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.
