The ideal dietary protein for growing pigs
The aim of this study was to determine the optimal amino acid balance in dietary protein for the growing pig. The method used was based on the principle that removal of any amino acid in excess of that required in this amino acid pattern should not affect nitorgen retention. A semi-purified diet based on casein and amino acids was used in a series of experiments. A proportion of each amino acid was removed in turn from the control amino acid pattern to determine how much could be removed without affecting nitrogen retention. The effect of changing the ratio of essential: non-essential amino acid was also examined. From these experiments an ideal dietary amino acid pattern for growing pigs was determined. This pattern was utilized significantly better than that proposed by ARC(1981). The property that in such an ideal pattern each amino acid should be equally limiting was tested and confirmed in both short and long term nitrogen balance trials. In the final experiment, the optimum ratios between lysine, methionine, threonine and tryptophan at two different rates of growth were determined using diets based on maize and soya bean meal. It was found that less threonine was needed than was deduced from the experiment with semi-synthetic diets. From the results of all the experiments the following amino acid pattern (g/160gN) is suggested: lysine 65, methionine + cystine 39, threonine 43, tryptophan 12, valine 49, isoleucine 39, leucine 72, phenylalanine + tyrosine 78, histidine 25.