Fertility of pasture bred mares in synchronized oestrus.

The researchers studied the fertility rates of pasture-bred mares that had their oestrus cycles intentionally synchronized over a span of three years. They found consistent high rates of pregnancy and foaling, with decreasing rates of abortion each year.

Methodology

• The research was carried out on a large scale, with the oestrus cycle of nearly 800 mares being synchronized over three years: 220 in 1983, 300 in 1984, and 272 in 1985.
• Fenprostalene was used to synchronize the mares’ oestrus cycle. The mares were given two injections of 250 micrograms 15 days apart to achieve this.
• During 1983 and 1984, a subset of the mares (56 in 1983 and 53 in 1984) was given a different treatment to delay and synchronize post-partum oestrus. This involved 1-10 daily injections of 150 mg progesterone and 10 mg oestradiol-17 beta.
• After receiving the second PG injection or at the end of the progesterone + oestradiol treatment, the mares were divided into groups of 15-21 and placed in separate pastures with a stallion for seven weeks.

Results and Findings

• The study found high rates of fertility in the mares, with pregnancy rates coming in at 87.7% in 1983, 93.7% in 1984, and 97.1% in 1985.
• The rates of foals being born, or foaling rates, were also high across the three years. These rates were 72.3% in 1983, 89.7% in 1984, and 94.1% in 1985.
• Despite the overall high rates of conception and birth, the study also recorded a number of abortions, mostly between the three to six-month gestation period. However, the number of abortions decreased annually from 34 (17.8%) in 1983, to 12 (4.3%) in 1984, and finally to 8 (3.0%) in 1985.

Conclusion

• The results from this study provided insights into the viability of large-scale, controlled breeding of mares. The high rates of pregnancy and foaling suggest that synchronizing the oestrus cycle can be a successful strategy for this purpose.
• The significant decrease in abortion rates over the years suggests that the health and fertility of the mares were not adversely affected by the synchronization protocol over time.