Relationships between stress indicators and sexual development during onset of maturity in broiler breeder roosters

The onset of sexual maturity involves extensive physiological and metabolic changes modulated by internal and external cues. To ensure reproductive efficiency, broiler breeder management relies on strict feed programs to prevent overweight. However, this strategy can induce chronic stress in susceptible individuals, potentially affecting the immune system (i.e., increased heterophil/lymphocyte (H/L) ratio), compromising both welfare and sexual development. At the same time, sexual maturity can lead to increased energy allocation, which itself may modulate immune parameters. In this context, we evaluated the network of relationships among welfare and morphophysiological variables (body weight, breast condition, health status, H/L ratio, plasma testosterone levels, and testicular width) in 29-week-old Cobb roosters under commercial broiler breeder farm conditions. Our aim was to improve the understanding of inter-individual variability during the onset of sexual maturation, with a focus on the H/L ratio as a classical indicator of chronic distress. Sixty roosters were assessed, all showing optimal plumage and no signs of pododermatitis or hock burns, indicating good health and general welfare. Histograms revealed high variability in plasma testosterone levels, with values ranging from 0 to 4 ng/mL. (Cobb Vantress, 2022). Noticeably, around 33 % of the birds showed relatively low testosterone concentrations (below 0.46 ng/mL) and small testes (width < 1.84 mm), consistent with sexual immaturity. As expected, strong positive correlations were found between body weight and breast conformation. Interestingly, a moderate but significant (ρ=0.57; P < 0.05) correlation emerged between H/L ratio and testicular width. To further investigate these relationships, roosters were categorized by high (>0.95), intermediate (0.54-0.90), and low (<0.51) H/L ratio groups. Roosters with high H/L ratios were heavier with higher testosterone levels than those with low ratios. Overall, these findings suggest that the onset of male sexual maturity in broiler breeders is not merely a period of distress but also a stage of adaptive physiological adjustments, where H/L ratio may serve as a key marker linking stress responsiveness to reproductive activation.