Abstract:
In a 7-year longitudinal study 292 Bovelder beef cows in a restricted breeding system inSouth Africa were observed from 1 to 2 days before their first breeding season, when repro-ductive tract scoring (RTS, scored from 1 to 5) was performed, until weaning their 5thcalves. The objective was to determine whether pre-breeding RTS in heifers is a valid toolto predict long-term reproductive performance. Outcomes measured were failure to showoestrus during the first 24 days of the first 50-day AI season (24-day anoestrus), failureto become pregnant during each yearly artificial insemination (AI) season (reproductivefailure), number of days from the start of each AI season to calving, and number of years toreproductive failure. The effect of RTS on each outcome was adjusted for year of birth, pre-breeding age, BW and body condition score (BCS), and for 24-day anoestrus, bull, gestationlength, previous days to calving and previous cow efficiency index, the latter two in thecase of the 2nd to the 5th calving season. During their first breeding season, heifers withRTS 1 and 2 combined were more likely to be in anoestrus for the first 24 days (OR = 3.0,95% CI 1.5, 6.4, P = 0.003), and were also more likely to fail to become pregnant even afteradjusting for 24-day anoestrus (OR = 2.1, 95% CI 1.1, 3.9, P = 0.025), compared to those withRTS 4 and 5 combined. Animals with RTS 1 and 2 combined were at increased risk of earlyreproductive failure compared to those with RTS 4 and 5 combined (HR = 1.4, 95% CI 1.0,1.9, P = 0.045) although RTS was not associated with calving rate or days to calving after thesecond calving season. Low RTS at a threshold of 1 had consistent specificity of ≥94% forboth 24-day anoestrus and pregnancy failure, however its predictive value was lower inthe age cohort with a higher prevalence of anoestrus. We conclude that RTS is a valid man-agement tool for culling decisions intended to improve long-term reproductive success ina seasonal breeding system, by excluding heifers that are likely to fail to become pregnantor likely to calve late during their first calving season.
