Reasons for performing study: Active immunisation against gonadotrophin-releasing
hormone (GnRH) provides a reversible method for control of oestrous behaviour and fertility
in mares. Previous reports failed to demonstrate the interval to resumption of cyclic ovarian
activity after GnRH-vaccination.
Hypothesis: Administration of the GnRH-vaccine ImprovacÒ1 in a large group of mares of
various ages will result in effective, reliably reversible suppression of ovarian activity within
a two-year period.
Methods: The mares, subdivided into three age categories were vaccinated twice (with a 35 d
interval) using ImprovacÒ1 and were monitored via blood samples until Day 720 after initial
vaccination for serum progesterone concentration determination by radio-immune assay and
anti-GnRH antibody titre by enzyme immuno-assay. Samples were collected until individuals
resumed cyclic ovarian activity.
Results: All mares showed suppression of cyclic ovarian activity (SPC <1 nmol/l) and 92.2%
resumed cyclic activity at Day 720 with a mean interval = 417.8 d (SD = 23.19) and median =
344 d. A significant age effect (P=0.028) on the interval, but not on GnRH-AB titre response,
was observed between the youngest (<4 years) and oldest (>11 years) categories. Conclusions: Immunising adult mares of all ages with ImprovacÒ1 resulted in a reversible
suppression of cyclic ovarian activity in most mares. An age effect, with the youngest mares
showing a longer interval to reversibility was observed.