Pregnancy diagnosis in cows; changes in milk progesterone concentration during the oestrous cycle and pregnancy measured by a rapid radioimmunoassay

Summary The progestagen concentration of milk was measured in cows, at various times of the reproductive cycle by a competitive protein binding procedure and by radioimmunoassay. The apparent progesterone concentration was found to be significantly greater in milk (P <0–05) than in plasma, probably due to the presence of an unidentified metabolite(s). A simple, rapid radioimmunoassay was developed to measure directly the progestagen concentration in milk. A mean concentration of about 1 ng/ml at oestrus (or mating) increased, 20–24 days later, to 3–4 ng/ml in cows subsequently found to be non-pregnant and to about 7 ng/ml in pregnant animals. The possible application of this analytical procedure as the basis of a method of pregnancy diagnosis is discussed.