Milk component ratios and their associations with energy balance indicators and serum calcium concentration in early-lactation spring-calving pasture-based dairy cows.

Indirect assessment of metabolic status using milk samples provides a noninvasive and objective tool for cow-level health monitoring. Milk fat-to-protein ratio (FPR) has been commonly evaluated as an indirect measure for negative energy balance in confined dairy cows. However, milk component ratios have not been explored for their association with pasture-based cows' metabolic status. The objectives of this observational study were to (1) describe milk component ratios from 0 to 45 d postpartum, (2) evaluate the associations between milk component ratios (FPR, fat-to-lactose [FLR], protein-to-lactose [PLR]) and indicators of energy balance (serum BHB concentration at 5-45 d postpartum and BCS change during the transition period), and (3) evaluate the associations between milk component ratios and serum Ca concentration 0 to 4 d postpartum in spring-calving dairy cows from pasture-based commercial farms. Milk component ratios were determined on samples collected before a.m. or p.m. milkings from 548 cows at 0 to 45 d postpartum (n = 970). Serum BHB and Ca determinations were performed in blood samples collected at the time of milk sample collection at 5 to 45 d postpartum (n = 918) and 0 to 4 d postpartum (n = 50), respectively; and BCS change was calculated using BCS assigned between 29 d prepartum and 45 d postpartum (n = 851). Cows' calving date, parity (first, second and third, or ≥fourth) and breed (Holstein-Friesian or dairy crossbred) information was obtained from the farm records. Data were analyzed by multiple linear regression. Average milk FPR, FLR, and PLR were 0.70, 0.53, and 0.72, respectively. Milk FPR linearly increased whereas milk FLR linearly decreased postpartum both at a rate of 0.004 units per day; milk PLR decreased 0.05 units per day for the first 30 d postpartum and moderately increased afterward. Milk FPR and FLR were 0.71 and 0.52 units lower before a.m. than p.m. milking, respectively; whereas milk PLR was similar before a.m. and p.m. milking. Milk FPR and FLR were 0.07 to 0.10 units higher for second and third compared with first and ≥fourth parity cows. Milk PLR was 0.03 units greater for ≥fourth compared with second and third and first parity cows. Further, crossbred cows had 0.07, 0.08 and 0.03 higher milk FPR, FLR, and PLR than Holstein-Friesian cows, respectively. Moderate to high P-values along with moderate to small estimated slopes and wide 95% confidence intervals were observed for the associations between milk component ratios and indicators of energy balance. A positive linear association was observed between milk FPR and serum Ca concentration within 4 d postpartum; milk FPR increased 0.31 units per each mmol/L increase in serum Ca concentration. Cows with low serum Ca concentration within 4 d postpartum had 0.27 units lower milk FPR compared with cows at or above the threshold (2.12 mmol/L), and tended to have 0.15 units lower milk FPR compared with cows at or above the threshold (2.00 mmol/L). In conclusion, further research is needed to reach conclusions on the association between milk component ratios determined before milking and energy balance indicators. The potential of milk FPR for monitoring blood Ca status warrants further investigation in early-lactation pasture-based dairy cows.