EFFECT OF TESTOSTERONE PROPIONATE ON PERFORMANCE AND CARCASS CHARACTERISTICS OF HEIFERS AND COWS D. B. Faulkner, F. K. McKeith, L. L. Berger, D. J. Kesler and D. F. Parrett University of Illinois 1, Urbana 61801 ABSTRACT Two experiments were conducted to evaluate the effect of testosterone on growth and composition of cattle. In the first experiment, crossbred yearling heifers (n = 48) were assigned to four treatments: 1) control (no implant), 2) Synovex-H implants on d 1 and 84, 3) one testosterone propionate implant administered on d 1 and a second on d 84 and 4) two testosterone propionate implants administered on d 1. Heifers were fed a high-energy diet for the 157-d study. Implanting with the high-testosterone treatment improved (P < .05) daily gain and feed efficiency compared with the other treatments. Marbling score was reduced (P < .05) with the high-testosterone treatment. In Exp. 2, mature cows (n = 36) were assigned to one of three feeding periods (0, 42, or 84 d) with the cows fed 42 or 84 d subdivided into two groups (implanted with testosterone propionate or nonimplanted control). Cows were fed a high-energy diet and slaughtered at the end of each feeding period. Testosterone did not influence (P > .05) feedlot performance. Increased time on feed reduced (P < .01) dally gain (live weight basis) and feed efficiency but did not influence feed intake. Testosterone treatment had little influence on the fat and moisture contents of the carcass soft tissue or on the palatability characteristics of loin steaks. Time on feed increased lean muscle mass and carcass fat (P < .05). Sensory traits were improved at 42 d on feed (P < .05), but no further sensory improvement was observed at 84 d. In summary, testosterone propionate improved the performance of heifers but not of cows. (Key Words: Cows, Heifers, Refeeding, Testosterone, Growth, Carcasses.) J. Anita. Sci. 1989.67:1907-1915 Introduction The efficient production of lean, whole- some, palatable and nutritious beef products will determine the future of the beef induslry. Several reviews (Bauman et al., 1982; Davis et al., 1984; Etherton and Kensinger, 1984; Schanbacher, 1984; Unruh, 1986) have shown the benefit, roles and relationships of several hormones involved in growth of meat animals. Much of this work has been with young animals; however, there is the potential of improved growth rate and carcass composition using these compounds in mature animals I Dept. of Anita. Sci. Received August 30, 1988. Accepted January 27, 1989. (Um'uh, 1986). Compounds that improve per- formance or alter composition of gain could improve the economics of feeding cows dramatically. Price and Makarecian (1982) implanted with zeranol and found an 8% improvement in efficiency of heifers, but no effect was observed with cows. Matulis et al. (1987) also observed no improvement in performance due to anabolic compounds. Evaluation of alternate compounds for mature cows is needed. Androgen (trenbolone acetate) treatment of heifers has improved both growth rate and feed efficiency (Galbraith, 1980; Henricks et al., 1982; Bouffault and Willemart, 1983). Evalua- tion of efficacy of other androgens may be important. These compounds should be evalu- ated in both young and old animals to determine whether their benefits are age- dependent. 1907