Periprandial Changes in Metabolite and Metabolic Hormone Concentrations in High-Genetic-Merit Dairy Heifers and Their Relationship to Energy Balance in Early Lactation

Sixteen high-genetic-merit Holstein-Friesian heifers were offered a complete diet of grass silage, maize silage, and concentrates ad libitum through mo 1 to 3 postpartum. Open-circuit calorimetry and a 6-d digestibility balance were performed on each heifer at the end of each month, and energy balance (EB) was calculated. After each digestibility balance, heifers were blood sampled hourly from 1 h before to 9 h after feeding. Prefeed plasma and 3-h composites of postfeed plasmas were analyzed for selected metabolites and hormones. Levels of nonesterified fatty acids (NEFA) decreased, whereas those of β-hydroxybutyrate (BHBA) increased after feeding in each month. Urea levels increased after feeding, whereas glucose levels decreased in each month. Insulin increased after feeding but with reducing significance as lactation progressed. Insulin was always lower before feeding, and mean insulin increased from mo 1 to mo 3. Levels of insulin-like growth factor-1 (IGF-I) increased across mo 1 to 3, but were unaffected by feeding except in mo 2, whereas leptin levels varied significantly on each sampling occasion, but showed no increase between mo 1 and mo 3. Multiple regression of all data showed no significant correlation between EB and either BHBA or NEFA levels. However, EB was negatively correlated with leptin levels (r = −0.632), which were themselves positively associated with IGF-I (r = 0.498) and glucose (r = 0.565). A relationship was established between overall mean EB and prefeed leptin, insulin, and urea (R2 = 0.63) in the first 3 mo of lactation. A potentially useful relationship was also established between EB and prefeed concentrations of leptin, IGF-I, urea, and glucose (R2 = 0.80) only in mo 1 of lactation.