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Introduction This report reviews the utilization of supplemental nonprotein nitrogen (NPN) compounds by ruminant and nonruminant animals. It is pre- sented as a guide for the successful feeding of these compounds in practical production programs. It also attempts to summarize research findings that established the present concepts of NPN utilization in the hope that they will suggest other means for making use of NPN. Only limited utilization of NPN compounds in nonruminant diets containing natural feedstuffs has been demonstrated to date. Whether future costs of protein and the availability of amino and alpha-keto acids make the inclusion of NPN compounds in nonruminant diets economically feasi- ble remains to be seen. Recently, the economic savings that occurs from substitution of NPN for plant protein has stimulated renewed interest in NPN compounds and effective ways to maximize their use. Nutritionists generally agree that protein is a major limiting nutrient for animal production around the world. This is especially true of rumi- nants subsisting primarily upon forages and roughages. Urea and other NPN compounds may be available in these areas and can be used to supplement the diets of ruminants under some conditions. Ruminants have evolved to the extent that their ruminal environment fosters a dense population of microorganisms in the forepart of the di- gestive tract. This allows these animals to obtain energy from the ligno- cellulose complexes that are poorly digested by nonruminants. In addi- tion, the ruminal microorganisms can utilize NPN compounds to synthesize protein, which can be converted by the host into meat, milk, 1
2 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition and fiber for human usage. The nonruminant, lacking appropriate fer- mentation capacity, must be provided dietary protein of higher quality. In this volume protein refers to crude protein (N X 6.25) unless other- wise specified. True protein refers to polypeptides of amino acids. Pro- tein of natural feedstuffs contains varying proportions of true protein and naturally occurring NPN constituents. The literature on NPN utilization is voluminous. References cited in this volume have been selected and do not cover all of the published literature on this topic. They give concepts, illustrate successful utiliza- tion of NPN compounds, and present information on controversial points. Some references will be used to delineate limitations in NPN utilization.
