The effect of time of implanting zeranol on the growth of suckling steer calves.

Three groups of beef steer calves were implanted in 1983 with 36 mg of zeranol at 2 months or 4.5 months age or at both these ages, and their growth rates were compared with an untreated group. At approximately 8 months age in December the 3 implanted groups were heavier than the control group by 14.6 kg (P less than 0.05), 23.4 kg and 22.1 kg (P less than 0.01). At peak animal weights recorded in March the differences were 18.7 kg (P less than 0.05), 31.3 kg and 31.0 kg (P less than 0.01). In June 1984, following loss of weight by the steers, differences between the implanted groups and the control groups had declined to 7.7 kg (P greater than 0.05), 19.4 kg (P less than 0.01) and 21.1 kg (P less than 0.001), differences similar to those recorded approximately 3 months after implantation. It is concluded that higher weight gains resulted from implanting suckling calves at 4.5 months of age, than by implanting calves at 2 months of age, and that implanting suckling calves twice is unlikely to be any better than implanting them once at 3 to 6 months age. Part of the maximal response can be lost if steers subsequently lose weight.