Effects of repeated embryo flushing without PGF administration on luteal function, percentage of unwanted pregnancy and subsequent fertility in mares.

Background: PGF is commonly given at the end of embryo flushing (EF) to shorten the interval to the next oestrus and ovulation.

Objectives: To determine the effect of repeated EF on plasma progesterone concentration, percentage of mares with endometritis, unwanted pregnancy and subsequent fertility in mares flushed without the use of PGF.

Study Design: Controlled experiments.

Methods: Nine mares were inseminated in seven consecutive cycles (n = 63), to either perform an EF (n = 54) 7-9 days after ovulation or left pregnant (n = 9). PGF was not used to induce oestrus. Ultrasound examination and blood sampling were performed just before the EF and 72 h later to determine changes in progesterone concentration and signs of endometritis.

Results: The overall percentage of positive EF/pregnancy was 55.5% (30/54) and 66.7% (6/9), respectively. The likelihood of pregnancy/positive EF in the first three cycles was 55.5% (15/29). This was not different (p > 0.1) from the fertility of the last four cycles (69.4%, 25/36). In five EF cycles (9.3%), mares had signs of endometritis and early luteolysis (progesterone <2 ng/mL) 72 h after EF. The reduction in progesterone concentration by 72 h after EF was greater (p < 0.05) for Day 9 (-2.3 ± 0.7 ng/mL) than Day 7 (-1.0 ± 0.8 ng/mL) or Day 8 (-1.3 ± 1.1 ng/mL) cycles. The progesterone concentration in non-flushed mares did not vary significantly during the sampled period (Day 7-12). There were 5 cycles in which the donor mare remained pregnant after the EF, although four were from a single mare.

Main Limitations: The mare population was limited to barren and maiden mares. The cycle order and operator allocation to each EF were not randomised.

Conclusions: EF induces a subtle, but significant reduction in progesterone concentrations compared with non-EF cycles. However, the percentage of mares with EF-induced full luteolysis is low (9.3%). The fertility of mares after repeated EF without administration of PGF was unaffected; however, there is a considerable risk of unwanted pregnancy (5/27 = 18.5%) in donors from which an embryo was not recovered.