Observations on reproductive performance in the yearling mare.

This study investigates the reproductive performance of 137 yearling mares over four breeding seasons. The results found a high incidence of early pregnancy loss, which could be attributable to factors such as immaturity, inadequate nutrition, and physical stress.

Study Methodology and Participants

• The research was conducted over four breeding seasons from 1968 to 1971.
• 137 yearling mares that were in small groups with stallions were studied for a period of 3 months between June and August each breeding season.

Observations and Test Results

• A pregnancy diagnosis was performed using multiple rectal palpations and tests for PMSG (Pregnant Mare Serum Gonadotropin), a hormone produced by pregnant mares.
• Of the 137 mares, ninety-five of them conceived whereas forty-four spontaneously aborted between 30 and 160 days of gestation.
• Upon laboratory examination, no infectious agents were found in the twenty-one aborted fetuses.
• Both qualitative and quantitative tests for PMSG in the mares that aborted showed similar results as those observed in mares with normal pregnancies.
• Plasma progestagen assays demonstrated high individual variations. The loss of the conceptus was always correlated with a drop in progestagen level which implies progestagen plays a significant role in maintaining pregnancy.

Possible Causes of Early Pregnancy Loss

• There was a high incidence of early pregnancy loss in these yearling mares. The potential causes suggested by the researchers were immaturity, physical stress and inadequate nutrition.
• They proposed that the early life stage, inadequate feed quality or quantity and physical stress from cohabitation with stallions or from environmental conditions could contribute significantly to pregnancy loss.
• The actual underlying cause however, was not definitively determined through the study and may require further investigation.