Maize-based diets were formulated to contain 208, 193, 179 and 165 g/kg crude protein (CP) were prepared and the last diet was supplemented with 4.33 g/kg glycine equivalents and 1.10 g/ kg threonine, individually and in combination, to provide seven dietary treatments. Each dietary treatment was offered to eight replicate cages (6 birds per cage) of off-sex, male Ross 308 chickens from 7 to 35 days post-hatch. The transition from 208 to 165 g/kg CP diets numerically compromised weight gain and feed conversion ratios (FCR) by 5.41% and 3.44%, respectively, and linearly increased relative fat-pad weights from 7.62 to 13.31 g/kg. The same transition linearly increased ileal starch:protein disappearance rate ratios from 2.01 to 3.23 (r = -0.823; P < 0.001) which were linearly related with relative fat-pad weights and quadratically related with FCR to significant extents across all seven dietary treatments. However, tandem inclusions of glycine equivalents and threonine in the 165g/kg CP diet significantly decreased dis- appearance rate ratios from 3.23 to 2.64; moreover, this tandem inclusion increased weight gain from 1994 to 2150 g/bird, decreased relative fat-pad weights from 13.31 to 11.65 g/kg and numerically improved FCR from 1.562 to 1.520. Instructively, birds offered the 165 g/kg CP diet supplemented with glycine equivalents and threonine performed comparably to their 208 g/kg CP diet counterparts with numerical differences of 1.99% more weight gain, 3.03% higher feed intake and a 0.66% higher FCR. That tandem inclusions of glycine equivalents and threonine in the 165g/kg CP diet condensed ileal starch:protein disappearance rate ratios diet largely stemmed from a 6.18% (0.842 versus 0.793; P < 0.005) increase in ileal protein (N) digestibility coefficients and an increase of 10.1% (21.39 versus 19.42 g/bird/day; P < 0.005) in ileal protein (N) disappearance rates. It is then relevant that the tandem inclusion increased mean apparent ileal amino acid digestibility coefficients by 7.99% (0.838 versus 0.776; P < 0.01) where increases in threonine, glycine and serine digestibilities were pronounced.
The possible mechanisms whereby tandem inclusions of glycine equivalents and threonine in a reduced-CP diet increased ileal amino acid digestibilities, and ultimately condensed starch:protein dis- appearance rate ratios, are discussed and may stem from the up-regulation of relevant amino acid transport systems with over-lapping specificities.