Immunocastration and testosterone supplementation alter testicular development and muscle collagen gene expression in male chickens

Abstract

Immunocastration using a gonadotropin-releasing hormone (GnRH) vaccine offers a potential alternative to surgical castration in poultry, yet its effects on growth performance and meat quality in chickens remain poorly characterized. Thirty-six male Sakura chickens were allocated to three groups (n = 12): control (C), immunocastration with Improvac at 4 and 8 wk (I), and immunocastration plus subcutaneous testosterone implantation at 8 wk (T). Birds were sampled at 10 and 16 wk. Immunocastration induced anti-GnRH antibody production in both I and T groups compared with C (P < 0.001 and P < 0.01, respectively). At 16 wk, serum testosterone was lower in I than in C (P < 0.05), whereas T showed levels comparable to C. Gonadosomatic index was markedly reduced in T at 16 wk compared with both C and I (P < 0.001 and P < 0.01, respectively), possibly reflecting combined HPG axis suppression by immunization and negative feedback by exogenous testosterone. Body weight was higher in C than in I and T from 7 wk onward (P < 0.01), but overall average daily gain did not differ among groups. Muscle color (L*, a*) differed between groups in the thigh but not in the breast. Raw shear force of the iliotibialis lateralis (ITL) was lower in I than in C and T (P < 0.05 and P < 0.001, respectively), suggesting that immunocastration may reduce connective tissue stiffness in thigh muscle. In the breast muscle, COL3A1 mRNA expression was elevated in T compared with C and I (P < 0.001 and P < 0.01, respectively), suggesting that testosterone may influence collagen gene expression in a muscle-specific manner. Further optimization of vaccination protocols specific to chickens is warranted to improve the consistency and efficacy of immunocastration effects.