Variability of the blood composition of dairy cows in relation to time of day

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Abstract
SUMMARY: A blood sample was taken at each hourly interval between 07.00 and 16.00 h on five consecutive days from each of two groups of five lactating and two groups of five nonlactating cows. The sampling was organized so that each cow was sampled twice daily and once at each time of day. Samples were analysed for blood glucose and haemoglobin, serum urea, inorganic phosphate, Ca, Mg, Na, K, total protein, albumin, Fe and Cu and plasma β-hydroxybutyrate. The sources of variation in the concentration of each constituent were determined.Definite patterns of changes in concentrations with time of day were observed for β-hydroxybutyrate, urea, Ca, Na, Mg and Cu. In lactating cows there was a 2½-fold increase in the concentration of β-hydroxybutyrate from the lowest at 07.00 to the highest at 11.00 h. Variation in the concentrations of the other constituents with time of day was a relatively small proportion of the total variance.It was concluded that diurnal variation in blood composition was unlikely to be a major source of error in interpretation for the usual constituents of a metabolic profile test. However, if β-hydroxybutyrate were to be included great care would be required in selecting a time for sampling.

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