Relationship between progesterone concentrations in milk and blood and time of ovulation in dairy cattle

The objective of this study was to investigate whether monitoring progesterone concentrations in milk and blood plasma can be used to predict time of ovulation in dairy cattle. Whole milk was sampled twice daily and blood samples were collected once a day before the morning milking. Ovulation was assessed by trans-rectal ultrasonography at 4 h intervals beginning from the end of estrus. For a parameter to be useful as predictor for time of ovulation, it should be precise (i.e. variation between animals should not exceed 12 h). In milk, progesterone concentration dropped <15 ng/ml at 97.7 ± 17.8 h (range: 54–126 h) before ovulation, to <5 ng/ml at 79.7 ± 11.2 h (range: 54–98) before ovulation to decline further to <2 ng/ml at 70.7 ± 16.8 h (range: 38–90 h) before ovulation (n = 20). In plasma, progesterone concentration dropped to <4 ng/ml 90.5 ± 19.6 h (range: 66–138 h) before ovulation and to <2 ng/ml at 75.0 ± 12.2 h (range: 50–98) before ovulation. These intervals were not influenced by parity, milk production or days in milk. In conclusion, monitoring of progesterone alone is not sufficient to predict ovulation because of the large variation in timing of decrease of progesterone concentrations relative to ovulation between animals. At best the range is about 2 days.