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Abstract The interactions between two different crude protein levels CP, 11 vs 14% of dry matter, DM and two urea sources US, feed-grade (FGU) vs. post-ruminal release (PRU) on finishing performance of Nellore bulls were evaluated. Nellore bulls n = 84; initial body weight (BW) = 448 ± 40.5 kg were stratified from light to heavy and allocated to 7 groups of 4 replicate pens (28 pens in total, with three bulls per pen). The following treatments were assigned: 1) 11% CP with UC; 2) 11% CP with PRU; 3) 14% CP with UC; 4) 14% CP with PRU. At the beginning of the study, 7 bulls were slaughtered to determine initial carcass dressing (452 ± 62.6 kg; 52.8%). Diets consisted of sorghum silage, ground corn, rehydrated and ensiled corn, vitamin-mineral mix, soyhulls (11% CP diets) and soybean meal (14% CP diets). Kaolin was incorporated in diets for reach 100%. The PRU source (Trouw Nutrition, Netherlands) contained 82% urea on DM basis. In an in vitro assay, 27% of this urea was released into the rumen. The total mixed rations were manually mixed and offered ad libitum twice a day (0800 and 1600 h). Daily feed intake was recorded. The experimental period lasted 90 d. The bulls were weighed individually after 16 h of feed restriction on d 0 and d 90 of the experimental period. The statistical model included the fixed effects of CP, US, and the CP × US interaction, as well as the random effect of pen nested within treatment. Effects of CP × US were not detected (P ≥ 0.39) for any feedlot growth performance variable. Bulls fed 14% CP presented greater BW, carcass weight, and ADG at d 90 (P 0.04) than those fed 11% CP (608 vs. 594 kg, 339 vs. 329 kg, and 1.93 vs. 2.14 kg/d, respectively; Table). Diets with 14% CP diets tended to increase feed efficiency (F:G) in 6% CP compared with diets with 11% CP (P = 0.10; 178.6 vs. 189.7 g/kg, respectively). Although there was not effect of urea source on growth performance, bulls fed PRU presented greater F:G and carcass gain efficiency when compared with those eaten diets with FGU (P ≤ 0.06; 190 vs. 178 g/kg, 118 vs. 109 g/kg, respectively). Diets with 11% CP for finishing bulls negatively affected DM intake, animal performance, and carcass weight. The PRU inclusion in the finishing diets increases feed efficiency for carcass gain.
Paula et al. (Sun,) studied this question.