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

Overview

• This study investigates how breeding stallions undergo seasonal changes in metabolism, hormone levels, and immune function throughout the mating season and subsequent negative photoperiod.
• It tracks these changes longitudinally in the same horses to identify biomarkers related to reproductive health and systemic energy demands.

Introduction and Objective

• Breeding stallions experience sustained metabolic, endocrine, and immune demands during the mating season, which requires systemic energy utilization.
• Despite this, longitudinal data monitoring the same stallions through different seasonal phases is limited.
• The study aims to characterize temporal changes in:
• Health and nutritional status
• Oxidative status and immune modulation
• Metabolic and endocrine biomarkers correlating with reproductive potential

Methods

• A prospective, longitudinal observational cohort study was conducted on ten healthy breeding stallions (ages 8-22 years, mixed breeds, bodyweight between 496-625 kg).
• Assessments were done at four time points:
• P0: Start of the mating season
• P1: Mid-season
• P2: End of the mating season
• P3: Negative photoperiod (post-season time)
• Data collected included:
• Bodyweight (bwt), body condition score (BCS; scale 1-9), muscle condition score (MCS; scale 1-5)
• Complete blood count (CBC)
• Plasma levels of leptin, testosterone, catalase, non-esterified fatty acids (NEFA), beta-hydroxybutyrate (BHB)
• Six derived leukocyte ratios related to immune function
• Statistical methods used non-parametric repeated-measures analyses with correction for multiple comparisons.

Results

• Bodyweight and Condition:
• Bodyweight remained stable across the seasons (no significant difference, p=0.86).
• Muscle condition score (MCS) peaked at mid-season (P1), indicating season-related muscle changes.
• Energy Metabolism:
• At the start (P0), NEFA levels were elevated, signaling a subclinical negative energy balance prior to mating season.
• No ketonaemia (presence of ketones in blood) was detected, so energy deficit was mild/subclinical.
• Hormones:
• Leptin levels dropped significantly from P0 (3.70 ng/mL) to P3 (1.11 ng/mL), reflecting changes in nutritional or fat stores.
• Testosterone also declined from P0 to P3, with a notable greater decrease in stallions older than 13 years.
• Immune System Changes:
• Leukocyte counts decreased markedly at P2 (end of season), indicating leukopenia (p < 0.001), but recovered by P3.
• Derived leukocyte ratios highlighted immune modulation:
• Neutrophil-to-lymphocyte ratio (NLR) remained stable early but doubled at P3, suggesting immunosuppression post-season.
• Platelet-to-lymphocyte ratio (PLR) increased 13-fold at P2, indicating inflammation or immune shifts at the season’s end.
• Correlations were observed:
• NLR negatively correlated with testosterone (r = -0.315)
• Eosinophil-leptin ratio (ELR) negatively correlated with leptin (r = -0.372), suggesting a link between fat metabolism and eosinophil count.
• A consistent negative relationship existed between testosterone and white blood cell counts across timepoints.

Conclusions and Implications

• The mating season triggers subclinical metabolic adjustments and marked yet reversible decreases in leukocyte counts (leukopenia) in healthy stallions.
• Immune changes are proportional—some immune cell ratios suggest transient immunosuppression during and after the breeding season.
• The data supports an axis involving leptin and eosinophils, linking metabolism/fat status with immune modulation.
• Overall, these findings advocate for a biomarker-based approach combining metabolic, hormonal, and immune parameters to monitor stallion health, optimize breeding management, and enable preventive veterinary medicine tailored to seasonal demands.