Review Article
Environmental Implications of Nitrogen Output on Horse Operations: A Review

https://doi.org/10.1016/j.jevs.2015.08.019Get rights and content

Highlights

  • Nitrogen (N) that is unused during digestion is then excreted into the environment.

  • Excess N may contribute to reduced quality of air and groundwater sources.

  • The potential impact of equine operations on N losses is explored.

Abstract

Nutrients such as nitrogen (N), which go unused during the digestive process, are then excreted into the environment via urine, gas, or fecal matter. Excess N released in this manner may contribute to a reduction of the quality of air and groundwater sources. Many states have introduced or developed legislation mandating nutrient management plans on livestock operations to reduce environmental N losses. Strategies for reducing the environmental impacts of N on equine operations are twofold, including a reduction in N inputs and better management of N outputs. The practice of precision feeding, or feeding to accurately meet, but not exceed the nutrients requirements of an animal is a plausible method for reducing N inputs. This approach is not widely implemented, as feeding protein in excess of requirements is a common practice in the equine industry. Also, precision feeding is predicated on a body of data containing the nutrient availability and digestibility in different feed sources; data which are not fully elucidated in the horse. Management of N outputs on equine operations is largely based on data extrapolated from other livestock operations as well as a few preliminary efforts on horse farms. The potential impact of equine operations on N losses is explored in this review, shedding light on areas where further research and management strategies are needed.

Introduction

Feeding practices with the goal of precisely meeting the dietary requirement of the first limiting amino acids (AA) have been implemented in many livestock systems to optimize performance and, recently, to minimize nitrogen (N) excretion into the environment, for example, the swine industry [1]. Paradoxically, feeding protein in excess of requirement has been a common practice in the horse industry [2]. Fecal and urinary N from livestock contributes to ground water contamination and decreased environmental air quality [3]. With increasing public awareness and emerging regulations to limit N losses to the environment, nutrient management plans may be mandated in the future to ensure sustainability of equine facilities, especially in increasingly urbanized areas. Strategies to mitigate the impact of equine-feeding practices on the environment are contingent on knowledge of factors that impact protein utilization of common feeds for equids. Because of the complexity of the equine digestive tract and the vast difference in feed types, including forages and cereal grains, prediction of feed protein digestibility and N output remains a challenge [4]. The goals of this review are to provide an overview of the current knowledge on N utilization in equids in relation to the environment and to identify knowledge gaps that preclude the progress in designing prediction models of N excretion in equids. The specific objective of this review is to assess the potential impact of N excretion from equids on the environment.

Section snippets

Environmental Implications of Excreted N

Nitrogen in the feces and urine must be managed to prevent consequences to the environment, such as water contamination and decreased air quality [5]. Excess N in the air and water will eventually reach larger bodies of water, where it can contribute to the deterioration of fisheries and fish habitats through harmful nutrient loading [6]. Manure nutrients (N, phosphorus, and organic matter) can be major pollutants in lakes and estuaries as well as rivers. Nitrogen attached to eroded soil

Defining a Need for Managing N Input and Output on Horse Operations

Animals acquire N from protein in the feedstuffs they eat. A substantial amount of organic N is excreted in urine or feces, as it has been estimated that less than 45% of consumed protein is used and made into animal protein [21], [22]. It has been shown that horse owners commonly over feed protein, up to 150% of the recommended requirements [2]. In a survey conducted by Harper [23], horse owners fed their horses 161% of National Research Council [24] crude protein (CP) requirements on average.

Nitrogen Content of Common Equine Feedstuffs

Digestibility of a protein and absorption of its AA constituents are the main determinants of protein quality and the predictor of utilization for maintenance or productive functions by the animal as compared. Domesticated equids are fed a wide variety of feeds ranging from herbage to seeds. The composition of proteins and their availability for digestion varies extensively across feeds, thus rendering prediction of N excretion difficult.

Nitrogen in equine diets comes from forages, grains, and

Nitrogen Balance

A Whole Farm Nutrient Balance [14] assessment is a tool that can be used to determine generation of excess nutrients on farm and can be useful in developing plans to manage nutrient buildups. This idea may be helpful for reducing N losses in equine production. The challenge is to manage the animals, crops, and other farm components to efficiently use available manure N and reduce potential losses to the environment [51]. Nutrient accumulation occurs when nutrient inputs exceed nutrient outputs

Challenges to Precision-Feeding N in the Horse Industry

In a Pennsylvania SARE Project Report [64], farm managers reported it was very difficult for horse owners to balance horse rations. Most farms used a commercial mixed feed concentrate that could not be adjusted. However, one farm with 70 horses reported that all horses were on individual diets. Most farms reported feeding hay from different weekly truckloads and did not produce hay on farm. In most cases, farm managers are unable to make major feed ration adjustments due to these reasons.

Programs and Regulations

In addition to the myriad of intrinsic reasons for managing horses to minimize nitrogenous impacts on the environment, horse owners, especially in populated areas near watersheds, recognize regulations imposed by government. Although all horse farms fall under the jurisdiction of the Federal Animal Feeding Operation, which is administered and regulated by the Environmental Protection Agency, many states have begun developing and passing laws which require higher standards than the federal

Conclusions

There are a myriad of factors at play in N utilization in horses and the impact on the environment. Management of horses and the associated N inputs and outputs is critical to prevent negative environmental consequences such as decreased air quality and water contamination. Precision-feeding and waste management are strategies that can minimize N excretion and environmental contamination. Feed choices and combinations affect N availability and absorption in the horse. Selection of feeds that

Acknowledgments

This project was supported in part by the US Department of Agriculture, Multi-State project NE-1041 and renewal NE-1441, Environmental Impacts of Equine Operations [73]. The authors thank Rozanne McGrath for editorial assistance.

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