Early determination of the reproductive potential of beef heifers using reproductive tract scoring, pelvimetry and transrectal ultrasonography

Abstract

A series of 4 observational studies was performed on a Bovelder herd at Johannesburg Water’s Northern Farm. The heifer and cow breeding seasons started on 15 October and 1 November every year, and consisted of 50 d and 60 d of oestrus observation respectively, with once daily artificial insemination (AI). Five to 7 days after the AI period bulls were added in multi-sire groups for a period of 42 d. Pregnancy diagnoses were performed between 23 March and 26 April every year. The farming system had been in place for >25 years, with few deviations. In the first study, reproductive tract scoring (RTS) by transrectal palpation on a 5- point scale was performed 1 day before the start of breeding on a group of heifers (n = 272) born in 2002. Heifers with RTS 1 or 2 were estimated to be pre-pubertal, those with RTS 3 pubertal and those with RTS 4 or 5 post-pubertal. Pre-breeding body weight (BW) and body condition score (BCS) were recorded, and Kleiber ratio (KR) was calculated (average daily gain/end weight0.75). Heifers were followed until they weaned their first calves. The objectives of this study were to determine if RTS is a valid tool to predict reproduction and production performance in limited bred beef heifers, and to compare its predictive value with that of BW, BCS, age and KR. After adjustment for BW and age, RTS was positively associated with pregnancy rate to the 50 day AI season (P < 0.01), calf weaning weight (P < 0.01) and pregnancy rate to the subsequent breeding season (P < 0.01), and negatively associated with days to calving (P < 0.01). RTS was a better predictor of fertility than was vi KR, and similar in its prediction of calf weaning weight. It was concluded that RTS is a predictor of heifer fertility and compares well with other traits used as a predictor of production outcomes. It was further hypothesised that RTS may predict long-term reproductive performance due to its association with pregnancy outcome and days to calving after first breeding, combined with reports that heifers calving early tend to calve early in subsequent seasons and have increased lifetime production. In study 2, a 7-year longitudinal study, 292 beef cows in two age cohorts were observed from 1-2 d before their first breeding season (day of RTS), until they had weaned up to 5 calves. Years to reproductive failure was defined as the number of years until a cow failed to become pregnant during the AI season. Animals with RTS 1 or 2 were at increased risk of early reproductive failure compared to those with RTS 4 or 5 (HR = 1.4, 95% CI 1.0, 1.9) despite the fact that RTS was not associated with calving rate or days to calving after the second calving season. The predictive value of RTS was lower in the age cohort with a higher prevalence of anoestrus. Most animals with RTS 1 or 2 that were subsequently detected in oestrus were in early to mid di-oestrus at the time of scoring. It was concluded that RTS is a valid culling tool to improve long-term reproductive success in a seasonal breeding system, by excluding poor performing animals. It was further concluded that the predictive value of RTS decreases with increasing prevalence of anoestrus and at certain stages of the oestrous cycle. Apart from failure to calve, dystocia as a result of foeto-maternal disproportion is the cause of significant production loss in beef heifers. Internal pelvis area (PA) can be calculated as the product of vertical and horizontal diameter of the pelvis measured transrectally using a calliper device. In study 3, the effects of five culling strategies using prebreeding PA data on calving and dystocia rates and on pre-breeding and calf birth weight (BWT) were compared in 484 heifers. Multiple regression analyses were used to determine independent predictors of PA, calf BWT and dystocia. Hypothetical culling of 10 or 20% of heifers was applied within age cohort after ranking by each of the following: unadjusted PA (PA); PA adjusted to 365 d of age (APA); PA:BW ratio (PA:BW); PA adjusted to the median BW of the group by the regression coefficient of PA on BW(BWPA) and PA similarly adjusted to the median lean BW (LBWPA). Dam parity, calf BWT and either BWPA or LBWPA were the only independent predictors of dystocia (P < 0.05), whereas the effect of PA tended to be significant only after adjusting for calf BWT (P = 0.08). After culling by PA or APA, retained heifers were heavier, had a higher calving rate and calves tended to be heavier at birth compared to culled heifers, but dystocia rates were not different. Although culling by PA:BW resulted in lower dystocia rate, it resulted in lower pre-breeding BW in retained than in culled heifers. Culling by BWPA and LBWPA had better sensitivity and specificity for dystocia than PA, and resulted in higher calving- and lower dystocia rates in the retained heifers, without affecting the pre-breeding BW or calf BWT. It was concluded that pelvimetry is a useful culling tool to aid in the management of dystocia in yearling heifers, and that adjustment of PA to median BW within age group improves its accuracy and avoids the undesirable side-effects of using unadjusted PA. The strong association of PA with calving rate resulted in the hypothesis for study 4, namely that PA can add prognostic value to RTS as a predictor of reproductive failure in beef heifers. Transrectal ultrasound (US) examination of the reproductive tract was also investigated in this study (n = 488) with the objective of identifying which ultrasonographic measures taken at a single point in time before the onset of breeding were independently associated with reproductive outcomes. In this study pregnancy failure was defined as the failure to become pregnant after the AI and bull breeding periods while anoestrus was defined as the failure to be detected in oestrus during the entire 50 d AI period. BCS, uterus horn diameter, absence of a CL, largest follicle of less than 13mm and PA were the pre-breeding examination variables that remained in prognostic models (P < 0.1). Combining either the model based on the three remaining US measures or RTS with PA provided more accurate prognostic models for pregnancy failure and anoestrus than using RTS alone (P < 0.05). It was concluded that US measures have prognostic value for pregnancy failure in restricted bred yearling heifers as a result of their association with anoestrus, and that smaller PA has additional value to identify poor performing heifers. In conclusion, pre-breeding examination is useful to identify beef cows with lower potential to reproduce successfully over the long term in a restricted breeding system. It is recommended that PA should always be included in such examination, either unadjusted or adjusted by BW, in herds with a low and high incidence of dystocia respectively, in combination with ultrasonography of the reproductive tract. When ultrasonography is not available, the accuracy of RTS by transrectal palpation can likely be improved by repeating it in low scoring animals after 7 days.