Michael Kreuzer, David R. Yáñez-Ruiz, Angela Schwarm, Pierre Noziere, Alexander N. Hristov, Ali R. Bayat, Jan Dijkstra, Pekka Huhtanen, Mark McGee, André Bannink, Ermias Kebreab, K. J. Shingfield, Christopher K. Reynolds, Zhongtang Yu, Les A. Crompton, Department of Animal Science, Pennsylvania State University (Penn State), Penn State System-Penn State System, Wageningen Livestock Research, Wageningen University and Research [Wageningen] (WUR), University of Reading (UOR), Department of Agricultural Science, University of Naples Federico II, Institute of Agricultural Sciences, Ecole Polytechnique Fédérale de Zurich, Teagasc Agriculture and Food Development Authority (Teagasc), Unité Mixte de Recherche sur les Herbivores - UMR 1213 (UMRH), Institut National de la Recherche Agronomique (INRA)-VetAgro Sup - Institut national d'enseignement supérieur et de recherche en alimentation, santé animale, sciences agronomiques et de l'environnement (VAS)-AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement, Natural Resources Institute Finland (LUKE), Consejo Superior de Investigaciones Científicas [Madrid] (CSIC), Animal Nutrition Group, University of California, Institute of Biological, Environmental and Rural Sciences (IBERS), Aberystwyth University, Department of Animal Sciences, University of Illinois at Urbana-Champaign [Urbana], University of Illinois System-University of Illinois System, United States Department of Agriculture (USDA) 2014-67003-21979 NH00616-R, Department of Animal Science and College of Agricultural Sciences, The Pennsylvania State University, Swiss Federal Office of Agriculture, INIA (Spain) MIT01-GLOBALNET-EEZ, Natural Resources Institute Finland (Luke), Ministry of Agriculture, Nature and Food Quality (the Netherlands), GRA project BO-43-003.02.004, Institut National de la Recherche Agronomique (INRA)-VetAgro Sup - Institut national d'enseignement supérieur et de recherche en alimentation, santé animale, sciences agronomiques et de l'environnement (VAS), Natural Resources Institute Finland, Ministry of Agriculture, Nature and Food Quality (The Netherlands), CSIC - Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA), National Institute of Food and Agriculture (US), Pennsylvania State University, Federal Office for Agriculture (Switzerland), USDA National Institute of Food and Agriculture, Swiss Federal Office of Agriculture, Ministry of Agriculture, Nature and Food Quality, The Netherlands, INIA, 2014-67003-21979, NH00616-R, MIT01-GLOBALNET-EEZ, and BO-43-003.02.004
Nitrogen 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 effect on subsequent processes in the intestines and body tissues make the study of N metabolism in ruminants challenging compared with nonruminants. 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) wk, 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 AA 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. Accurate, precise, and inexpensive laboratory assays for feed protein availability are still needed. Techniques used for direct determination of rumen microbial protein synthesis are laborious and 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 AA metabolism are also laborious, expensive (especially the methods that use isotopes), and results can be variable, especially the methods based on measurements of digesta or blood flow. Volatile loss of N from feces and particularly 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, methods to study ruminant N metabolism have been developed over 150 yr of animal nutrition research, 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 more accurate and reliable methods to determine manure N emissions in the context of feed composition and ruminant N metabolism., The authors acknowledge support for preparation of this manuscript by the USDA National Institute of Food and Agriculture, grant/award number: 2014-67003-21979, NH00616-R, the Department of Animal Science and College of Agricultural Sciences, The Pennsylvania State University; the Swiss Federal Office of Agriculture, INIA (Spain, project MIT01-GLOBALNETEEZ); Natural Resources Institute Finland (Luke); and Ministry of Agriculture, Nature and Food Quality (the Netherlands), GRA project BO-43-003.02.004