Effect of delayed castration on the growth rate, behaviour and serum insulin-like growth factor-1 concentration of beef cattle on tropical pasture

Male beef calves in northern Australia are generally castrated under 6 months of age as an aid to management of animal behaviour and for the purpose of producing sufficiently fat carcasses at slaughter. It is also common for beef producers to administer hormone growth promotants, such as oestradiol, to increase the growth rates of steers at pasture. This experiment tested the hypothesis that delaying castration by 9 months would produce an increase in growth rate for the period while the cattle are retained as bulls without compromising management of animal behaviour. Bulls of three genotypes were castrated at 7 months or 16 months of age and half were treated with oestradiol after castration. Body weight (BW) and serum insulin-like growth factor-1 concentrations (IGF-1) were measured and behaviour was observed before and after the 16 month castration. At both 7 and 16 months of age, surgical castration of bulls caused no animal welfare problems. BW gain was affected by genotype with Bos taurus types having a lower BW gain than Bos indicus and F1-cross cattle. IGF-1 was similarly affected by genotype. Oestradiol treatment increased growth of the steers (early castrates) during the 9 months between castration times compared with non-implanted steers and bulls. Oestradiol implanted steers had similar BW gain from the late castration to slaughter, regardless of age at castration, which was greater than non-implanted early castrates which, in turn, was greater than the late castrates. Similar differences were observed for IGF-1 concentrations. Behaviour was not affected by genotype, oestradiol treatment or age of castration but this may have been an artefact of the design where all treatment groups were grazed together. Although delayed castration resulted in no behavioural or management problems, it did not produce a commercially useful increase in growth rate.