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Summary Two mares being monitored in a project concerning early foetal death failed to maintain their pregnancies despite improved management. They repeatedly lost their conceptuses so one was treated with allyl trenbolone and the other with flunixin meglumine from Day 10 of gestation. Treatment with allyl trenbolone until Day 70 of gestation maintained pregnancy despite regression of the primary corpus luteum and the resulting low levels of endogenous progesterone in peripheral plasma during one month; secondary corpora lutea developed at the expected time. In the other mare with a history of premature release of prostaglandin F2α, treatment with flunixin meglumine until Day 60 of gestation inhibited prostaglandin F2α synthesis and luteal function was maintained.

Summary In the mare, there is a high incidence of early embryonic mortality which corresponds to early conceptus development and the time of maternal recognition of pregnancy. The need for synchrony between the developing conceptus and the uterine environment emphasises the delicate processes of establishment and maintenance of pregnancy. Further research is necessary to investigate the environments of the conceptus and uterus, and their interaction, for a better understanding of these processes and ultimately a reduction in early embryonic loss.

Summary The overall live foaling percentage per cycle on 4 Kentucky Thoroughbred farms was 42.0%. The rate varied with mare reproductive status, mare age, cycle number and stallion. The most pronounced effect was associated with mare age. Estimates of the relative importance of embryonic loss as a factor limiting reproductive success for young and old mares. suggest that young mares lose half of their embryos from fertilisation to term, and the loss rate increases with mare age to the point that old mares lose all or nearly all of their embryos.

Summary Data were compiled regarding the success of commercial embryo recovery rate, transfer pregnancy rate, and embryonic loss in recipients, with respect to influence of donor parity and age, between 1985 and 1988. There were significant differences between maiden and subfertile mares for embryo recovery rate (40/66 versus 81/282), transfer pregnancy rate (34/49 versus 38/78), and embryonic loss in recipients (4/34 versus 13/38). There were also significant differences between recently foaling and subfertile mares (146/276 versus 81/282) and for younger mares (two to eight and nine to 17-year-old) compared with older mares (18 to 28-year-old) for embryo recovery rates (80/132, 94/183 and 93/309, respectively). Transfer pregnancy rates differed signifiantly between mares aged two to eight-years-old compared with those aged nine to 17 and 18 to 28-years-old (59/84, 50/97 and 50/90 respectively). No differences were observed among any of the age groups regarding embryonic loss in recipients. There was an effect of year on embryo transfer pregnancy rate in 1985 compared with subsequent years that could have been attributed to any of a wide variety of external factors.

The effects of pre-ovulatory and post ovulatory insemination on pregnancy rate and embryonic-loss rate were studied in 268 mares in two experiments. Within each experiment mares were randomised within replicates as follows: to be inseminated on the day the pre-ovulatory follicle reached 35 mm (pre-ovulatory group), to be inseminated on the day of ovulation (Day 0 group), and to be inseminated on the day after ovulation (Day 1 group). Ultrasonic pregnancy diagnoses were performed on Days 11, 12, 13 and 14 (Experiment 1) and Days 11, 12, 13, 14, 15, 20 and 40 (Experiment 2). Combined for the two experiments, pregnancy rates were different (P less than 0.01) among the three groups. For Experiment 2, pregnancy rate within the pre-ovulatory group was lower (P less than 0.05) for insemination 4 days or more before ovulation than for up to 3 days before ovulation. Pregnancy rate was lower (P less than 0.05) for the Day 0 group than for the pre-ovulatory group inseminated up to 3 days before ovulation. In Experiment 2, ovulation was detected by examinations every 6 h; pregnancy rate was greater (P less than 0.05) for mares inseminated 0 to 6 h after ovulation than for mares inseminated at 18 to 24 h. No pregnancies occurred when mares were inseminated 30 h or more after ovulation. The mean day of first detection of the embryonic vesicle was different (P less than 0.0001) among the three groups. Diameter of embryonic vesicle averaged over Days 11 to 15 also differed (P less than 0.0001) among groups.(ABSTRACT TRUNCATED AT 250 WORDS)