Effect of conceptus removal and intrauterine administration of conceptus tissue on luteal function in the cow.

Two experiments were conducted to determine the day on which the bovine embryo first affects luteal function. Experiment I was to determine the sole effect of flushing the uterine lumen and involved 12 non-inseminated heifers flushed 9, 14 and 16 days after estrus (4 animals per group). The same procedure was used to remove conceptuses from 15 pregnant heifers on days 9 (n = 4), 14 (n = 5) and 16 (n = 7) after the onset of estrus. Progesterone concentrations were measured daily throughout the cycle corresponding to conceptus removal (or flushing alone) and throughout the preceding cycle. In non-inseminated animals and when the conceptuses were removed on days 9 or 14, neither the progesterone pattern nor the inter-estrous interval was altered. By contrast, when the embryo was removed on day 16, the time of luteolysis was delayed by 4 to 7 days and the heifers were observed to be in estrus 26 to 29 days after AI. In experiment 2, 21 virgin heifers were distributed over four treatment groups. Animals in groups 2 to 4 received the following treatments twice a day between day 15 and day 19 of the estrous cycle in the horn ipsilateral to the corpus luteum: group 2 (n = 3), 0,25 ml saline; group 3 (n = 3), two 12-day-old conceptuses; group 4 (n = 5), one 16-day-old conceptus. progesterone concentrations were determined from day 13 of the cycle to 4 days after the following estrus. No antiluteolytic effect was found with intrauterine administration of either saline alone or day-12 embryos. In contrast, administration of day-16 conceptuses through the cervix (group 4) lengthened the estrous cycle by 6 to 7 days. The results of these experiments suggest that the antiluteolytic and/or luteotropic factor(s) originating from the conceptus is fully potent by day 16. Embryonic mortality occurring on day 16 or later may induce an extension of corpus luteum lifespan.