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7 General Conclusions Despite hundreds of experiments carried out between 1940 and 1970, limited progress and understanding was made toward a more com- plete characterization of those dietary conditions under which added NPN would be beneficial. Within the past 5 years, new insights and de- velopments, though controversial at this time, are stimulating research that will provide much more definitive guidelines for added NPN use. Already it is apparent that the old guidelines-urea up to (a) one-third of the dietary nitrogen, (b) 1 percent of the total diet dry matter, and (c) 3 percent of the grain mixture-need major modifications. Because high levels of natural NPN and/or high levels of rumen-soluble true pro- teins occur in some feedstuffs, there may be dietary conditions under which no added NPN would be used. In the future, to answer the question, "How much NPN can be added to the diet of a ruminant?", it will be necessary to ask: (a) What is the level of protein in the basal diet relative to the animal's requirement? (b) What is the energy level (soluble or available carbohydrate) in the basal diet? (c) What level of soluble nitrogen is in the basal diet? (d) Is the level of other dietary components (especially required mineral elements) high enough to sustain maximum rumen microbial growth? (e) What feeding system will be used? (f) What is the expected level of intake? (g) Does the growth or production requirement for amino acids exceed the capacity of rumen microbial synthesis so that some rumen bypass of natural protein is needed? The answers to these questions and probably others will be needed to define when and how much added 95
96 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition NPN will be useful. Specific suggestions and guidelines are found at the end of each section for individual classes of livestock. Except by starvation prior to feeding, improper formulation, or use of a supplement not intended for direct feeding, no clear cases of urea toxicity or long-term detrimental effects have been reported. Research results of the next few years will provide much-needed answers for those questions that must be resolved in order to define those specific conditions under which added NPN will be beneficial.
