Amino acid interactions in pigs
This thesis examines the influence of moderate excesses of protein, amino acid mixtures or single amino acids on the utilisation of dietary protein and amino acids by growing pigs. The work focused mainly on amino acid interactions affecting the utilisation of lysine, threonine and methionine, amino acids frequently limiting in pig diets. In the first part of the thesis the influence of dietary excesses of single amino acids (methionine, phenylalanine, tyrosine, tryptophan, histidine and lysine) were investigated. The results showed that an excess of about twice the requirement of one of these amino acids in a diet with an 'ideally' balanced amino acid pattern did not affect animal performance. The objective of the following experiment was to investigate the effect of different levels of protein on lysine utilisation in growing pigs given diets limiting in lysine. The effect was assessed by comparing the regression of daily nitrogen retention on daily lysine intake (using ileal digestible lysine) at three different protein levels (100, 200 and 300 g CP/kg diet). The regressions were not significantly different, showing that excess protein had no effect on lysine utilisation. Independent of the amount of excess protein, lysine was utilised with an efficiency of 0.58-0.65 and an increase of 1 g ileal digestible lysine enhanced protein accretion by about 9 g. The next series of experiments examined the effects of moderate excesses of essential- and non-essential amino acids on threonine and methionine utilisation, using nitrogen retention measured in pigs fed diets deficient in threonine or methionine. The findings showed that moderate excesses of essential amino acids increased the utilisation of threonine and methionine by about 7 to 22% but excesses of non-essential amino acids had no significant effects. A 50% excess (above requirement) of methionine in a threonine-deficient diet did not affect threonine utilisation.