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Three experiments were conducted to determine the day at which the bovine embryo first affects luteal lifespan and if intrauterine infusion of homogenates of bovine embryos would extend luteal lifespan in nonpregnant heifers. Exp. I was a 2 x 2 factorial design and utilized 23 pregnant or nonbred Holstein heifers that were subjected to embryo removal procedures on day 15 or day 18 postestrus (estrus = day 0). Exp. II was a 4 x 2 factorial design and utilized 39 pregnant or nonbred Holstein or beef x Holstein heifers that were subjected to embryo removal procedures on day 13, 15, 17 or 19 postestrus. Removal of embryos at day 18 (Exp. I) and day 17 or 19 (Exp. II) prolonged (P<.01) luteal lffespan as compared to nonbred controls or when embryos were removed on days 13 or 15 of pregnancy. In Exp. III, seven heifers received twice daily intrauterine infusions of a homogenate of 17-and 18-day-old embryos and seven received control infusions. As compared to the control group, intrauterine infusions of homogenized 17-and 18-day-old embryos lengthened (P<.01) the interestrous interval by delaying regression of the CL. The results of these experiments indicate that between days 15 and 17 of pregnancy the bovine embryo produces a substance that prolongs the functional lifespan of the CL.

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Bovine embryos were produced in vitro using a 2 x 2 design of modified medium (KSOM or SOF) and oxygen concentration (5% or 20%). Day 7 blastocysts were transferred in bulk (n = 11, on average) to recipient heifers and recovered non-surgically at Day 14. In two replications of a Latin square, eight heifers received embryos from each combination of factors. Recovered embryos were evaluated for trophoblast length and width, as well as the presence and diameter of an embryonic disc (ED). An ED was detected in a higher percentage of embryos that had been cultured in KSOM than SOF (72% v. 46%, respectively; P < 0.05). The aim of a second series of experiments was to associate Day 14 morphology with subsequent developmental capacity. In vitro-produced blastocysts were transferred (n = 17-20) on Day 7 to each of eight heifers and recovered at Day 14. Thirty-eight blastocysts were retransferred to heifers following morphological evaluation. Embryos in which an ED with no signs of degeneration had been detected maintained more pregnancies than other embryos in which an ED had either shown signs of degeneration or had not been detected (5/8 v. 2/30, respectively; P < 0.01). Further investigation into ED integrity at the elongating stage may contribute to our understanding of pregnancy establishment and maintenance.