Physiological mechanisms regulating metabolic efficiency in commercial broilers.

Authors: ELLESTAD, LAURA - University Of Georgia; LATZSCH, SAMUEL - University Of Georgia; Rothrock, Michael; Guard, Jean

Submitted to: Poultry Science Association Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 4/30/2019
Publication Date: 7/15/2019
Citation: Ellestad, L., Latzsch, S., Rothrock Jr, M.J., Guard, J.Y. 2019. Physiological mechanisms regulating metabolic efficiency in commercial broilers. Poultry Science Association Meeting Abstract. 233.

Interpretive Summary: Recent implementation of the Veterinary Feed Directive and increased demand from the public for food produced without antibiotics have created a need for development of strategies to maintain broiler production efficiency in the absence of antibiotic growth promoters. As feed accounts for almost 70% of total broiler production cost, understanding basic mechanisms regulating efficiency of feed nutrient use is crucial to developing these strategies and preserving production margins, particularly in antibiotic-free systems. In any given population of broilers raised under the same conditions, there is a natural variation in metabolic efficiency as measured by feed conversion ratio [FCR; g feed intake (FI)/g body weight gain (BWG)]. The objective of this study was to identify physiological parameters associated with hormonal regulation of nutrient uptake and utilization differing between high efficiency (low FCR) and low efficiency (high FCR) commercial broiler chickens. Male Ross 708 broilers (108 birds) were reared in single-bird battery pens between post-hatch day (D) 7 and D35, and BWG, FI, and FCR were calculated on an individual bird basis from D7- D35. On D36, blood, liver, and muscle were collected from the six highest (HE) and lowest (LE) efficiency birds based on FCR for measurement of circulating hormones and expression of genes associated with hormonal action and nutrient partitioning (n=6). Data were analyzed with a two-tailed Student’s t-test. While there were no differences between HE and LE groups in terms of D35 final body weight, BWG, or FI, LE birds had a significantly higher FCR than HE birds (1.69±0.03 vs 1.41±0.007; P<0.0001). Circulating levels of thyroxine (T4) were higher in the LE group (17.4±0.42 vs 13.6±0.62 ng/mL; P<0.05), and levels of corticosterone (CORT; 558.8±70.0 vs. 364.1±72.8 pg/mL; P=0.0787), growth hormone (GH; 1532±251.9 v. 1043.9±35.3 pg/mL; P=0.1030), and insulin (INS; 7.0 ±1.0 vs. 4.61.69±0.4 µIU/mL; P=0.0788) tended to be higher in LE birds. Plasma levels of insulin-like growth factor 1 (IGF1) and triiodothyronine did not differ between groups. In muscle, mRNA levels of IGF1 receptor (2-fold), cationic amino acid transporter 1 (CAT1; 5-fold), and glucose transporter (GLUT) 5 (2-fold) were higher in HE birds (P<0.05). Levels of mRNA for CAT1 (2-fold) and GLUT2 (1.2-fold) were also higher in the liver of HE birds (P<0.05), while expression of sodium-glucose transporter 1 (1.8-fold) was higher in LE birds (P<0.05). These data suggest that plasma T4, and to a lesser extent CORT, GH, and INS, may contribute to improved feed efficiency by altering expression levels of key amino acid and sugar transporters in metabolically important tissues such as liver and muscle.

Technical Abstract: Recent implementation of the Veterinary Feed Directive and increased demand from the public for food produced without antibiotics have created a need for development of strategies to maintain broiler production efficiency in the absence of antibiotic growth promoters. As feed accounts for almost 70% of total broiler production cost, understanding basic mechanisms regulating efficiency of feed nutrient use is crucial to developing these strategies and preserving production margins, particularly in antibiotic-free systems. In any given population of broilers raised under the same conditions, there is a natural variation in metabolic efficiency as measured by feed conversion ratio [FCR; g feed intake (FI)/g body weight gain (BWG)]. The objective of this study was to identify physiological parameters associated with hormonal regulation of nutrient uptake and utilization differing between high efficiency (low FCR) and low efficiency (high FCR) commercial broiler chickens. Male Ross 708 broilers (108 birds) were reared in single-bird battery pens between post-hatch day (D) 7 and D35, and BWG, FI, and FCR were calculated on an individual bird basis from D7- D35. On D36, blood, liver, and muscle were collected from the six highest (HE) and lowest (LE) efficiency birds based on FCR for measurement of circulating hormones and expression of genes associated with hormonal action and nutrient partitioning (n=6). Data were analyzed with a two-tailed Student’s t-test. While there were no differences between HE and LE groups in terms of D35 final body weight, BWG, or FI, LE birds had a significantly higher FCR than HE birds (1.69±0.03 vs 1.41±0.007; P<0.0001). Circulating levels of thyroxine (T4) were higher in the LE group (17.4±0.42 vs 13.6±0.62 ng/mL; P<0.05), and levels of corticosterone (CORT; 558.8±70.0 vs. 364.1±72.8 pg/mL; P=0.0787), growth hormone (GH; 1532±251.9 v. 1043.9±35.3 pg/mL; P=0.1030), and insulin (INS; 7.0 ±1.0 vs. 4.61.69±0.4 µIU/mL; P=0.0788) tended to be higher in LE birds. Plasma levels of insulin-like growth factor 1 (IGF1) and triiodothyronine did not differ between groups. In muscle, mRNA levels of IGF1 receptor (2-fold), cationic amino acid transporter 1 (CAT1; 5-fold), and glucose transporter (GLUT) 5 (2-fold) were higher in HE birds (P<0.05). Levels of mRNA for CAT1 (2-fold) and GLUT2 (1.2-fold) were also higher in the liver of HE birds (P<0.05), while expression of sodium-glucose transporter 1 (1.8-fold) was higher in LE birds (P<0.05). These data suggest that plasma T4, and to a lesser extent CORT, GH, and INS, may contribute to improved feed efficiency by altering expression levels of key amino acid and sugar transporters in metabolically important tissues such as liver and muscle.