Short communication: Effect of sampling time relative to the first daily feeding on interpretation of serum fatty acid and beta-hydroxybutyrate concentrations in dairy cattle.

Serum nonesterified fatty acid (NEFA) and beta-hydroxybutyrate (BHBA) concentrations are used to evaluate energy status in peripartum dairy cows. Blood samples from 37 cows in the week before parturition and 47 cows in the first week after parturition from 3 dairy herds were taken 1h before the first feeding (-1h) as well as 4 and 10h after the first feeding. Nonesterified fatty acid concentrations were measured in samples from cows before calving and BHBA was measured in samples from lactating cows. Mean NEFA concentrations in the prepartum cows were significantly higher at -1h (0.20 mmol/L) than at 4h (0.14 mmol/L), but were not different between 4 and 10h (0.17 mmol/L). Using a cutpoint of NEFA > or = 0.4 mmol/L, 32% of cows had high concentrations at -1h compared with 16% of the same cows at 4 and 10h. There were no differences in mean BHBA between -1h (646 micromol/L) and 4h (596 micromol/L), but mean BHBA was higher at 10h (711 micromol/L) than at -1h. Using a cutpoint of BHBA > or = 1,400 micromol/L, there were no differences in the proportions of high BHBA, which were 9, 11, and 13% of cows at -1, 4, and 10h, respectively. Prandial effects on serum NEFA may affect interpretation of this analyte. In order not to misclassify cows when assessing energy status, samples for NEFA must at least be taken at a consistent time relative to feeding within a given herd. When sampling cows to monitor elements of energy metabolism in the prepartum period, there was twice the probability of detecting animals with NEFA values > or = 0.4 mmol/L if they were sampled 1h before the first feed delivery compared with sampling the same cows 4 or 10h after feeding. Copyright 2010 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.