1686 Performance of lambs fed high concentrate-diets containing monensin or narasin

The objective in this trial was to determine the effect of ionophore inclusion (monensin or doses of narasin) on performance of lambs fed high-concentrate diet. Forty-five (30 males and 15 females) Dorper × Santa Inês lambs were assigned to a randomized complete block design, defined by age (90.5 ± 0.8 d old), sex, and initial BW (25.3 ± 0.5 kg). Lambs were housed in “tie stalls” and fed a TMR composed of 90% concentrate. Diets were isonitrogenous (15.5% CP, DM basis) and treatments were Control (without ionophore), monensin (25 mg/kg DM), and three doses of narasin (5, 10 or 15 mg/kg DM), corresponding to the experimental diets C, M, N5, N10, and N15, respectively. Feed ingredients (ground corn, ground hay, soybean meal, limestone, mineral salt, urea, and ammonium chloride) and ionophores were mixed using a horizontal mixer. The experiment lasted 56 d and lambs were weighted after a fasting period of 14 h on Days 0, 14, 28, 42, and 56. Average daily gain (ADG), dry matter intake (DMI), and feed efficiency (FE) were determined in each period. Data were analyzed using the MIXED procedure (SAS Inst. Inc.). There were two contrasts previously defined (I: control vs. ionophores; II: monensin vs. narasin). The effects of levels of narasin (control, N5, N10, and N15) were evaluated using linear and quadratic orthogonal contrasts. The effects were considered significant when P < 0.10. Inclusion of ionophores did not affect BW 14d after start receiving the experimental diets. However, on d 28 animals fed the diets containing narasin (5, 10, or 15 mg/kg) were heavier (P = 0.01) than those fed monensin (C: 32.7; M: 31.3; N5: 32.2; N10: 33.2; N15: 33.2 kg). On d 42 there was an increased linear effect (P = 0.04) for levels of narasin and animals fed monensin were lighter than animals fed narasin (C: 36.8; M: 36.1; N5: 36.1; N10: 38.0; N15: 37.8 kg; P = 0.07). There was an increased linear effect (P = 0.02) on final BW (d56) and animals fed narasin were heavier than those fed monensin (C: 40.9; M: 40.2; N5: 40.5; N10: 42.4; N15: 42.5 kg; P = 0.02). There was no effect (P = 0.40) on DMI (1.09 ± 0.04 kg/d). Narasin increased (P = 0.04) feed efficiency (C: 0.25; M: 0.25; N5: 0.25 N10: 0.26; N15: 0.27). Narasin improved lamb performance compared to a monensin dosage of 25 mg/kg.