FIGURE 2-4

Chemical structures of the endogenous steroids estradiol, progesterone, and testosterone and of synthetic anabolic steroids.

response in feedlot bulls (Richards et al., 1986) to a 69.9 percent increase in average daily gain in heifers treated with TBA (Bouffault and Willemart, 1983). Gender determines which anabolic steroid will be administered. The estrogenic compounds are generally more effective in steers. The response in females is more variable and less consistent, but the androgenic steroids are superior. Use of a combination of anabolics generally produces an additive response compared to use of either estrogenic or androgenic implant alone. Response in bulls is generally less than that of steers, and implanted steers often achieve the growth performance observed in nonimplanted bulls (Fisher et al., 1986).

Rate of live-weight gain is increased 10 to 20 percent on average by anabolic steroids (Bradley et al., 1957; Sharp and Dyer, 1971; Griffiths, 1982; O'Lamhna and Roche, 1984; Gill et al., 1987; Keane and Drennan, 1987; Perry et al., 1991), but responses approaching 50 percent have been observed in lambs (Sulieman et al., 1988), steers in calorimetry chambers (Lobley et al., 1985), and steers compensating for earlier growth restriction (Keane and Drennan, 1987). It appears that young animals may respond better to steroid implants than older animals (Mader et al., 1985; Whittington, 1986; Simms et al., 1988), and this may be particularly true in bulls (Richards et al., 1986). In some studies it was observed that the rate of mass increase was higher during the early period following implantation and then decreased, which may reflect the declining circulating concentration of the anabolics after the first few weeks (Schanbacher, 1984; Lobley et al., 1985; MacVinish and Galbraith, 1988; Hayden et al., 1992).

Results from TBA-estradiol combinations appear to be superior to either implant alone. Dose-response efficacy trials involving 1,296 steers conducted for FDA approval of TBa-estradiol combinations indicate that the ADG plateaued at 118 mg TBA combined with 24 mg estradiol, but the feed efficiency plateaued at 139 mg TBA combined with 28 mg estradiol (Bartle et al., 1992). ADG was increased 18 percent and the feed: gain ratio was reduced 9.5 percent, both of which exceeded the response to 30 mg estradiol alone. Implants of 140 mg TBA alone did not improve growth performance in this and other studies (Apple et al., 1991; Hayden et al., 1992). The authors further indicated that a TBA-estradiol ratio of 5:1 is optimum for feedlot steers fed a high-grain diet. Feed conversion efficiency is usually improved with anabolic steroids, but the magnitude of the response is variable. Improvements of 5 to 14 percent have been reported (Rumsey, 1978; Greathouse et al., 1983; Mathison and Stobbs, 1983; Steen, 1985; Thonney, 1987). TBA-estradiol combinations have been shown to decrease feed: gain ratios by 10 to 13 percent (Trenkle, 1987; Eversole et al., 1989; Perry et al., 1991; Bartle et al., 1992). The degree of response to anabolic steroids is clearly influenced by dry-matter intake changes, which are also variable, and by the degree of change in composition of gain. The majority of studies in which large increases in gain were observed also showed 5 to 10 percent increases in feed intake (Thonney, 1987; Perry et al., 1991; Bartle et al., 1992) and proportional increases in lean mass in cattle (Keane and Drennan, 1987) and lambs (Sulieman et al., 1986, 1988). However, no significant changes in feed intake were observed in several studies (Griffiths,