The effects of threonine, glycine and serine on growth performance, fat deposition, nutrient utilisation and starchprotein digestive dynamics in male broiler chickens offered reduced-crude protein diets

Abstract

Reducing dietary crude protein (CP) significantly reduces soyabean meal use, fecal output and nitrogen (N) emissions. However, tangible reductions in CP typically result in compromised broiler performance. In the present study, reduced CP diets (maize/soyabean) were offered to Ross 308 off-sex male broilers from 14 to 35 d post-hatch and supplemented with glycine (Gly), serine (Ser) and threonine (Thr). Seven iso-energetic (12.9 MJ/kg, AMEn), 11.0 g/kg standardized ileal digestible lysine (SID Lys) diets, maintaining minimum "ideal" essential AA ratios, were offered to 336 broilers (6 birds/cage × 8 replicates) fed a common starter to 14 d. CP in diets 1 to 4 was reduced incrementally from 205 to 165 g/kg and the remaining 3 diets (165 g/kg CP) were supplemented with either Thr, Gly + Ser or Thr + Gly + Ser. Gly and Ser were supplemented in diets 6 and 7 at levels equal to the SID values of the highest CP diet (6.66 and 8.46 g/kg respectively). SID Thr was supplemented to provide a ratio of 0.75 to SID Lys in diets 5 and 7 but kept at a ratio of 0.65 to Lys for the remainder of the diets. Data were analyzed by one way ANOVA and pairwise comparisons where appropriate. Decreasing dietary CP from 205 to 165 g/kg increased (<i>P</i> < 0.001) relative fat pad weights by 75% from 7.6 to 13.3 g/kg of body weight. Addition of Thr or Gly + Ser to the 165 g/kg CP diet with had no impact on growth performance. However, supplementation with all 3 AA improved (<i>P</i> < 0.025) liveweight gain (2150 vs. 1994 g), feed intake (<i>P</i> < 0.025; 3269 vs. 3114 g), reduced relative fat pad weights (<i>P</i> < 0.006; 11.7 g/kg vs. 13.3 g/kg) and numerically increased FCE (<i>P</i> = 0.315; 640 to 658 g gain/kg feed) compared with the un-supplemented 165 g/kg diet. The supplemented 165 g/kg diet performed as well as the 205 g/kg CP diet except for higher (<i>P</i> < 0.001) relative fat pad weights (11.7 vs. 7.6 g/kg) with no impact on AME or starch digestibility coefficients at the distal jejunum or ileum. Starch disappearance rates (g/bird/day) were lower (<i>P</i> < 0.001) for the 205 g/kg CP diet compared with the reduced CP diets by an average 15.7% in the distal jejunum and ileum. Furthermore, the 165 g/kg diet supplemented with Thr, Gly and Ser resulted in a higher (<i>P</i> < 0.001) N digestibility coefficient and disappearance rate at the distal ileum (0.842 vs. 0.793 and 21.39 vs. 19.42 respectively) and thus reduced (<i>P</i> < 0.001) starch:protein disappearance rate ratios in the distal jejunum and ileum (3.85 vs. 4.24 and 2.64 vs. 3.23 respectively) that have been associated with improved broiler performance. This study demonstrated that reduced CP diets supplemented with Thr, Gly and Ser resulted in broiler performance comparable to the 205 g/kg diet.