Validation of a direct time-resolved fluoroimmunoassay for progesterone in milk from dairy and beef cows

The aim of this study was to validate a direct time-resolved fluoroimmunoassay (TR-FIA) for quantifying progesterone concentrations in milk during the bovine oestrous cycle. Holstein–Friesian and suckled and non-suckled Japanese Black cows were used to demonstrate the relationship between milk and plasma progesterone concentrations and to monitor progesterone profiles in milk and plasma during the oestrous cycle. The minimum detection level of the assay was 1.53 ng/mL. Progesterone concentrations in milk and plasma changed in a similar manner throughout the oestrous cycle in dairy and beef cows, and milk and plasma progesterone profiles were significantly correlated (P < 0.001). The study confirmed that a direct TR-FIA can be used to monitor the oestrous cycle in cattle and to quantify progesterone concentrations in whole milk.