Introduction
Castration alters growth rate and carcass characteristics due to a change in hormonal status (Anderson et al., 1988; Lee et al., 1990; Hunt et al., 1991). Implants alter growth rate and carcass composition in steers due to a change in hormonal status as well (Hayden et al., 1992; Hongerholt et al., 1992; Johnson et al., 1996b). Managing bulls and aggressively implanted steers in an early-weaned system enables early intramuscular fat deposition and allows for rapid and efficient growth, as well as a lean, high quality carcass (Schoonmaker et al., 1999c). Early-weaned bulls deposit fat more favorably than do early-weaned steers (Schoonmaker et al.,1999c) possibly due to a gradual (Lee et al., 1990) increase (as testicles grow) in hormone secretion rather than large fluctuations (as cattle are implanted) in hormone secretion. After steers are implanted, trenbolone (Johnson et al., 1996a) and Insulin-like Growth Factor-1 (IGF-1) (Johnson et al., 1996b) concentration peaks, then gradually decreases As a result muscle deposition slows (Johnson et al., 1996a), and the opportunity for intramuscular fat deposition may be delayed to a later age for implanted steers compared to bulls. However, waiting until 205 or 365 days of age to place bulls in the feedlot may hinder their ability to deposit intramuscular fat because the testicles are larger, hormone secretion is higher, and thus muscle deposition is more likely. Therefore, delaying feedlot entry until 205 or 365 days of age in bulls may compromise carcass quality to a greater extent in bulls than aggressively implanted steers. Our objective was to determine the effect age at feedlot entry on performance and carcass characteristics of bulls and aggressively implanted steers.