From bodyweight to biomarkers: Season‐associated metabolic, endocrine and haematological shifts in breeding stallions

Abstract Background Breeding stallions are subjected to sustained seasonal metabolic, endocrine, and immune demands requiring systemic energy engagement. Prospective longitudinal data in the same individuals are limited. Objectives To screen adaptive responses of breeding stallions of clinical relevance through longitudinal characterisation of temporal changes across the mating season and negative photoperiod: (i) health and nutritional status; (ii) oxidative status and immune modulation; and (iii) metabolic and endocrine biomarkers as correlates of reproductive potential. Study Design Prospective longitudinal observational cohort study. Methods Ten clinically healthy breeding stallions (mixed breeds, age 8–22 years; bwt 496–625 kg) were assessed at P0 (start), P1 (mid‐season), P2 (end of season) and P3 (negative photoperiod). Bodyweight, body condition score (BCS; Henneke 1–9) and muscle condition score (MCS; 1–5) were recorded alongside complete blood count, plasma leptin, testosterone, catalase, non‐esterified fatty acids (NEFA) and beta‐hydroxybutyrate (BHB). Six derived leucocyte ratios were calculated. Non‐parametric repeated‐measures tests were applied (Friedman/Kruskal–Wallis; Wilcoxon post hoc with Bonferroni correction). Results Bodyweight was stable ( p = 0.86), yet pre‐season NEFA were increased (P0: 0.354 mmol/L; p < 0.05 vs. subsequent time points) without ketonaemia, indicating subclinical negative energy balance. MCS peaked at mid‐season ( p = 0.005). Leptin declined from P0 (3.70 ng/mL) to P3 (1.11 ng/mL; p = 0.001); testosterone fell from P0 to P3 ( p = 0.033), with greater attenuation in stallions aged >13 years. Leucocytes fell from 7.59 to 0.83 × 10 9 /L at P2 ( p < 0.001), recovering at P3. Derived ratios confirmed proportional immunosuppression (NLR stable P0–P2, doubling at P3; p = 0.0004) and a 13‐fold PLR rise at P2 ( p < 0.001). NLR correlated with testosterone ( r = −0.315; p = 0.048) and ELR with leptin ( r = −0.372; p = 0.018). Intra‐timepoint correlations between testosterone and WBC were consistently negative (P0: r = −0.090; P1: r = −0.202; P2: r = −0.298; P3: r = −0.559, p = 0.093). Conclusions The mating season elicits subclinical metabolic adaptations and marked reversible leukopenia in clinically healthy stallions. Derived ratios demonstrate proportional immunosuppression and a leptin–eosinophil axis, supporting an integrated biomarker‐based approach to preventive medicine.