Glycine Supplementation Improves Body Weight Gain In Slow-growing Broiler Chickens

Nutrient-specific appetite has been defined as a dietary selection developed as a response to a specific nutritional requirement (Niknafs and Roura 2018). In chickens, specific appetites offer the opportunity to determine individual differences in nutrient requirements (Roura and Navarro 2018). In a double-choice feed model, slow compared to fast-growing chicks showed a high preference for feed supplemented with Glycine (Gly) versus an iso-energetic balanced diet (Roura and Navarro 2019). Based on this specific amino acid preference, a 96 floor-pen experiment with 576 day-old chickens (Ross 308) was performed at the Queensland Animal Science Precinct (University of Queensland-Gatton, Australia). Chicks were tagged and sexed by extracting DNA from individual feather tips. At three weeks old, chickens were re-grouped into 8 categories based on body weight (BW) and assigned 1 of the 4 experimental treatments (standard diet (ST) + 1% ideal protein -caseinate-; ST + 1% Gly, ST + 1% Asp, and ST + 0.5% Gly + 0.5% Asp) following a randomized block design. ANOVA analysis was used to generate the least-squares means for each treatment with a covariate representing the sex ratio in each pen. Homogeneity was defined as all pens having a similar BW at day 42, with a regression line of chick performance at 42 days relative to BW at day 21. Pens supplemented with Gly 1% had consistently higher intercepts and lower slope values for BW (P=0.014), ADG (P=0.011) and ADFI (P< 0.001) compared with the control or the other supplemented diets Regarding BW, the slope for the glycine group (diet 3) was 0.890 while for diets 1 (control), 2 (aspartic) and 4 (glycine + aspartic) were 1.834, 2.295, and 1.923, respectively. The higher the slope the poorer the homogeneity. Gly supplementation (1%) improved the uniformity of BW at 42 days of age in broiler chickens by improving the growth in the poor-growing categories.

Gly has a pivotal role in amino acid metabolism sitting in the center of several pathways involving essential (especially sulphur amino acids, threonine, and arginine) and non-essential (serine, alanine, glutamic acid, and glutamine) amino acids (Li and Wu 2018). Our results indicate that a higher level of dietary Gly has a growth-boosting effect in slow-growing birds. These results agree with previous literature supporting conditional essentiality of Gly in chicken diets (Dean, et al., 2006). A cost-effectiveness analysis of Gly supplementation should be addressed to incorporate higher levels of Glycine in broiler diets.