Review Article
Gastrointestinal Nitrogen Metabolism of Equids and Impact on Protein Requirement

https://doi.org/10.1016/j.jevs.2016.06.002Get rights and content

Highlights

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    Evolutionary basis of gastrointestinal function and nitrogen metabolism in equids.

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    Pitfalls and challenges behind estimation of protein digestibility in equids.

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    Potential role of the large intestine in defining protein requirement of equids.

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    Gaps and future direction in the estimation of amino acid requirements of equids.

Abstract

Equids evolved grazing forage of low-protein and high-fiber content. However, present day horse feeding management typically consists of higher protein and less fiber, often exceeding protein requirements. The impact of feeding excessive proteins to equids on nitrogen (N) excretion and contamination of ground water is of particular concern and relevance in areas close to water ways. A review was prepared as part of an initiative by the USDA Multi-State project NE-1041 committee on “Environmental Impacts of Equine Operations” to build programs aimed at mitigating N excretion from equine feeding operations. This review presents information on dietary protein utilization in equids and identifies knowledge gaps for potential key future research areas to build upon. The review addresses the gastrointestinal (GIT) anatomy of equids with an emphasis of evolutionary dietary and anatomic adaptations. Challenges in assessment of protein quality of feeds are emphasized in particular in regard to the significance of prececal and postcecal protein digestibility and the contribution from hindgut N and amino acid (AA) metabolism and absorption. The need for greater understanding of GIT protein digestion processes, anatomic site of N and AA absorption, and systemic access to protein and AA digestibility estimates of equine feeds to refine current CP and generate AA requirement estimates is discussed.

Introduction

It is common in the horse industry to feed proteins in excess of requirement [1]. The potential impact of feeding excessive proteins to equids on nitrogen (N) excretion and contamination of ground water is of particular concern and relevance in areas close to water ways [1]. Designing and implementing plans to mitigate the impact of equine feeding practices on the environment (reviewed in [1]) entails accurate predictions of dietary N utilization and excretion. In livestock, successful predictions of N utilization and excretion are primarily based on knowledge of feed protein quality and understanding of the digestive processes. Equids are fed a wide diversity of forages, cereal grains, and oil seeds that vary in protein quality, but there is poor systematic availability of such information. In addition, processes of protein digestion and amino acid (AA) absorption are less well documented in equids compared to other livestock. The shortage of information has precluded building models of N and AA utilization that are needed for the factorial estimation of N and AA requirement for equids. This review was prepared as part of an initiative by the USDA Multi-State project NE-1041 committee on “Environmental Impacts of Equine Operations” to build programs aimed at mitigating N excretion from equine feeding operations. Our goal was to assemble information on dietary protein utilization in equids and identify knowledge gaps for potential key future research areas to build upon. The review begins by describing the gastrointestinal (GIT) anatomy of equids, highlighting evidence of their evolutionary dietary and anatomic adaptation to high-fiber, low-protein plant material. Challenges in assessment of protein quality of feeds are emphasized in particular in regard to the significance of prececal and postcecal protein digestibility and the contribution from hindgut N and AA metabolism and absorption. The need for greater understanding of GIT protein digestion processes, anatomic site of N and AA absorption, and systemic access to protein and AA digestibility estimates of equine feeds to refine current CP and generate AA requirement estimates is discussed.

Section snippets

Evidence of Relatively Low Dietary Protein Requirement of Equids Among Herbivorous Mammals

Unlike many herbivorous animals, which eat the reproductive products of plants including fruits, seeds, and berries, the equidae subsist on the structural parts of the plant. These include the stems and leaves, which contain significant amounts of structural polysaccharides, including cellulose, hemicelluloses, and lignin [2]. Foraging ungulates comprising the artiodactyl and perissodactyl orders have adopted a unique strategy in the herbivore community. They select different plant taxa in the

Defining Protein Digestibility

Horses are hindgut fermenters; thus, they rely on microbial degradation of fibrous dietary components to synthesize VFA [26]. Unlike ruminants, where fermentation of structural polysaccharides and microbial protein synthesis occur before passage through the stomach and small intestine, fermentation in equids mainly occurs distal to the small intestine in the cecum and proximal colon [16], [27] as discussed above, thus complicating the measurement of protein digestibility or “availability.”

Protein Digestibility of Common Forages Fed to Horses

Forage protein digestibility depends on the relative proportion of the cell content fractions (lipids, non–structural carbohydrates, pectin, starch, non–protein nitrogenous compounds, and soluble proteins) and of the cell wall constituents (fiber-bound protein, cellulose, hemicelluloses, lignin and lignified nitrogenous compounds) [2], [65]. In hindgut fermenters and ruminant animals, cell protein components are considered highly digestible [2], fiber-bound proteins are only partially

Conclusion

Equids have uniquely and successfully evolved with dentition and hindgut fermentation that have allowed them to use a diet higher in fiber and lower in protein compared to other grazing herbivores. Less than half of the equid's stomach is composed of proper gastric mucosa which supports an evolutionary strategy favoring dietary selection for lower protein content herbage. Feeding equids to meet their minimum N requirements is the most effective and simple approach to mitigate N excretion and

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    Financial disclosure: 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 [81].

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