Diurnal and postprandial changes in key metabolic markers in high- and low-yielding early-lactation Saanen dairy goats

Although diurnal variation in circulating hormones and metabolites is well-characterized in dairy cattle, limited information exists for dairy goats, despite its importance for accurate metabolic assessment in dairy species. This experiment investigated diurnal variation in key metabolites and hormones over a 48-h period in early-lactation Saanen dairy goats under controlled feeding conditions, comparing high- and low-yielding individuals. Twenty-nine multiparous goats (25 ± 4.7 DIM) were classified as high- (HP; n = 15) or low- (LP; n = 14) producers based on average milk yield over the previous 7 d, using fixed thresholds (<3 L/d for LP; > 4 L/d for HP). Blood samples were collected every 2 h via jugular catheters and analyzed for nonesterified fatty acids (NEFA), glucose, BHB, insulin, creatinine, BUN, insulin-like growth factor-1, and growth hormone. Circulating NEFA, glucose, BHB, creatinine, and BUN displayed diurnal variation. The HP goats exhibited higher circulating concentrations of NEFA and BHB, but lower concentrations of glucose and insulin compared with the LP goats. These findings highlight the differences in energy metabolism and nutrient partitioning between production groups. Overall, sampling before morning feeding is recommended when assessing energy metabolism or basal nitrogen metabolism because there is less variability in metabolite concentration due to postprandial fluctuations, and metabolites are at their most extreme concentrations for assessing energy balance and metabolism. This experiment provides initial insights into the transient daily variation in blood metabolites during early lactation in dairy goats fed 1.4× maintenance requirements, which can assist in improved metabolic profiling in future experiments.