Effects of a slow-release urea product on performance, carcass characteristics, and nitrogen balance of steers fed steam-flaked corn

Abstract

Two experiments were conducted to examine the impact of source, urea (U) or Optigen II (OP), and level of dietary NPN on performance (Exp.1) and N balance (Exp. 2) of growing cattle. Sixty Angus crossbred steers (initial BW = 353 ± 13.9 kg) were used to evaluate performance, and fed 1 of 3 steam-flaked corn based diets: U (U(1.2), 1.2% NPN), OP (OP(1.3), 1.3% NPN), or OP without cottonseed meal (OP(3.1), 3.1% NPN). U(1.2)and OP(1.3) contained cottonseed meal and NPN as CP sources, whereas OP(3.1) contained only NPN. For Exp. 1, steers were blocked by postweaning BW and assigned to treatment (TRT) and pen within block. Body weight was collected every 14 d during the 105-d trial. Six steers from each TRT were selected based on residual feed intake (RFI) and harvested. Carcass and organ measurements were obtained. Cumulative animal performance was evaluated in 3 periods (0 to 35, 0 to 70, and 0 to 105 d) using a mixed coefficient model with initial BW as a covariate. For Exp. 2, 5 ruminally cannulated Holstein steers in a 5 × 5 Latin square design were used to evaluate N balance. Steers were fed a steam-flaked corn based diet with either no NPN, 0.75% U or N equivalent OP, or 1.5% U or N equivalent OP. Intake was measured, and feed, orts, urine, and fecal samples were obtained and composited for each steer by period. Data were analyzed using a mixed coefficient model. Orthogonal contrasts were used to evaluate differences between OP and U, and high and low level of NPN. For Exp. 1, there were no differences (P > 0.10) in initial BW, final BW, ADG, or DMI among TRT for any of the periods. However, for period 1 steers on OP(3.1) had reduced F:G than U(1.2) (5.71 kg/kg vs. 7.39 kg/kg; P = 0.03), and steers fed OP(1.3) tended to have less F:G than those fed U(1.2) (6.07 kg/kg vs. 7.39 kg/kg; P = 0.07). In period 2, OP(3.1) had reduced F:G than U(1.2) (5.58 kg/kg vs. 6.56 kg/kg; P = 0.03), but did not differ from OP(1.3) (5.97). For Exp. 2, steers fed OP tended (P = 0.09) to have less apparent N absorption than those fed U. Apparent N absorption differed (P < 0.05) for level of NPN. There were no differences (P > 0.10) in intake or digestibility among source or level of NPN. No major differences (P > 0.10) on performance and carcass composition were observed between U and OP diets. Steers had better initial F:G (Period 1 and 2) when OP was used as the only source of feed N (OP(3.1)), suggesting that OP may replace true protein feeds in finishing cattle diets, minimizing feed use with no impact on carcass quality.