LIMITING AMINO-ACIDS IN DAIRY-COWS GIVEN CASEIN AS THE SOLE SOURCE OF PROTEIN

The object of the two experiments undertaken was to identify which amino acids (AA) in a test protein, casein, were limiting for production in the lactating dairy cow. Experiment 1 was designed to compare the response of the lactating cow to supplementation of a basal protein input with supplements of (a) casein, (b) mixtures of 18 synthetic essential (EAA) and non-essential (NEAA) amino acids formulated to mimic casein or (c) the same 12 EAA as in (b) but excluding NEAA. In Experiment 2, the effect of removal of AA from the synthetic EAA mixture on production responses allowed the identification of the order and extent of AA limitation. Nine British Friesian dairy cows maintained by total intragastric infusion of nutrients, with casein as the sole source of protein, were used. The daily basal casein nitrogen (N) (g/kg W0.75) and total energy (kJ ME/kg W0.75) inputs for Experiments 1 and 2 were 1.17, 920 and 1.27, 1130, respectively. The results of Experiment 1 showed that retained N and productive N (milk N + retained N) did not differ significantly between the three supplements. Therefore, it was concluded that the supplement of EAA alone was an adequate substitute for additional casein. In Experiment 2, statistical tests showed that, relative to the response obtained with the full complement of EAA, productive N values were estimated to decrease by 91, 76 and 54% when lysine, methionine and histidine, respectively, were removed individually from the EAA mixture. Therefore, lysine, methionine and histidine were identified as the first, second and third limiting AA in the lactating cow, where casein was the sole protein source.