The impact of canola meal and enzyme supplementation on growth performance, carcass traits and digestibility of starch and protein in broiler chickens offered corn-based diets

Canola meal is a cost‑effective alternative to soybean meal in broiler diets in Australia and parts of Southeast Asia, but its inclusion rate is often limited due to its lower protein digestibility. Supplementation of enzymes, particularly phytase, is a common practice in broiler chicken industry. However, the additive effects of protease and carbohydrases on top of phytase on growth performance and nutrition digestibility remain unclear. A total of 576 off-sex male Ross 308 broiler chickens were assigned to eight dietary treatments in a 2 by 4 factorial design, including two diet types [corn-soybean meal (SBM) based and corn-SBM-canola meal (CM) based diets)], and four enzyme regimes [phytase, protease, phytase+protease, phytase+ enzyme cocktail (EC)]. Including CM in con-SBM diets compromised feed intake and weight gain (P < 0.05) in broilers from 11-24 days post-hatch but did not alter feed conversion (P = 0.134). The impact of CM inclusion on growth performance was not significant in broiler chickens aged 25 to 35 days, nor was enzyme supplementation. CM inclusion also compromised apparent Lys digestibility at day 35 (P = 0.014) and reduced total tract nitrogen retention in broiler chickens (P < 0.001), whereas protease-supplemented diets resulted in the highest N retention regardless of diet type (P = 0.038). There were interactions between diet type and enzyme supplementation for starch digestibility. In the distal ileum on day 24, the corn-SBM phytase-only diet showed the highest starch digestibility, but this pattern was not observed on day 35. In general, the phytase‑alone treatments resulted in higher apparent protein digestibility in the distal ileum of broiler chickens offered corn-SBM diets on day 24. For distal ileal apparent amino acid digestibilities at day 35, supplementing protease on top of phytase numerically improved the digestibility of most amino acids, whereas supplementing a protease and carbohydrase cocktail significantly improved digestibilities of all 16 tested amino acids in birds offered corn-SBM diets (P < 0.05). The extent of the response to additional enzymes beyond phytase was correlated with the inherent amino acid digestibilities in the baseline phytase-only diets (P < 0.05). By comparing the slope of the linear regressions, the additional benefit of supplementing mono-protease was greater in corn-SBM-CM diets, whereas the additional benefit of supplementing the protease and carbohydrase cocktail was greater in corn-SBM diets.