Impact of crude protein level and urea source on finishing performance of Nelore Bulls

Abstract

This study aimed to evaluate the effects of different levels of crude protein (11% vs. 14% DM) associated with different sources of urea in the diet (conventional vs. post-ruminal release) on finishing performance, carcass characteristics and apparent digestibility of dietary constituents of Nelore bulls. Eighty-four Nelore bulls with initial body weight (BW) of 448 ± 40.5 kg were blocked by BW, from lightest to heaviest, allocated in 7 groups of 4 pens (28 pens in total, with three bulls per pen). of equal PC and randomized block design (initial PC), in a 2 x 2 factorial arrangement (2 levels of crude protein and 2 sources of urea) to test interactions between urea sources. The following treatments were assigned: (1) 11% crude protein (CP) with conventional release urea (UC) (11% CP + UC); (2) 11% crude protein (CP) with post-ruminal release urea (PRU) (11% CP + PRU); (3) 14% crude protein (CP) with conventional release urea (UC) (14% CP + UC); (4) 14% crude protein (CP) with post-ruminal release urea (PRU). The experimental period lasted 90 days. The fully mixed rations were mixed manually and offered ad libitum twice a day (08:00 and 16:00 h), so that the surplus did not exceed 5%. Daily food consumption was recorded. Bulls were weighed individually (not fasted) on days 0, 50 and 90 of the experiment. Carcass ultrasound was performed together with weighing. Blood samples were obtained individually before feeding on day 49 of the experiment to measure energy metabolism, protein metabolism parameters, and glucose concentrations. Differences were considered significant at P ≤ 0.05 and trends were discussed at 0.05 < P < 0.10. Bulls fed 14% CP had higher BW and ADG on day 90 (P < 0.04) than those fed 11% CP (608 vs. 594 kg and 1.93 vs. 2.14 kg/day, respectively). In average, 14% CP diets promoting an increase of 6% on feed efficiency compared to 11% CP diets(P = 0.10; 178.6 vs. 189.7 g/kg, respectively). Dietary CP concentration affect positively the hot carcass weight (P = 0.03; 329 vs. 338 kg), but ultrasound measurements were not affected (P > 0.15). PRU treatments showed lower fecal N loss (P = 0.04) than those fed conventional urea (79.2 vs. 87.2 g/d). The N retention efficiency per unit of N ingested and absorbed was higher (P < 0.001) for treatments with 11% CP (20.3 vs. 17.1 g/g and 32.2 vs. 24.2 g/ g). The N retention efficiency per gram of N ingested was higher (P = 0.03) with the inclusion of PRU (19.5 vs. 17.9 g/g). The reduction in CP content of diets negatively affect DM intake, animal performance, and carcass weight.