Evaluating the Rumen Microbial Community of Genetically Divergent Spring-calving Dairy Cows Grazing Grass-only or Grass-clover Swards at Different Stages of the Grazing Season

Charles Dwan^1,2*Anubhav Das^3,4Paul W O'Toole^3,4Tom F O'Callaghan⁵Dara Meehan^3,4Deirdre Hennessy²Hannah Irish⁶Frank Buckley^2,6Ben Lahart¹

• ¹Teagasc Animal and Grassland Innovation Centre, Moorepark, Fermoy, Ireland
• ²University College Cork School of Biological Earth and Environmental Sciences, Cork, Ireland
• ³University College Cork School of Microbiology, Cork, Ireland
• ⁴University College Cork APC Microbiome Institute, Cork, Ireland
• ⁵University College Cork School of Food and Nutritional Sciences, Cork, Ireland
• ⁶Teagasc Animal and Grassland Innovation Centre , Moorepark, Fermoy, Ireland

The current study used culture-independent methodology to investigate the rumen microbiome composition in two genetically divergent groups of spring-calving dairy cows, high (€218) and low (€157) Economic Breeding Index (EBI), grazing two sward treatments, perennial ryegrass (grass-only; n = 14) or perennial ryegrass and white clover (grass-clover; n = 14) at three time points across the spring, summer and autumn of a single grazing season. The analysis indicated that EBI status had no significant effect on the rumen microbial community within the statistical power of this study. Beta diversity between microbiomes was only different (P < 0.001) between the two sward treatments in the autumn when clover proportion was greatest (50.2%). Season had a significant effect on microbiome beta diversity across both sward treatments (P <0.001). There were only minor differences in the composition of the rumen microbiomes between the two sward treatments. Many bacterial genera were differentially abundant between spring and the two later time points. Bacterial genera more abundant in the spring were positively correlated with rumen propionate levels while those more abundant in summer and autumn were negatively correlated with propionate and positively correlated with acetate and butyrate. Methanogenic archaeal abundance was greater in summer and autumn compared to spring and they were negatively correlated with propionate and positively correlated with methane (CH4) production. The results from this study demonstrate that genetic selection using the EBI does not affect the rumen microbial community and the core rumen microbial community is similar in cows grazing grass-only or grass-clover swards. The results also demonstrate that the rumen bacterial community shifts across the grazing season, providing more favourable conditions for methanogenesis in the summer and autumn compared to spring.