Prolonged dismount latency and its relationship with pregnancy outcomes in Thoroughbred horses

Abstract

In equine reproductive management, understanding the factors that influence pregnancy outcomes is essential for optimising breeding success. In the Thoroughbred breeding industry, dismount semen (residual post-copulatory urethral semen) is frequently collected and assessed as a proxy for ejaculate quality. Given that stallion ejaculate is fractionated, we hypothesised that the composition of dismount semen is influenced by the duration of time the stallion remains mounted on the mare following ejaculation, and that prolonged post-ejaculatory intromission would allow more of the ejaculate to be deposited into the uterus, potentially improving pregnancy rates. The aim of this study was to investigate the relationship between dismount latency—defined as the time a stallion remains mounted on a mare post-ejaculation—and both dismount sample quality and pregnancy outcomes. Dismount latency was measured as the time from the start of ejaculation to the removal of the penis from the mare. Dismount samples were collected from 62 Thoroughbred stallions and analysed for sperm concentration (NucleoCounter) and motility parameters (iSperm). There was a negative correlation between dismount latency and sperm concentration in the dismount sample (R² = -0.131; P ≤ 0.05). In terms of motility, total motility was negatively correlated with dismount latency (R² = -0.058, P = 0.09), while progressive motility (R² = -0.004, P = 0.67), straightness (R² = 0.041, P = 0.18), and average path velocity (R² = 0.02, P = 0.3) demonstrated no significant correlation with dismount latency. Additionally, a one-tailed t-test revealed that dismount latency was significantly longer when pregnancies were achieved (11.1 ± 0.42 seconds for positive pregnancies vs. 10.2 ± 0.67 seconds for negative pregnancies; P ≤ 0.05). Based on these findings, we conclude that higher-quality dismount samples—those with greater sperm concentrations and motility—are typically obtained when the stallion spends less time on the mare post-ejaculation. Conversely, poorer-quality dismount samples, characterised by lower sperm concentrations and motility, are collected when the stallion spends more time on the mare. This suggests that in cases of prolonged dismount latency, more sperm is being deposited directly into the mare's uterus rather than into the vagina, which is typical of premature dismounts. Thus, prolonged dismount latency appears to improve pregnancy outcomes, and management strategies encouraging longer dismount latency should be considered to enhance breeding success. However, the negative correlation between dismount sample quality and pregnancy success raises concerns about using dismount samples as a reliable indicator of ejaculate quality, highlighting the need for caution when evaluating these samples as proxies for fertility.