Assessment of the Rumen Bacterial Community in Beef Cows Differing in Feed Efficiency Across Four Feeding and Grazing Scenarios

Abstract In Alberta Canada, the mature cow-herd consumes a variety of forage-based diets; whether that be grazing pastures in spring, summer, and into the fall and winter, or preserved forage during late winter and very early spring. Residual feed intake (RFI) is a measure of the feed efficiency of individual cattle and is typically measured in a feedlot setting. We sought to evaluate how rumen bacterial communities fluctuate when animals progressed through several forage and grazing scenarios throughout the year and whether cattle with different feed efficiency have varied shift patterns of rumen bacteria. Rumen fluid samples were collected from thirty-seven Kinsella composite hybrid beef cows at the end of different feeding or grazing regimes (March: silage fed in the feedlot, June: grazing on seeded pasture, August: grazing on native grassland, and November: grazing on stockpiled native grassland). RFI values of the beef cows were measured during the feedlot period and cows were divided into three groups: high-RFI (inefficient; RFI > 0.33), medium-RFI (0.33 > RFI > -0.33), and low-RFI (efficient; RFI < -0.33). Volatile fatty acid (VFA) profiles and total bacterial populations were analyzed using gas chromatography and qPCR with universal bacteria primers, respectively. Bacterial communities were assessed using 16S rRNA gene amplicon sequencing, and the data were analyzed using QIIME2 (version 2022.11) with the SILVA 138.1 database. Rumen samples collected in June (seeded pasture) had greater (P < 0.05) concentrations of total VFAs, acetate, propionate, butyrate, isobutyrate, valerate, and isovalerate, compared with those collected at later dates while grazing native grassland. Rumen samples collected in March (feedlot) and June (seeded pasture) had greater (P < 0.05) total bacterial populations compared with samples collected in August (native grassland) and November (stockpiled native grassland). Higher microbial species richness (P < 0.05) was observed for rumen samples collected in March (feedlot) than those under other feeding regimens. Beta-diversity analysis revealed the rumen bacterial community of cows during the feedlot period was clearly separated (P < 0.05) from those grazed on pasture. A total of 22 phyla and 279 genera were identified from the rumen bacteriome. The relative abundances of five genera: Anaerosporobacter, Oribacterium, Pirellula, Robinsoniella, and uncultured taxa in Absconditabacteriales_SR1 were significantly different (P < 0.05) in the rumen during the different feeding or grazing periods. There were no differences in the rumen bacterial diversity and composition between RFI groups. The findings imply that feeding a homogenous diet in seeded pasture may enhance rumen fermentation and bacterial proliferation, and different grazing regimens could affect rumen bacterial diversity and abundance. Further studies are needed to identify major factors that drive the changes between different feeding and grazing regimes and to find ecological indicators of the rumen bacterial community related to feed efficiency.