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The metabolizable protein (MP) feeding standard for lactating cows represents a balance between animal requirements for metabolizable amino acids (MAAs) and their fulfillment by diets composed of a wide variety of feedstuffs. One additional measurement for feedstuffs, the .urea-fermentation-potential (UFP), is utilized when urea is a part of the diet. Although the MP standard is an amiho kei'd (AA) standard, nevertheless, an alte/'fiativd ~if~gle-value MP standard was approximated ~H~out resorting to calculations for each ess~iatlal 'AA. The net protein (NP) of body origin ifi the feces was estimated to be 12 g/kg 0~ dry matter (DM) consumed. This amount Was regarded as a feedstuff expense of digestion reducing the quantity of MP available for productive purposes such as lactation. The estimation of rumen microbial protein synthesis as 10.4% of total digestible nutrients (TDN) is based upon three evaluations. The first is that 52% of ration TDN undergoes digestion within the rumen. The second is that 25% of the digested TDN is transformed into microbial crude protein when adequate nitrogen is present. The third evaluation is that 80% of the microbial protein is alpha-amino. The 80% true digestibility value for abomasal microbial protein synthesized within the rumen is based upon cattle and sheep metabolism trials. Semi-purified diets 1

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