Nutrient Balance on Nebraska
Livestock Confinement Systems

Rick Koelsch
Livestock Environmental Engineer
Biological Systems Engineering Department
Chase Hall, University of Nebraska-Lincoln

Gary Lesoing
Research Assistant Professor
Center for Sustainable Agricultural Systems
University of Nebraska-Lincoln

Managing the environmental risk associated with livestock production is a significant challenge. The degree of imbalance between the nutrient inputs and the managed nutrient outputs for a livestock operation provides insight as to the underlying causes of nutrient related environmental challenges. A nitrogen and phosphorus balance is presented for 33 Nebraska livestock operations (including 17 swine operations).

Nitrogen and phosphorus losses to surface and ground water are critical water quality issues associated with livestock manure. In Nebraska, approximately 320,000,000 pounds of nitrogen and 230,000,000 pounds of phosphorus are excreted annually by livestock and poultry. A 1995 GAO report to the U.S.Senate suggested that manure was the source of 37% of all nitrogen and 65% of all phosphorus into watersheds in the central states including Nebraska.

An underlying cause to the environmental problems associated with livestock production is the accumulation of nutrients. A large fraction of the nutrients consumed by livestock does not leave the farm as meat instead remaining on the farm in manure. The intent of this study is to define a whole farm nutrient balance on Nebraska livestock operations. The study also identifies characteristics or management practices that minimize the accumulation of nutrients on farm.

An accounting of nutrient inputs (purchased feed, fertilizer, animals, biologically fixed nitrogen, and nitrates in irrigation water) and managed nutrient outputs (animals, crops and other products moved off farm) was completed for 33 livestock operations (Figure 1). Changes in farm inventory were also included in the analysis. The accounting period was for one year (1995 for six operations and 1996 for 27 operations). The degree of imbalance was estimated based upon the differences in inputs, managed outputs, and inventory changes. The calculated imbalance in nutrients can either be lost to the environment (i.e. nitrate leaching to ground water, or ammonia volatilization) or added to soil storage mechanisms (i.e. increasing soil phosphorus levels).

The average nutrient balance for of all 33 farms is summarized in Table 1 for three distinct size groupings. The magnitude of nutrient inputs, managed outputs, and imbalance increases with livestock operation size. The relative nutrient imbalance (% of inputs) also increases with size of the operation. This measure more than double for those farms with more than 2500 animal units as compared to farms with less than 250 animal units (see "% of inputs" in Table 1).

Phosphorus balance provides the better indication as to when a sustainable nutrient balance has been achieved from a water quality perspective. Nitrogen can be lost through volatilization (a relatively benign environmental loss) or to surface and ground water (a more damaging environmental loss). Substantial losses of ammonia nitrogen by volatilization often masks when a reasonable nutrient balance is achieved. For this reason, the following comparisons will focus primarily on phosphorus balance.

The observed nutrient imbalance cannot be explained strictly by the size of the livestock operation (see Figures 2). Although larger livestock units tend to have greater nutrient imbalances, farm size alone provides only a limited explanation for the observed variation. Some of the largest nutrient imbalances were observed for farms with 100 to 1000 animal units.

A neutral or negative phosphorus balance was observed for several of the smaller livestock operations indicating equal or greater managed outputs than inputs of phosphorus (Figure 2). These farms tended to have fewer livestock numbers and larger land bases. Farms with negative phosphorus balances were commonly drawing upon soil phosphorus reserves during the one year for which the nutrient balance was estimated.

Several larger livestock operations were also observed to have a relatively small phosphorus imbalance (see Figures 2). A closer review of data from three of those farms indicates an active effort to move manure off-farm to neighboring crop producers. Marketing of manure nutrients increases the managed outputs of nutrients thus contributing to an improved nutrient balance.

The degree of integration of crop and livestock enterprises is often considered an indicator of the relative potential for environmental problems (Figures 3). For the 33 participating farms, the nutrient balance again shows substantial variation when plotted against the density of livestock to land base. Lower phosphorus imbalances were more common for livestock operations with larger relative land bases. However, the three previously mentioned cattle feedlots, all with very limited land resources, were capable of achieving a reasonable balance in phosphorus inputs and managed outputs by actively marketing manure nutrients(see Table 2).

The source of nutrient inputs to livestock operations is illustrated in Figures 4. Purchased animal feeds were a significant source of the nitrogen and phosphorus inputs. Nitrogen inputs as feed varied from 33 to 77% of total nitrogen inputs for farms with less than 250 animal units and more than 2500 animal units, respectively. Phosphorus inputs as feed show less variation, ranging from 62 to 71% of total inputs for the same livestock groupings. Feed was often the most significant source of nutrients arriving on livestock farms.

Commercial fertilizer was the most significant nitrogen input for livestock operations with less than 2500 animal units. Fertilizer was also an important source of phosphorus input for these same farms. Commercial fertilizer was an insignificant nutrient input for the livestock operations with more than 2500 animal units (2% of nitrogen inputs and 1% of phosphorus inputs).

This study has several implications relative to nutrient management of livestock production:

1. Evaluating livestock systems nutrient balance from a whole farm perspective provides a more complete picture of the driving forces behind nutrient related environmental challenges. Accumulation of nutrients resulting from an imbalance of nutrient inputs and outputs is a problem for many, but not all, Nebraska livestock operations.

2. An assessment of environmental risk based strictly on factors such as livestock herd size or livestock to crop land density oversimplifies a complex issue. Neither smaller sized livestock operations or operations better integrated with crop production insured that a "sustainable" nutrient balance resulted.

3. New strategies are needed for addressing the risk associated with nutrient accumulations on livestock operations. Nutrient management planning focused on minimizing commercial fertilizer purchases addresses only part of the nutrient inputs to most livestock operation. Future nutrient planning efforts should improve the whole farm nutrient balance by reducing purchased feed nutrient inputs and expanding managed outputs by marketing of manure nutrients to off-farm customers.

Table 1. Average characteristics and nutrient balance for 33 Nebraska livestock farms.

Table 2. Impact of manure marketing on nutrient balance for three livestock operations.

Figure 1. A whole farm nutrient balance for a livestock farm.

Figure 2. Phosphorus balance versus livestock system size.

Figure 3. Phosphorus balance versus cropland to animal density. 

Figure 4. Relative sources of nitrogen and phosphorus inputs.

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