Effects of reducing dietary crude protein and whole grain feeding on performance and amino acid metabolism in broiler chickens offered wheat-based diets

A total of 336 off-sex, male Ross 308 chicks were offered seven dietary treatments from 7 to 35 days post-hatch; each treatment was offered to eight replicate cages with six birds per cage. Three wheat-based diets were formulated to declining crude protein (CP) levels of 215, 190 and 165 g/kg but with a constant energy density (12.70 MJ/kg), electrolyte balance (250 mEq/kg) and digestible lysine level (11.00 g/kg). In a 2 x 3 factorial arrangement birds were offered either 215 or 165 g/kg CP diets to which 0%, 12.5% and 25.0% whole gain was incorporated postpelleting. In addition, a ground grain, 190 g/kg CP diet served as a seventh treatment. The assessed parameters included growth performance, relative gizzard, pancreas and abdominal fatpad weights, nutrient utilisation, concentrations of free amino acid in portal (anterior mesenteric vein) and systemic (brachial vein) plasma and apparent jejunal and ileal amino acid digestibility coefficients and disappearance rates. The CP reduction from 215 to 165 g/kg compromised FCR by 5.99% (1.576 versus 1.487; P < 0.005) and increased relative fat-pad weights by 12.2% (8.02 versus 7.15 g/kg; P < 0.025). Whole grain feeding (25.0%) significantly decreased relative fat-pad weights by 14.6% (6.91 versus 8.09 g/kg) but did not otherwise improve the performance of birds offered reduced-CP diets. Whole grain feeding enhanced energy utilisation (AME, ME:GE ratios, AMEn) but the CP reduction significantly compromised AME and AMEn. The transition from 215 to 165 g/kg CP in ground grain diets significantly depressed ileal digestibilities of histidine, alanine, aspartic acid, glycine, serine and tyrosine and generally depressed jejunal and ileal amino acid disappearance rates. However, methionine disappearance rates were increased by 28.6% (0.485 versus 0.377 g/bird/day; P < 0.005) and 22.8% (0.539 versus 0.439 g/bird/day; P < 0.001), respectively. The effects of 215, 190 and 165 g/kg CP diets on portal free amino acid concentrations were diverse as lysine was linearly increased but histidine, leucine, tryptophan, asparagine, glycine, serine and tyrosine were decreased. The effects of reduced CP diets and whole grain feeding on systemic free amino acid concentrations were also diverse as significant treatment interactions were observed for isoleucine, leucine, lysine, valine, glutamine and proline. Reducing dietary CP compromised feed conversion efficiency which was not attenuated by whole grain feeding although whole grain feeding did reduce relative abdominal fat-pad weights.