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EARLY EFFECT OF CONCEPTUS ON PLASMA PROGESTERONE LEVEL IN THE COW

Progesterone levels in jugular venous blood are being measured by us in cycling (unmated) and inseminated (pregnant or non-pregnant) cows as part of a comparative study of cows with normal and difficult breeding histories. Hawk, Wiltbank, Kidder & Casida (1955) reported a high incidence of embryonic death in cows during the period 16 to 25 days after insemination. The 3rd week of gestation may indeed be a critical phase in the regulation of luteal function, since towards the end of this week regression of the corpus luteum normally occurs in the absence of a conceptus. This note records that in a herd of normal Friesian cattle with a cycle length of 21·2 days±1·5 S.D. the presence of a conceptus in the uterus first exerted a significant