The limiting amino acids for milk production in the dairy cow
1) A review of the literature relating to the amino acid requirements in ruminants is presented. It was concluded from this review that the amino acids most likely to be limiting for milk production were lysine and methionine. 2) The object of this project was, by using the approach developed by Storm (1982), to identify which amino acids in a test protein were limiting for production in the lactating dairy cow. The nutritional requirements of the animals were met by the total intragastric infusion technique; this system eliminated the problems of rumen fermentation, and allowed precise control of the amino acid supply to the small intestine of the experimental animals. 3) The identification of the limiting amino acids involved several steps which included:a) the estimation of the efficiency of nitrogen and amino acid nitrogen utilisation by the cow,b) calculation of the supplement required to increase the efficiency of N utilisation to a maximum of 1.0,c) the demonstration that a similar response in production could be obtained from a supplement of casein, or a mixture of synthetic essential and non-essential amino acids in similar proportions to those in casein,d) to establish that the removal of non-essential amino acids had no effect on production responses ande) measurement of the changes in production obtained by removal of individual essential amino acids from the amino acid supplement, according to the theoretical basis assumed by Storm (1982). 4) The efficiency of nitrogen utilisation of the test protein, casein was estalished under nitrogen limiting conditions from the regression of productive nitrogen (milk nitrogen and retained nitrogen) on nitrogen input for cows in early, mid and late lactation and cows in the dry period. The values obtained for the four stages were 0.693, 0.683, 0.583 and 0.607 for early, mid and late lactation and the dry period respectively. Analysis of the data revealed that the slopes of these relationships did not differ significantly. It appears, therefore, that under nitrogen limiting conditions, the efficiency of nitrogen utilisation was not effected by stage of lactation. The overall value obtained for the efficiency of utilisation of casein nitrogen supplied at the abomasum was 0.66. 5) Assuming that the proportion of amino acid nitrogen in total nitrogen was 0.88 and the true digestibility of amino acid nitrogen was 0.95, the efficiency of amino acid nitrogen utilisation (UAAN) could be calculated as follows:U_AAN = rm Efficiency of Nitrogen utilisationover rm (Proportion of AAN in total nitrogen) (True digestibility)Therefore, the supplement of amino acids required to raise the efficiency of amino acid nitrogen utilisation to the optimum of 1.0 was rm 1-UAANover rm UAAN of the basal casein input of amino acid nitrogen. 6) Using the value of 0.693 for the efficiency of utilisation of casein nitrogen, the amino acid supplement required was 0.206 of the basal casein input of amino acid nitrogen. Using this supplement it was established that a similar response in productive nitrogen could be obtained by supplementing the basal input of casein with either casein or a mixture of essential and non-essential amino acids in similar proportions to that of casein or a mixture of essential amino acids alone. The non-essential amino acids were excluded from the supplement in further experiments. 7) The efficiency of nitrogen utilisation used in the final experiment was that of mid lactation cows 0.683, as the cows were approaching mid lactation. Essential amino acids were removed from the supplement either singly or in pairs and changes in productive nitrogen were measured. Significant decreases in productive nitrogen were obtained by the removal of lysine, methionine and histidine. Removal of phenylalanine resulted in decreases in productive nitrogen, although this did not reach significance (p < 0.10). The average percent decrease in productive nitrogen with.