Review ArticleEnvironmental Implications of Nitrogen Output on Horse Operations: A Review
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.
References (73)
- et al.
Ration evaluation of Chesapeake Bay watershed horse farms from a nutrient management perspective
J Equine Vet Sci
(2009) - et al.
ADSA Foundation Scholar Award: implementing waste solutions for dairy and livestock farms
J Dairy Sci
(2006) - et al.
Control of gaseous emissions from livestock buildings and manure stores
J Agric Eng Res
(1994) - et al.
Dietary protein affects nitrogen and ammonia excretion in horses
J Equine Vet Sci
(2011) - et al.
Effect of phone book versus sawdust and straw bedding on the presence of airborne gram-negative bacteria, fungi and endotoxin in horse stalls
J Equine Vet Sci
(1998) - et al.
Measurement of ammonia concentrations in horse stalls
J Equine Vet Sci
(2000) - et al.
Dietary protein restriction and fat supplementation diminish the acidogenic effect of exercise during repeated sprints in horses
J Nutr
(2001) - et al.
The amino acid and protein content of corn as related to variety and nitrogen fertilization two figures
J Nutr
(1953) - et al.
Effects of feeding grass or red clover silage cut at two maturity stages in dairy cows. 1. Nitrogen metabolism and supply of amino acids
J Dairy Sci
(2009) - et al.
Nitrate accumulation in crops and nitrate poisoning in animals