RECOVERY RATE AND QUALITY OF EMBRYOS FROM MARES INSEMINATED AT THE FIRST POSTPARTUM ESTRUS

The pregnancy rate is lower in mares inseminated at the first postpart um (p.p.) oestrus (40-50%) compared with pregnancy rates in subsequent oestrous cycles (55-65%). The causes of the lowered pregnancy rate ar e not fully understood. The aim of the present study was to examine if embryonic defects could be one of the reasons for lowered pregnancy r ate. A total of 23 p.p. and 14 non-lactating control mares were flushe d 7 days after detection of ovulation. Embryo recovery rate was 48% an d 71% in p.p. and control mares, respectively (p=0.16). Embryos were p hotographed, measured, graded and stained with fluorescein diacetate t o assess their viability Thereafter embryos were bisected and stained with Hoechst 33342 to count the cell nuclei. Embryos in both groups we re equally viable and the cell numbers were not significantly differen t. According to morphological evaluation all embryos were classified a s excellent or good. Embryos aged 7.3 to 7.6 days (+/- 0.25 days) were smaller in the p.p. group than in the control group (p<0.05). Forty-s even (9/19) and 8% (1/13) of the uterine swabs, taken before the first insemination, yielded bacteria and neutrophils in p.p. and control ma res, respectively. The amount of neutrophils and/or bacteria had no st atistically significant effect on embryo recovery rate (p>0.10). Recov ery of embryos was not related to histological findings in uterine bio psies taken after embryo recovery. Embryo recovery rate in p.p. mares (48%) was similar to previously reported fear heat pregnancy rates (40 -50%). Hence, early embryonic death in utero would not be the most lik ely reason for lowered pregnancy rate in mares inseminated at the firs t p.p. oestrus. Sperm transport and oviductal conditions by the time o f the first p.p. oestrus would need to be studied to clarify the role of fertilisation failure as the cause of lower pregnancy rate in mares inseminated at foal heat.