Ultrasonographic reproductive tract measures and pelvis measures as predictors of pregnancy failure and anestrus in restricted bred beef heifers

Abstract

Previous reports have shown that reproductive tract score (RTS) can predict reproduction outcomes in seasonally bred beef heifers, although the accuracy can vary. Some ultrasonographic measures of the female reproductive tract and pelvis area have also been associated with reproductive outcome in young heifers. The objectives of this study were to determine which transrectal ultrasound or pelvis measures taken at a single examination are independent predictors of reproductive failure, and if the RTS system can be optimised with this information. In this observational study 488 year-old beef heifers in 2 birth cohorts were followed from prior to first breeding until confirmation of pregnancy. A single pre-breeding examination included BCS, RTS, ultrasound measures of the reproductive tract (length and diameter of the left and right ovaries, presence and diameter of a CL, largest follicle diameter and left uterus horn diameter) and transverse and vertical diameters of the pelvis. Additional farm records including dam parity, sire, birth weight and birth date, weaning weight, weaning date, pre-breeding BW, AI dates and semen used were available. Breeding consisted of 50 d of AI, followed 5 to 7 d later by a 42 d bull breeding period. Pregnancy failure was defined as the failure to become pregnant after the AI and bull breeding periods while anestrus was defined as the failure to be detected in estrus during the 50 d AI period. From the pre-breeding data and farm records independent predictors of pregnancy failure and anestrus were identified using step-wise reduction in multiple logistic regression models. Age at the onset of breeding was the only consistent independent predictor of pregnancy failure and anestrus in both cohorts of this study (P < 0.05). BCS, uterus horn diameter, absence of a CL, largest follicle of less than 13 mm and pelvis area (PA) were the pre-breeding examination variables that remained in prognostic models (P < 0.1). Combining either the model based on the three remaining ultrasound measures or RTS with PA provided more accurate prognostic models for pregnancy failure and anestrus than using RTS alone (P < 0.05). It is concluded that ultrasound measures have prognostic value for pregnancy failure in restricted bred yearling heifers as a result of their association with anestrus, and that smaller pelvis area has additional prognostic value for poor performing heifers.