The implementation of the MP system to replace CP was a major step forward in terms of biology and nutrition. The NRC committee used a large number of research papers to evaluate the relationship between CP and milk production.1-2-1 In this large data set, CP accounted for <30% of the differences observed in milk production. This is due mainly to the inability of the CP system to account for differences in protein fractions contained in feeds. Two feeds may have the same level of CP, but vary in the proportion in the RDP and RUP (rumen undegradable protein). This difference in the RDP and RUP fractions will result in a different milk production potential.

MP is the sum of microbial CP (MCP), RUP, and endogenous CP. One definition of MP is that it consists of the true protein that is digested in the intestine plus the amino acids (AA) absorbed in the intestine. The absorbed amino acids are the precursors used for synthesis of protein in the cow. Lysine and methionine appear to be the most limiting essential amino acids in dairy cattle. Even though the exact AA requirements have not been defined for dairy cattle, it is suggested that expressing requirements as a percentage of MP is the best current way to describe AAs in rations. The optimum values from literature data are 7.2% for lysine and 2.4% for methionine as a percentage of MP.[2] It is difficult to attain these levels in practical rations without the use of protected amino acids. A more practical approach is to target lysine at 6.6% of MP and methionine as 2.2%.[3- It is suggested that the target ratio of lysine:methionine is 3:1.

The protein fractions in feeds have also been divided into A, B, and C fractions.1-2-1 Fraction A is the percent of the total CP that is in the nonprotein nitrogen (NPN)

fraction. This fraction is assumed to be very rapidly available in the rumen. Fraction C is the portion of the CP that is undegradable in the rumen. Fraction B is the total CP minus that present in the A and C fractions. Tables in the Dairy NRC contain the protein A, B, and C fractions for most common feeds.[2]

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