Genetic Parameter Estimation for Plasma Biomarkers Associated With Energy Reserves During Critical Physiological...

The ability of ruminants to mobilise and restore body reserves (BR) over time, referred to as BR dynamics, is currently considered an interesting biological component to be included in breeding programs targeting enhanced BR resilience. However, genetic studies of proxies for BR levels and BRD remain scarce, particularly in small ruminants. The aim of this study was to estimate the genetic parameters for key plasma biomarker concentrations in sheep at critical physiological stages (PhySt i.e., mating; mid-pregnancy, before-lambing, after-lambing, and weaning), and their changes over time. Non-esterified fatty acids (NEFA), β-hydroxybutyrate (BHB), triiodothyronine (T3) and insulin (INS) were monitored at those PhySt in successive production cycles. A total of 659 productive Romane ewes were phenotyped for one (n = 252, multiparous) or two (n = 407, primiparous and multiparous) cycles. BR mobilisation was observed from the second half of pregnancy and during suckling while BR accretion was more evident from weaning until the next mid-pregnancy. Considering biomarkers concentrations as repeated measurements through the whole production cycle, heritability estimates were 0.07, 0.09, 0.15, and 0.10 for NEFA, BHB, T3, and INS, respectively. Heritability estimates for plasma biomarkers at key PhySt ranged from 0.08 to 0.16 for NEFA, 0.07 to 0.12 for BHB, 0.09 to 0.18 for T3, and 0.04 to 0.15 for INS. Heritability estimates for biomarker changes over time ranged from 0.01 to 0.23. Genetic correlation estimates between different PhySt were positive for each plasma biomarker and ranged from 0.19 to 0.87 for NEFA, from 0.50 to 0.89 for BHB, from 0.54 to 0.95 for T3 and from 0.34 to 0.90 for INS. Most genetic correlation estimates between biomarkers at a given PhySt were generally low to moderate (r_g = -0.52 to 0.59), with few showing strong negative or positive values beyond 0.60 in magnitude. Considering changes over time, correlations were similarly low to moderate ( $r_g$  = -0.59 to 0.53), with only a few estimates reaching high values ( $r_g$  = -0.60 to -0.97 and 0.67 to 0.97). This study demonstrates that blood biomarkers related to energy BR have genetic variation, indicating their potential for implementation in sheep breeding programs aimed at improving BR use and build-up.