Effects of cyclic monensin feeding on ruminal function in cannulated beef steers consuming low-quality forage

Abstract

The aim of this research was to evaluate the effects of cyclically feeding monensin (Rumensin 90, Elanco Animal Health) to ruminally cannulated steers (12 Bos taurus; 260 kg of BW) consuming low-quality forage (4.9% CP) on measures of intake, digestion, and ruminal function. Twelve steers were randomly assigned to 1 of 3 treatments in a completely randomized design: (1) no monensin (CON); (2) 200 mg·head−1·d−1 monensin (MON); or (3) 200 mg·head−1·d−1 monensin for 14 d and 0 mg·head−1·d−1 monensin for the subsequent 14 d (CYC; recurrent cycle). Hay and ort samples were collected d 10 through 13 for intake determination during each of the four 28-d replicated cycles. Rumen fluid was collected with a suction strainer 0, 2, 4, 8, and 12 h after feeding on d 14 for pH and VFA analysis. Data were analyzed using the MIXED procedure of SAS 9.4 (SAS Institute Inc.). Treatment × cycle interactions or treatment effects were not observed for any measure of intake (OM intake, NDF intake; P ≥ 0.17) or pH (P = 0.13). A tendency for a treatment × cycle interaction was observed (P = 0.08) for OM digestion but not for NDF digestion (P = 0.37). Treatment × cycle interactions were observed (P < 0.01) for molar proportions of acetate, propionate, and acetate:propionate ratio. Because these treatment × cycle interactions occurred, treatment × hour interactions, treatment effects, and hour effects were evaluated within cycles for these parameters. Treatment × hour interactions (P ≤ 0.02) occurred for molar proportion of propionate in cycle 1 and propionate and acetate:propionate ratio in cycle 3, and there was a tendency (P = 0.09) for a treatment × hour interaction for acetate:propionate in cycle 4. Although treatment effects were observed for molar proportions of acetate, propionate, and acetate:propionate ratio in cycle 1 (P ≤ 0.01), these effects had diminished by cycle 4 (P ≥ 0.39). An effect of hour after feeding was observed (P ≤ 0.02) in all 4 cycles for acetate, propionate, acetate:propionate, and ruminal pH. Further evaluation of monensin feeding methods that vary duration of feeding and withdrawal periods may be warranted. Additionally, further study concerning differences in the effect of these methods on a diet bases on concentrate versus low-quality forage warrant investigation.