Factors affecting accuracy of pregnancy diagnosis in cattle.

When pregnancy was diagnosed by concentration of progesterone in milk 23 days postinsemination in 3014 cows, 76.9% of positive and 93.8% of negative diagnoses were confirmed by uterine palpation or return to estrus. Agreement between diagnosis by uterine palpation and diagnosis by milk progesterone test increased for at least 50 days after insemination. Accuracy of laboratory diagnosis of pregnancy increased as concentration of progesterone in the milk sample increased, making it possible to identify cows likely to return to estrus by a low concentration of progesterone in milk 23 days after insemination. There were 352 cows (11.7%) for which concentration of progesterone in the milk sample collected on the day of insemination was higher than normal during estrus. Exclusion of these cases increased the accuracy of pregnant diagnosis by 7.1% and not pregnant diagnosis by 3.1%. From a pragmatic viewpoint, pregnancy diagnosis by concentration of progesterone in milk was accomplished most effectively by quantitative assaying of only one sample collected 23 days after insemination and assigning to the absolute concentration of progesterone a probability that pregnancy will be verified after 50 days or more of gestation by uterine palpation.