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Abstract
Subtherapeutic levels of antibiotics have historically been used in the broiler industry as antibiotics growth promoters (AGPs) to enhance production. Implementation of the Veterinary Feed Directive has barred this practice, generating a need to establish AGP alternatives. However, the mechanisms by which AGPs influence broiler growth and feed efficiency remain unknown, and there is a lack of information as to how AGPs affect physiological systems of broilers that differ in feed efficiency. The objectives of this study were to (1) determine how AGPs influence growth and metabolic efficiency in broilers with high or low feed efficiency and (2) determine how AGPs influence intestinal physiology of broilers with high or low feed efficiency. Male broilers were individually caged and received one of two diets: an antibiotic-free control diet or a diet containing an AGP. Birds and feed were weighed to determine individual feed conversion ratio [FCR; feed intake/body weight gain (BWG)]. Blood was collected on D21, D28, and D36 to assess circulating hormones and metabolites. On D36, the 8 birds with the lowest total FCR (high efficiency; HE) or highest total FCR (low efficiency; LE) from each diet were sampled, resulting in four groups: LE-C, LE-A, HE-C, and HE-A. Data were analyzed by ANOVA and post hoc comparisons were made with Fisher’s test of least significant difference when ANOVA indicated significance (P≤0.05). LE-A broilers were found to have improved growth and FCR over LE-C broilers. AGP-fed broilers of both HE and LE were found to have elevated somatotropic signaling, reduced thyrotropic signaling, upregulated gene expression associated with nutrient partitioning, increased concentrations of metabolites related to intestinal growth and differentiation, reduced ileal villi heights, and reduced expression of pro-inflammatory cytokines. HE broilers had reduced endocrine signaling of the thyrotropic axis, elevated skeletal muscle paracrine signaling of the somatotropic axis, and improved intestinal inflammatory response. These data suggest enhanced growth performance when feeding AGPs may be a result of more efficient nutrient partitioning and improved intestinal health and function. Differences in feed efficiency may be attributed to alterations in somatotropic and thyrotropic hormonal axes, as well as changes in the intestinal inflammatory response.