Synchronization of estrus in goats: the relationship between PMSG binding in plasma, time of occurrence of estrus and fertility following artificial insemination

Radioimmunoassay (RIA) was used to measure plasma eCG binding in dairy goats (n = 524) at the beginning of a progestagen/eCG treatment and 25 d after eCG administration. The eCG binding was not dependent on the age of the females but increased with the number of treatments they had previously received (3.4 % ± 4.8, N = 47 vs 9.6 % ± 13.2, N = 249; mean ± SD; P < 0.01 for goats treated 0 and 1 time vs those treated 2 to 5 times, respectively). The synchronization treatment led to an increase in the binding of eCG (7.1 % ± 10.9 before vs 28.3 ± 24.5 after treatment; P < 0.01). When eCG binding before treatment was higher than 5 % the onset of estrus was delayed: 37.9 % of goats came into estrus more than 30 h after sponge removal vs 7.4 % when eCG binding was lower than 5 % (P < 0.01). Fertility was significantly decreased when eCG binding was higher than 10 %. These results show that the repetition of treatment with eCG to induce estrus in goats increases eCG binding. This could explain the lowered efficiency of the hormonal treatment to synchronize estrus and the associated decrease in fertility when goats are inseminated at a predetermined time.