Effects of Monensin or Narasin on Rumen Metabolism of Steers during the Period of Adaptation to High-Concentrate Diets

Narasin is a coccidiostatic and antibacterial agent and may change rumen fermentation. The objective in this trial was to determine the effects of monensin or narasin on rumen metabolism of steers during the period of adaptation after abrupt transition from high-forage to high-concentrate diet. Eight Nellore steers (BW 475.7 ± 3.7 kg), cannulated in the rumen, were used in a replicated 4x4 Latin Square design. The trial lasted 96 days, divided in 4 periods of 24 days each. Steers were fed daily with coastcross haylage during the first 8 d of each period. From day 9 to 24, steers were fed with the experimental diets, which were composed by 90% of concentrate. The experimental diets were: C (Control: no additives added), Mon (25 mg of monensin/kg of DM), N10 (10 mg of narasin/kg of DM) and N20 (20 mg of narasin/kg of DM). The rumen fluid was collected every four hours (starting prior feeding, 4, 8 and 12 hours after feeding), from days 8 to 14 of each period. Short chain fatty acid (SCFA) and rumen pH were analyzed as repeated measures over time using MIXED procedure. The effect of the experimental diets was evaluated using the Tukey test. There was no effect of experimental diets for acetate (59.86 ± 0.86 mM/100 mM, P = 0.20), propionate (22.15 ± 0.77 mM/100 mM, P = 0.14), butyrate (13.14 ± 0.67 mM/100 mM, P = 0.38), isovalerate (2.43 ± 0.16 mM/100 mM, P = 0.36) and acetate-to-propionate ratio (2.77 ± 0.12, P = 0.19). However, steers fed Mon had a greater molar proportion of isobutyrate (C: 1.03^b; Mon: 1.11^a; N10: 1.01^b; N20: 1.07^ab ± 0.05 mM/100 mM, P = 0.03) than C and N10. The Mon and N10 experimental diets decreased molar proportion of valerate (C: 1.18^a; Mon: 1.01^b; N10: 1.05^b; N20: 1.11^ab ± 0.06 mM/100 mM, P = 0.02). Diet containing monensin decreased total SCFA (C: 143.9^a; Mon: 132.6^b; N10: 145.8^a; N20: 140.4^ab ± 6.70 mM/L, P = 0.03) than C and N10. Steers submitted to Mon and N20 diets had a greater ruminal pH than steers fed C and N10 diets (C: 6.18^b; Mon 6.36^a; N10: 6.18^b; N20: 6.30^a ± 0.12, P = 0.01). The inclusion of 20 mg of narasin or 25 mg of monensin/kg of DM had a promising effect by increasing the ruminal pH during adaptation to high-concentrate diets.