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Protein Physiology and Its Application in the Lactating Cow: The Metabolizable Protein Feeding Standard^{1, 2,}

Iowa State University^,4, Ames 50010

Abstract

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 amino acid (AA) standard, nevertheless, an alternative Single-value MP standard was approximated without resorting to calculations for each essential AA. The net protein (NP) of body origin in the feces was estimated to be 12 g/kg of 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 devoid. Of alpha-amino protein were fed in these trials from which the metabolic protein in the feces was computed and subtracted using an average nonruminant value. Additional MP and MAAs arising, from diets balanced with positive and negative UFP feedstuffs are of significance and are illustrated with an example diet. The conclusion is reached from examination of UFP values assigned to 100 feedstuffs that approximately 20% are capable, and 80% incapable, of being benefited by urea or other forms of nonprotein nitrogen, (NPN). This is based upon the results of the feeding standard that only those feedstuffs or diets containing less natural protein than 13 to 14.5% of TDN are capable of being benefited by NPN supplementation. The amount of the benefits is in direct proportion to how much the natural protein falls below the 13 to 14.5% level, provided this level supplies lactation requirements and the UFP of the diet is not exceeded. Where protein requirements exceed 13 to 14.5% of TDN and the diet contains less natural protein than this amount, the standard predicts that urea supplementation will be beneficial but less so than natural protein.

¹ Journal Paper No. J-7972 of the Iowa Agriculture and Home Economics Experiment Station, Ames. Project No. 1789.

² Invitational paper presented at the Livestock Sections-Dairy Cattle-Protein Physiology and Its Application in the Lactating Cow Symposium held during the 66th Annual Meeting of the American Society of Animal Science, University of Maryland, College Park, July 28 to 31, 1974.

³ The authors are grateful for the mathematical assistance of Mr. Allen Nipper in the preparation of this paper.

⁴ Department of Animal Science.

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