An evaluation of the performance of Simmental x Hereford and Hereford cattle in the brigalow region of Queensland

Abstract

This thesis presents the results of an eight-year program to evaluate the performance of Simmental x Hereford and Hereford cattle in the Brigalow (Acacia harpophylla) region of central Queensland, There were two phases in the program. In Phase 1, Hereford heifers and cows were mated to either Simmental or Hereford sires to produce progeny for sire breed and other comparisons in a number of performance traits. Then in Phase 2, these progeny were themselves mated in a reciprocal crossbreeding program to determine dam breed differences in pregnancy rates, calving performance and maternal ability.A preliminary study of the effects of calving supervision was undertaken to assist with the interpretation of results in subsequent calving performance studies. The supervision regime involved the inspection of heifers twice daily, so that assistance could be rendered to any dystocia-affected heifers. Results demonstrated that supervision significantly reduced the incidence of dam mortality in two-year-old Hereford heifers bearing Hereford-sired calves, compared with their unsupervised contemporaries. All heifers survived in the supervised group whereas 30% of heifers died in the unsupervised group. Calving supervision was justified as a management practice by the higher survival rates of both heifers and calves, as well as humane considerations. There was no evidence that supervision procedures increased the incidence of birth complications. Unsupervised Simmental-sired calvings were not observed. However, it was inferred from the results of this and the subsequent study of calving performance in Phase 1, that much higher incidences of dam and calf mortality would have occurred in the absence of supervision in Simmental-sired calvings,compared with Hereford-sired calvings.In the main Phase 1 study, 354 Fl Simmental x Hereford (SH) calves and 531 straightbred Hereford (HH) calves were borne by Hereford dams over three calving seasons in 1973, 1974 and 1975. The calving performance of two- and three-year-old heifers and two classes of multiparous cows was monitored using a twice-daily supervision regime. The incidence of difficult births averaged over both sire breeds was 45.5, 13.0, 6.3 and 4.5% for 2-, 3-, 4- and >5-year-old Hereford dams respectively. In each of the four dam age classes the incidence of difficult births in Simmental-sired calvings was at least 1.6 times the incidence of Hereford-sired calvings. However, these differences were only significant in two-year-old calving heifers, where 60.7% of Simmental-sired calves were either aided or stillborn, compared with 30.3% of Hereford-sired calves. The incidence of difficult births in two-year-old Hereford heifers was approximately three times the incidence in three-year-old Hereford heifers in each of the sire breeds. The degree of foeto-pelvic disproportion, as indicated by the ratio of calf birth weight/dam pelvic area, was significantly greater in Simmental- than in Hereford-sired calvings in all dam age classes. This was mainly attributable to the increased birth weights of the SH calves which were approximately 4 kg heavier than the HH calves.The findings of this study indicate that the increased incidence of difficult births and birth complications in Simmental-sired calves in Hereford herds could be largely overcome by management changes in heifers. Either Simmental sires should not be mated to yearling or two-year-old Hereford heifers in unsupervised herds, or they should be mated only to two-year-old Hereford heifers which calve under supervision.The SH calves suckled by Hereford dams grew significantly faster and were heavier at weaning than HH calves. Similarly SH steers grew faster than HH steers in a post-weaning study which involved either a crop or pasture finish. The carcasses of the SH steers at turnoff ages of 35 months (crop) or 45 months (pasture) were heavier and exhibited a thinner layer of subcutaneous fat than HH carcasses. Furthermore, a significantly greater proportion of SH carcasses dressed heavier than 273 kg and thereby attracted a premium price on the export market.