Results and Discussion
Early-weaned cattle spent the most amount of time in the feedlot, followed by NW, while Y calves spent the least amount of time in the feedlot (P < 0.01) to achieve a similar backfat depth at slaughter (Table 2). While in the feedlot, Y calves gained faster (P < 0.01) than NW and EW calves, resulting in the shorter stay in the feedlot. However, when measured from 110 days of age until slaughter, ADG was greatest (P < 0.01) for EW, intermediate for NW, and lowest for Y calves. Thus, EW cattle were the youngest at slaughter, by 58 days compared to NW cattle, and Y cattle were the oldest at slaughter, by 143 days compared to NW cattle (P < 0.01) due to faster overall gains, and smaller mature size. Increased overall ADG for EW cattle was a result their gaining approximately 50% faster (P < 0.01) than NW and Y calves early on (110 to 202 days of age). When ADG was measured from 202 days of age until slaughter, NW, followed by EW cattle, gained faster (P < 0.01) than Y cattle. Normal-weaned calves may have experienced some compensatory gain upon entering the feedlot, while Y cattle were still on pasture from 202 to 371 days of age. At slaughter, Y cattle were 221 lb heavier than NW cattle, and NW cattle were 143 lb heavier than EW cattle (P < 0.01). Hip height was similar (P > 0.30) between age groups at 110 and 202 days of age, but at slaughter Y calves were 1.8 inches taller than NW calves, and NW calves were 1.4 inches taller than EW calves (P < 0.01). Total DMI was similar (P > 0.22) between age groups, but daily DMI was the lowest for EW, intermediate for NW, and the highest for Y calves (P < 0.01). Early-weaned calves were the most efficient, followed by NW, and Y calves were the least efficient (P < 0.01). Lower daily intakes and greater feed efficiency for younger calves is in agreement with Myers et al. (1999), Schoonmaker et al. (1999a, 1999c), and Story et al. (2000) and is a result of their consuming more feed when their maintenance energy requirements were low (NRC, 1996).
Both bulls and steers had an average daily gain of 3.8 lb/day while in the feedlot and an average daily gain of 3.3 lb/day from 202 days of age until slaughter. As a result, bulls and steers were slaughtered at a similar age (P > 0.42) and spent a similar (P > 0.37) number of days on feed. However, from 110 to 202 days of age bulls gained 7.9% faster (P < 0.01) than aggressively implanted steers. However, a weaning status x castration interaction existed for ADG from 202 days of age until slaughter, and for slaughter weight (P < 0.01). Early-weaned bulls gained 8.3 % slower (3.41 versus 3.72 lb/day), and were slaughtered at a lower weight (1123 versus 1159 lb) compared to aggressively implanted EW steers, which is in contrast to Schoonmaker et al. (1999c). Normal-weaned bulls and steers gained similarly from 202 days of age until slaughter (3.71 vs 3.72 lb/day) and slaughter weight was not different (1293 vs 1274 lb); however, Y bulls gained 5.6 % faster (2.84 versus 2.69 lb/day), and were slaughtered at a heavier weight (1574 versus 1435 lb) compared to aggressively implanted Y steers. This change in the pattern and extent of growth between bulls and steers at different ages may be due to a gradual (as testicles grow) increase in hormone secretion in bulls rather than large fluctuations (as steers are implanted) in hormone secretion. At 110 and 202 days of age, bulls and steers were both 40 and 45 inches tall (P > 0.28), respectively; however, steers were 0.9 inches taller (P < 0.04) than bulls at slaughter. Daily DMI, total DMI, and feed efficiency were similar (P > 0.16) between bulls and steers during all phases of the trial.
Even though cattle were all slaughtered at a similar backfat depth (P > 0.13) as was planned, EW calves had the smallest (P < 0.01) rib-eye area, followed by NW, and Y had the largest rib-eye area due in part to lower carcass weights for EW cattle (Table 3). Yearling calves produced carcasses that were 123 lb heavier than NW carcasses, and NW calves produced carcasses that were 96 lb heavier than EW carcasses (P < 0.01), indicating that early feedlot placement may compromise product yield. Dressing percentage was similar between age groups (P > 0.25). The pattern of backfat and rib-eye area deposition was altered depending on age at feedlot entry. Similar to Schoonmaker et al. (1999b), EW cattle were the fattest over the rib by 0.10 inches (P < 0.01), and had the largest rib-eye area by 1.0 in2 (P < 0.01) at 202 days of age due to the higher energy diet. Backfat depth and rib-eye area for NW and Y cattle at 202 days of age was not different (0.10 and 7.4 inches2, respectively) and had changed very little in 92 days. Early-weaned and NW calves had lower yield grades (P < 0.04), higher quality grades (P < 0.01), and produced fewer select carcasses compared to Y calves (P < 0.01), indicating that earlier feedlot placement results in higher quality beef. The percentage cattle grading low choice, average choice, and high choice was similar (P > 0.15) between EW, NW and Y cattle.
Bulls had a greater (P < 0.09) hot carcass weight, a higher dressing percent (P < 0.08), and a larger (P < 0.01) rib-eye area at a similar (P > 0.93) backfat depth compared to steers; however, rib-eye area and backfat depth were not different (P > 0.28) at all other stages of growth (110 or 202 days of age). Yield grade, quality grade, and quality grade distribution did not differ (P > 0.23) for bulls compared to aggressively implanted steers. However, a weaning status x castration interaction occurred (P < 0.04) for percentage of cattle grading high choice. All of the bulls grading high choice entered the feedlot at 110 days of age, while all of the steers grading high choice entered the feedlot at 202 days of age. This indicates that for bulls to achieve a high level of intramuscular fat deposition, they must be fed a high energy diet at a young age rather than at an older age, while aggressively implanted steers still have the ability to grade high choice when entering the feedlot at an intermediate age. The gradual release of testosterone, estradiol 17b, and growth factors (Lee et al., 1990), and eventual completion of puberty in bulls compared to the rapid release and gradual decrease of trenbolone and growth factors in implanted steers (Johnson et al., 1996b) may explain this interaction.