Seasonal and individual effects on ovarian and endocrine responses of mares to a synchronization treatment with progestagen-impregnated vaginal sponges.

The 8 saddle-type mares were treated with progestagen-impregnated vaginal sponges for 8 days and prostaglandin on the day of sponge removal. The treatment was given at random days of the cycle in April, May, July and September. Sponge insertion induced a sharp decrease in LH levels; at sponge removal, there was an immediate increase in LH and a large FSH rebound. Ovulation was synchronized 11 +/- 2 . 8 days (s.d.) after the end of treatment. In April the interval from sponge removal to ovulation was longer (14 days compared with 10 . 1, 10 . 7 and 10 . 2 days), the basal and peak LH levels lower, and follicular growth more delayed than in the 3 other months. In July, there was a prolonged oestrogen preovulatory rise and reduced FSH levels. Numerous individual differences were shown, but none was significantly linked to the interval from sponge removal to ovulation. Within the 3 months when response was unaffected by season, the interval from end of treatment to ovulation was correlated with progesterone (r = 0 . 45 and r = 0 . 50) and FSH (r = -0 . 48 and r = -0 . 49) levels at Days 5 . 6 and 7 . 8 after sponge insertion. This means that in treated mares that have an active corpus luteum at the end of treatment there will be low FSH levels at this time and ovulation will be late. A good prediction of the ovulation time after such a synchronization treatment must therefore take into account season, animal and time of treatment in the oestrous cycle.