Productivity of Hereford, highgrade Simmental and Belmont Red beef herds in central Queensland. 4. Liveweight and age of heifers at puberty

Abstract

Liveweight and age at first and second oestrus of Hereford, highgrade Simmental, and Belmont Red heifers were measured over 4 cohorts born from 1979 to 1982. Heifers were observed for standing oestrus or chin-ball markings by vasectomised bulls from about 7 months of age (May) to about 26 months (December). During this period they grazed predominantly improved pastures growing on cleared brigalow country in a subtropical environment. Hereford heifers demonstrated first and second oestrus at lower (P<0.05) liveweights (277 and 301 kg) than Belmont Reds (293 and 319 kg), which were lighter (P<0.05) than highgrade Simmentals (322 and 345 kg). Belmont Red heifers were younger (P<0.05) at first oestrus (527 days) than either Hereford (565 days) or highgrade Simmental (550 days), which were not significantly different. There was a similar pattern for age at second oestrus, except that only Belmont Red (583 days) and Hereford (617 days) differed significantly (P<0.05). However, 13% of heifers did not record a second oestrus during the observation period, in addition to the 7% that failed to record first oestrus by the end of the experiment. There was no difference between cohorts for liveweight at which heifers expressed first oestrus, but the cohort born in 1981 had higher liveweights at second oestrus than the 1979 and 1980 cohorts. Average age ranged from 476 to 613 days at first oestrus and 542 to 620 days at second oestrus over the 4 cohorts. This variation was a reflection of seasonal conditions, particularly during the period May-October following weaning at about 7 months of age. Only 6% of Hereford, 19% of highgrade Simmental, and 16% of Belmont Red heifers had reached puberty in time for seasonal mating as yearlings (about 15 months), and 88% of these were from the 1982 cohort.