26 Effect of Monensin Inclusion on the Rumen Prokaryote Microbiome inBos Indicus andBos Taurus Steers Consuming Bermudagrass Hay

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Abstract Monensin is a carboxylic polyether ionophore that has the ability to alter ruminal fermentation and microbial populations in the rumen. Current research shows monensin to have beneficial effects on feed efficiency, specifically, a reduction in average daily feed intake and an increase in gain. Ten ruminally cannulated steers [5 Bos indicus (BI), 5 Bos taurus (BT); 398 kg BW] were used to measure both the effects of monensin inclusion and subspecies’ differences when provided a forage-based diet (bermudagrass hay; 13.7% CP). Subspecies were concurrently subjected to a two period, two treatment crossover design. Each 70-day period, consisted of 20-days for treatment adaptation followed by 22-days for sample collection. Treatments included 0 (CON) or 200 (MON) mg·animal-1 monensin (Rumensin 90; Elanco Animal Health, Greenfield, IN) fed daily within 0.91 kg dried distiller’s grains with solubles. A withdrawal period (28 days) was allowed before the second period, to prevent carryover effects of monensin. Rumen fluid was collected via suction strainer from the dorsal and ventral sacs of the rumen at feeding on day 42, then subsequently frozen for later analysis. Upon completion of the trial, rumen fluid was thawed and analyzed for microbial populations using massively parallel sequencing. The V4 region of the 16S rRNA gene was sequenced to analyze and compare rumen prokaryotic microbes within treatment groups and subspecies. Sequences were processed using QIIME 2 (q2) version 2021.2 and imported into R v4.1 for statistical analysis and graphical displays. Bray Curtis-based principle coordinate analysis (PCoA) provided visualization of microbial community clustering by monensin treatment and by subspecies, facilitating identification of microbial community shifts resulting from the treatments. There was no significant interaction between monensin and subspecies (P = 0.42); however, effects of monensin (P ≤ 0.01) and subspecies (P = 0.04) did exist. Due to possible links to feed utilization and methane production, this alteration of rumen microbial communities in cattle is of great interest.