Individual variability in metabolic and hormonal profiles for body reserve dynamics in ewes reared under indoor or outdoor farming system conditions

The ability of ruminants to cope with energy imbalances through alternating body reserves (BR) mobilization and accretion is a key mechanism to improve animals' resilience and/or robustness. This study aimed to characterize individual variability in BR dynamics using plasma concentrations of key biomarkers in productive ewes. Non-esterified fatty acids, β-hydroxybutyrate, triiodothyronine, insulin, and body condition score traits were monitored longitudinally throughout the productive cycles. The study included primiparous and multiparous ewes, reared under 2 contrasting farming systems (FS), indoor (173 ewes) and outdoor (234 ewes), belonging to 2 cohorts (Coh17/18). We used functional principal component analysis and unsupervised clustering to capture biomarker variation. The optimal number of clusters was selected using the Bayesian information criterion (BIC), integrated complete-data likelihood (ICL), normalized entropy criterion (NEC), and a minimum cluster size of >5% of ewes. The decrease in BR indicated that BR mobilization occurred from mid-pregnancy (P) until weaning (W), regardless of FS or parity (Par). On the contrary, the increase in BR suggested that BR accretion occurred from W until the next P in both FS. Between 2 and 3 distinct trajectories were identified for each biomarker (BIC < 3214.38; ICL < 3237.82; NEC < 6.450), depending on Par and/or FS. Most trajectories were characterized by transient increases in biomarker concentrations during the BR mobilization period, followed by declines that sometimes continued into the BR accretion phase. Such trajectories differed mainly in biomarker concentrations at different stages and/or the time point when peaks were observed. Greater individual variability in biomarker trajectories was particularly observed around lambing. Maintaining similar trajectories across cycles in major clusters for 53% to 100% of ewes suggested that biomarker trajectories might be repeatable. In addition to individual variability, BR levels, their temporal changes, and litter size contributed to the distribution of ewes across clusters for each of the traits, with low (P ≤ 0.05), moderate (P < 0.01), and high (P < 0.001) levels of significance. These findings highlight the potential of plasma biomarkers for characterizing individual variability in BR variations in ruminants reared in different FS conditions.