Effects of rotating antibiotic and ionophore feed additives on enteric methane and rumen microbial populations of steers consuming a high forage diet

Abstract Text: Ionophore and antibiotic feed additives have been shown to decrease ruminal methanogenesis, but evidence of long-term mitigation is lacking. We proposed a rotation of feed additives as an alternative to reduce methane (CH₄) production. Rumen-cannulated steers (n = 12) were fed a basal high forage diet at 2% of BW (DM) for 13 wk in a Calan gate facility receiving 1 of 6 treatments (trt): 1) control (Con) no additive, 2) bambermycin (B) = 20 mg B/hd/d, 3) monensin (M) = 200 mg M/hd/d, 4) B7M= rotating B and M treatments weekly, 5) B14M = rotating B and M treatments every 14 d, and 6) B21M= rotating B and M treatments every 21 d. Steers were blocked by weight in a RCBD with repeated measures. Rumen fluid was collected weekly for analysis (n = 13) and results were normalized according to organic matter intake (kg OMI). Trt did not significantly affect CH₄ production (P = 0.60), but tended to affect CH₄ to Propionate ratio (CH₄:Pro) (P = 0.06) being highest for Con and lowest for M, B21M, and B14M (0.42 vs. 0.36, 0.36, and 0.33, respectively). Week affected both CH₄ and CH₄:Pro (P < 0.05) with significant reductions by wk 3 but this effect was not sustained beyond wk 6. Microbial analysis revealed rotationally treated steers had greater populations of gram positive (G⁺) bacteria than continuously fed steers and Con (P < 0.01) and wk 0 populations were different from wk 5 and 6 but similar to wk 12 (51.1 vs.  37.5 and 35.1 vs. 44 %, respectively; P < 0.01). A class of G^- bacteria (Sphingobacteriia), phylum Bacteroidetes, was not affected by trt or wk but was positively correlated with CH₄ production (r = 0.24, P = 0.04). Archaeal populations of Methanobrevibacter spp. and Methanosphaera sp. correlated with CH₄ production (r = 0.22 and r= 0.37, respectively) and were not affected by trt. Wk tended to affect Methanobrevibacter spp. populations being lowest during week 3 and highest during week 12 (53.64% vs. 68.89% of Archaea; P = 0.06). Methanosphaera sp. populations were lowest during week 5 and higher during week 0 and 12 (0.02 % vs. 1.31 and 0.53 % of Archaea respectively; P< 0.05). Our results suggest microbial adaptation to trt between 4 and 6 wk.  Rotating monensin and bambermycin did not reduce CH₄ or delay microbial adaptation, more than continuously fed steers.

Keywords: CH₄, feed additives, microbes