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.
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