The objective of this study was to determine the validity of the blood-collecting frequency in lactating cows by evaluating the repeatability of measurements of blood plasma constituents. Blood samples were obtained from 15 Holstein lactating cows under a feeding trial. Blood drawing 4 hours after morning feeding was performed 19 times during a period of 44 days to analyze blood plasma constituents based on a metabolic profile test. The cows ingested adequate nutrients for their requirements. The repeatabilities were estimated by analysis of variance of blood plasma constituents. Low repeatability was observed in inorganic phosphorus and total bilirubin (0.149 and 0.041, respectively); while high repeatability was observed in cholesterol, calcium, free fatty acids, γ-glutamyl transpeptidase (0.843, 0.784, 0.702 and 0.699, respectively). Respectively, repeatabilities of urea nitrogen, glucose, magnesium and aspartate amino transferase were 0.603, 0.571, 0.543, and 0.341. For blood constituents with a high repeatability exceeding approximately 0.7, repeated measurement has little value. However when ascertaining the eigenvalues of blood constituent values such as inorganic phosphorus with low repeatability, multiple measurement should be considered.

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