represents all nutrients brought onto the farm from all sources for which we cannot account in the managed products leaving the farm. Thus, replacement of all of the fertilizer with manure use will only partially address the whole farm nutrient imbalance on farms where manure nutrients exceed crop needs.

Swine manure from deep pit and earthen basin facilities typically can be applied to a corn/soybean rotation at rates that meet the corn's nitrogen needs and supply sufficient phosphorus for the two-year rotation from one manure application. Many of the participating producers utilize deep pit manure as their only crop nutrient source. However, for some operations that feed with diets containing synthetic amino acids and/or phytase as feed additives, the reduced levels of nutrients in the manure can create situations where supplemental commercial fertilizer may be necessary to meet crop nutrient requirements.

Nitrogen and phosphorus commercial fertilizers were not used for 15 and 19 of the 25 crop years, respectively, for which farm data was shared (see Figure 1 and 2). In these situations, further efforts to develop or implement an NMP would produce few additional on-farm environmental improvements for these farms currently not using commercial fertilizers. Of those farms that used no phosphorus fertilizer, WFNB ranged from a ratio of less than 1.0 to 1 (farm is exporting more P than importing) to greater than 2.0 to 1 (farm is importing and thus accumulating more than 1 lb of excess P for every lb of P exported). Thus, a nutrient management plan that eliminates all commercial fertilizer use does not eliminate the risk of excess nutrients for all situations or farms.

Commercially purchased fertilizer contributes to an increasing imbalance as illustrated by Figures 1 and 2. As fertilizer use per acre increased, WFNB ratio³ also increased. However, other factors contributed to the imbalance as suggested by the variation of the individual farm values from the straight line (low R²).

Figure 1. Comparison of whole farm nitrogen balance with nitrogen fertilizer use per acre.

Figure 2. Comparison of whole farm phosphorus balance with phosphorus fertilizer use per acre.

Impact of Manure Offset to Commercial Fertilizer

Multiple factors influence WFNB. The influence of fertilizer purchases can be understood by considering a case study farm where only commercial fertilizer use is allowed to vary. Farm B5 used no commercial fertilizer in their 2006 NMP. Table 2 compares three scenarios, one where no fertilizer is purchased (2006 NMP) and two scenarios where less efficient crediting of manure nutrients to meet crop nutrient needs is assumed. These last two scenarios cause the WFNB ratio to increase. For this farm (and most others), full implementation of an NMP produces a more favorable WFNB. However, the WFNB imbalance is too great on some farms to be corrected by elimination of fertilizer use alone. Thus additional nutrient management measures may be necessary.

Impact of Diet on Fertilizer Replacement

Swine diet influences the amount of nutrients in the manure and the available nutrients for fertilizing crops with manure. Historically, nutrients were considered to be in excess on many animal feeding operations and so dietary strategies that reduced manure nutrients were considered preferable. However, three of the participating farms exhibited a negative whole farm phosphorus imbalance during at least one of the two years monitored. Two farms exhibited nitrogen imbalances than may also be inadequate to meet crop nitrogen requirements. If these situations persists, detrimental impacts on the farms performance may result. As animal feeding operations have achieved greater efficiencies in nutrient management, application of dietary strategies to reduce nutrients in the manure could be detrimental under some circumstances.

A preferred dietary nutrient level should be considered to avoid both an excess and a deficit of nutrients within a farm as well as meet animal nutrient requirements. This preferred level varies with farm. A reduction of dietary nutrient levels may not always be beneficial to a farm that has already reduced or eliminated purchased fertilizer through implementation of an NMP. Deep pit and earthen basin facilities should carefully monitor diet change effects on manure N and P to insure that manure application will meet crop nutrient needs.

Table 3 illustrates the impact of changing the dietary nutrient concentrations on WFNB. A change of Farm A2 diet from 16.9% to 14.0% crude protein or from 0.46% to 0.40% P creates a negative balance. In other words, the farm is mining nutrients for export most likely from soil nutrient reservoirs. Such conditions are not sustainable and will eventually need to be offset by fertilizer purchases. For this farm, reducing dietary crude protein to 14% and phosphorus to 0.4% will eventually cause a nutrient shortfall in the cropping system. The cost of fertilizer purchase will need to be compared with the cost (or savings) of maintaining the higher dietary nutrient content need.

Results of On-Farm Measurement of WFNB

2006 Results - Appendix A

2007 Results - Appendix B

Return to Introductory Page for WFNB Resources

Authors: Rick Koelsch, University of Nebraska; Joe Lally, Iowa State University; Alan Sutton, Purdue University

This project was funded by The National Pork Board Project

¹NMPs also include practices designed to limit movement of nutrients from cropland. However, an NMP’s primary role is to insure a balance between all nutrient sources and crop nutrient removal. Accurate crediting of manure to replace commercial fertilizer additions is fundamental to an NMP on animal feeding operations.
²A positive imbalance (or an Input:Output ratio greater than 1) represents a direct loss to the environment or an accumulation of nutrients that may add to an environmental risk (e.g. high soil P levels). A negative imbalance (or an Input:Output ratio less than 1) suggests a farm is not importing sufficient nutrients to meet the needs of the products it is producing.
³For explanation of WFNB Ratio, refer to Introduction to WFNB.
⁴200 acres of corn (400 acres of crops in corn/soybean rotation) multiplied by fertilizer application rate.

Browse related Articles by tag: animal manure management, manure nutrients, pig manure, whole farm nutrient balance