Real-time transrectal ultrasonographic measurement of the fetal eye (vitreous body) to predict parturition in bucking horse mares

Monitoring gestation and predicting parturition in mares with unknown breeding dates is challenging, especially in pasture bred herds of bucking horses. Transrectal ultrasonographic measurement of fetal vitreous body length is a validated predictor of foaling dates, but previous research suggested a need for breed-specific parameters to develop accurate indices for respective breeds. This study aimed to develop a scale associating fetal vitreous body length with days before parturition (DBP) in bucking horses. Biannual data collection over 2 years identified 199 pregnancies out of 240 reproductive examinations, of which 155 fetal eyes were successfully identified and measured (78 %). Foaling dates confirmed for 131 successful fetal eye examinations (n = 131) enabled DBP calculation. A single transrectal ultrasonographic measurement was collected of the length of the fetal eye measured to the maximum dimensions of the vitreous body at the level of the lens, ranging from 9 to 35 mm. Multilevel mixed-effects linear regression methodology yielded the curvilinear equation DBP= -0.163(fetal eye length measurement in mm) ² + 268.949 (r = 0.881, p < 0.001, concordance=0.994, average difference -0.077±17.214 days), which accurately calculates DBP in bucking horses based on transrectal ultrasonographic measurement of the fetal eye. Although this newly developed equation is similar to a previously reported equation for use in small ponies, applying the equation used for DBP prediction in small ponies to bucking horse pregnancies resulted in a small, but significant underestimation in predicting bucking horse DBP.