Real-time transrectal ultrasonographic measurement of the fetal eye (vitreous body) to predict parturition in bucking horse mares.
Authors: Gonzalez S W, Espy B M K, Stefanovski D, Turner R M
Journal: Journal of equine veterinary science
Summary
Predicting foaling dates in bucking horse mares managed on pasture breeding systems presents a genuine clinical challenge, particularly when breeding records are unreliable or absent; transrectal ultrasonographic measurement of fetal vitreous body length has emerged as a validated method, though previous work suggested breed-specific calibration was necessary. Over two years, Gonzalez and colleagues collected transrectal ultrasound measurements of fetal eye length (ranging 9–35 mm) from 131 bucking horse pregnancies with confirmed foaling dates, using multilevel mixed-effects linear regression to develop a predictive equation. The resulting curvilinear model (DBP = −0.163 × [eye length]² + 268.949) demonstrated excellent accuracy with an r value of 0.881 and concordance of 0.994, predicting days before parturition with an average error of just ±17.2 days. Critically, applying the previously published equation derived from small pony populations to these bucking horses produced statistically significant underestimation of parturition timing, confirming that breed-specific parameters are essential for reliable prediction. For practitioners managing bucking horse breeding herds, this equation provides a robust tool to time mare monitoring, prepare for foaling complications, and coordinate breeding management decisions when traditional dating methods are unavailable.
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Practical Takeaways
- •Use this breed-specific equation for bucking horse mares with unknown breeding dates rather than equations developed for other breeds, as the pony equation underestimates foaling time
- •A single transrectal ultrasound measurement of fetal eye length provides reliable prediction of parturition timing, enabling better management of pasture-bred herds with unknown breeding histories
- •Expect to successfully identify and measure fetal eyes in approximately 78% of examinations, allowing practical parturition prediction in most mares presented for evaluation
Key Findings
- •A breed-specific curvilinear equation (DBP = -0.163(fetal eye length)² + 268.949) accurately predicts days before parturition in bucking horses with r = 0.881 and concordance = 0.994
- •Transrectal ultrasonographic measurement of fetal vitreous body length (9-35 mm) is a reliable predictor of foaling dates in bucking horses with average prediction difference of -0.077±17.214 days
- •The previously published small pony equation significantly underestimates parturition timing in bucking horses, demonstrating breed-specific parameters are necessary for accurate prediction
- •Of 240 reproductive examinations, 155 fetal eyes were successfully identified and measured (78%), with 131 foaling dates confirmed for validation