Body reserves dynamic of suckling ewes across successive production cycles under outdoor and indoor contrasting farming system conditions

Body reserves (BR) mobilization (BRM) and accretion (BRA) are crucial biological processes in ruminants that help them manage negative energy balance and adapt to changing environments. The BR dynamics (BRD) is affected by the interplay of key factors such as the farming system (FS) characteristics, physiological stage (PhySt), and parity (Par) or cohort (Coh) of the ewes, as well as litter size (LSi) at lambing and during suckling. This study aimed to evaluate the effects of contrasting FS (intensive, indoor (IND) vs. extensive, outdoor (OUT)) on the BRD of Romane ewes. Two flocks were monitored: 173 ewes in IND and 234 in OUT, belonging to two cohorts (Coh17/18). Ewes were monitored for body weight (BW), body condition score (BCS), backfat thickness (BFT), back muscle thickness (BMT), non-esterified fatty acids (NEFA), β-hydroxybutyrate (BHB), triiodothyronine (T3), and insulin (INS) at five key PhySt (Mating, M; mid-pregnancy, P; before-lambing, bL; after-lambing, aL; and weaning, W) in primiparous (PRIM) or multiparous (MULT) ewes during successive production cycles. Data were analyzed using linear mixed models, with significance threshold set at P ≤ 0.05. There was no isolated effect (P > 0.05) of FS on the BR traits evaluated. However, significant interactions were observed between FS and PhySt and Par (P < 0.001), LSi (P < 0.01), or Coh (P < 0.001) for all BR traits, indicating that FS may influence BR through factors such as feed availability, energy demands, or stress levels across different PhySt. MULT ewes showed better BR recovery than PRIM ewes regardless of FS, suggesting improved metabolic efficiency with maturity. With larger LSi, greater BRM, and slower recovery, was observed regardless of FS, emphasizing the need for tailored nutritional strategies., and Coh18 ewes showed greater capacity to mobilize and rebuild BR than Coh17. BR dynamics was similar in both FS as indicated by comparable levels of BHB (but not NEFA), T3, and BCS around lambing. In both FS, BRM was observed from P until W, and BRA was observed from weaning until next P. In conclusion, BRD in sheep is strongly shaped by PhySt, and to a lesser extent by environmental factors, influencing FS resilience and productivity. These insights support the importance of improving animal adaptive capacities through BR management for enhancing sustainability in diverse FS, particularly in the face of climate variability and rising production costs.