Effects of implants of oestradiol plus progesterone on the growth rate and intramuscular fat composition of steers raised in southern Australia

Abstract

At 3 experimental sites in South Australia and Victoria beef steers at pasture received subcutaneous implants of oestradiol (20 mg) plus progesterone (200 mg) in the ear and were raised with untreated animals for about 100 days. Implanted animals showed responses in liveweight gain 10-23% greater than untreated animals. The least response (0.10 kg/ steer.day) was recorded at Roseworthy Agricultural College in South Australia, which has the shortest growing season (7 months). The maximum response (0.19 kg/steer.day) was recorded near Mt Pleasant in South Australia which has a more temperate climate than Roseworthy, but a shorter growing season than a site in Victoria (near Molesworth) where untreated animals had the highest growth rates (1.07 kg/ steer.day). Analysis of intramuscular lipid showed that the ratio of saturated to polyunsaturated fatty acids was significantly correlated (P< 0.01) with the final liveweight of both the treated and untreated steers (r = 0.82). Our results demonstrated that oestrogenic implants were effective in steers in southern Australia. They were most effective in older animals that weighed more than 180 kg at implantation and were entering the fattening phase of their development. Oestrogenic hormone treatment increased growth rate but did not increase (nor decrease) fat deposition in animals of the same weight (P> 0.01).