Abstract/Summary

Nitrogen (N) is a component of essential nutrients critical for the productivity of ruminants. If excreted in excess, N is also an important environmental pollutant contributing to acid deposition, eutrophication, human respiratory problems, and climate change. The complex microbial metabolic activity in the rumen and the impact on subsequent processes in the intestines and body tissues make the study of N metabolism in ruminants challenging compared to non-ruminants. Therefore, using accurate and precise measurement techniques is imperative for obtaining reliable experimental results on N utilization by ruminants and evaluating the environmental impacts of N emission mitigation techniques. Changeover design experiments are as suitable as continuous ones for studying protein metabolism in ruminant animals, except when changes in body weight or carryover effects due to treatment are expected. Adaptation following a dietary change should be allowed for at least 2 (preferably 3) weeks, and extended adaptation periods may be required if body pools can temporarily supply the nutrients studied. Dietary protein degradability in the rumen and intestines are feed characteristics determining the primary amino acids available to the host animal. They can be estimated using in situ, in vitro, or in vivo techniques with each having inherent advantages and disadvantages. There is still a need for accurate, precise, and inexpensive laboratory assays for feed protein availability. Techniques used for direct determination of rumen microbial protein synthesis are laborious, expensive, and data variability can be unacceptably large; indirect approaches have not shown the level of accuracy required for widespread adoption. Techniques for studying postruminal digestion and absorption of nitrogenous compounds, urea recycling, and mammary amino acid metabolism are also laborious, expensive (especially the methods that utilize isotopes), and results can be variable, especially the methods based on measurements of digesta or blood flow. Volatile loss of N from feces and urine can be substantial during collection, processing, and analysis of excreta, compromising the accuracy of measurements of total tract N digestion and body N balance. In studying ruminant N metabolism, nutritionists should consider the longer-term fate of manure N as well. Various techniques used to determine the effects of animal nutrition on total N, ammonia- or nitrous oxide-emitting potentials, as well as plant fertilizer value, of manure are available. Overall, over 150 years of animal nutrition research have developed methods to study ruminant N metabolism, but many of them are laborious and impractical for application on a large number of animals. The increasing environmental concerns associated with livestock production systems necessitate a more accurate and reliable methods to determine manure N emissions in the context of feed composition and ruminant N metabolism.