Effect of chronological age of beef steers of different maturity types on their growth and carcass characteristics when finished on natural pastures in the arid sub-tropics of South Africa
In the arid sweetveld regions of South Africa producers are marketing beef steers increasingly as long weaners (ca. 12 months of age) or finishing them on the natural grazing to a ready-to-slaughter stage at between 18 and 30 months of age. Limited production norms in this regard are available since most growth and carcass studies have been conducted in the sourveld regions of the country. In this study steers from four different beef maturity types which differ in body frame size were used, viz. Simmentaler crosses (large, >500 kg mature weight), Bonsmara crosses (large-medium, 450-500 kg mature weight), the Afrikaner (small-medium, 400-450 kg mature weight) and the Nguni (small, <400 kg mature weight). After weaning the steers in each type were randomly allocated to three groups, viz. groups slaughtered at 18, 24 or 30 months of age after raising them on natural sweetveld pasture. Live weight, cold carcass weight, carcass fat classification code and number of visible incisors were recorded. Growth rates from weaning to 24 months of age were similar for the different maturity types, though the Afrikaner steers gained significantly less than the Bonsmara crossbreds. Periods where high growth rates occurred (at 12 to 18 months of age and 24 to 30 months of age) coincided with the rainy season. Relatively low dressing percentages were noted and could be attributed to the fact that all internal organs and fat were removed at slaughtering, as well as a possible high level of gut fill. Due to genetic variation within maturity types it was not possible to predict the market readiness of a particular individual from its live weight. The carcass weights were heavier for steers slaughtered at 24 months of age than those at 18 months of age, but had a lower fat classification code. This seems to be due to the fact that these steers were slaughtered at the end of the winter period when the quality of the grazing was at its lowest. The carcasses of all maturity types had the highest fat classification codes at 30 months of age. However, the carcasses of 77.8% of the Simmentaler crossbreds were graded 1 (lean) in the fat classification. Furthermore, at 30 months of age 23.8% of all steers had more than two permanent incisors. This increases their carcass age classification, which lowers their carcass grading according to the South African grading standards, and thus their price per kg relative to the younger ages. The results of this investigation indicated that steers of all frame sizes would have to be fed additional energy to ensure that they finish with a fat classification code of at least 2 before the age of 30 months to ensure optimal financial returns.