Developments in Amino Acid Nutrition of Dairy Cows.

Sloan, B.K. 1997

Feed Manufacturers Conference

AA requirements are estimated using two approaches, either the factorial approach or the empirical "dose response" approach. In dairy cows given conventional diets it has been demonstrated that lysine (Lys) and methionine (Met) are the two most limiting amino acids. This is not surprising given that 1) Met and Lys are first and second limiting in ruminally synthesised microbial protein for nitrogen retention of growing cattle 2) most feed proteins have lower amounts of Lys and Met relative to total essential amino acids (EAA) than ruminally synthesised bacterial protein 3) the contribution of Lys to total EAA in rumen undegradable protein (RUP) is often slightly lower than in the same feeds before exposure to ruminal fermentation and 4) Lys and cystine, the latter which can be synthesised in the body from Met, probably have lower intestinal digestibilities than other EAA in RUP. Requirements for amino acids can be expressed either in daily amounts (g/d) or on the basis of profiles or patterns.

The author prefers the latter because 1) they can be determined more accurately, 2) it is easier to formulate a diet for a desired pattern of absorbed AA than a given quantity of AA 3) the field nutritionist is in a better position than the researcher to fine tune on farm diets for amounts of RUP and ruminally degraded protein and 4) the approach is consistent with the concept of "ideal protein" as proposed in pig and poultry nutrition. It seems reasonable to conclude that the required percentage of Lys and Met in duodenal digesta for the maximum content and yield of milk protein approximates to 15 and 5 or more of total EAA respectively when conventional diets are fed. A comparison of the requirements for Met and Lys with the contributions of Met and Lys to total EAA in feeds and with the calculated levels of Lys and Met in duodenal digesta of high producing early lactation cows indicates the difficulty of meeting simultaneously the required contributions of both Lys and Met for maximum content and yield of milk protein without an exogenous supply of Met and Lys. Production studies which are designed to determine the value of optimising intestinal AA profiles must be initiated at of before parturition in order to realise the full effects on herd health and lactational performance.