A system biology approach reveals differences in the dynamics of grass vs hay colonization and utilization by rumen microbes with minor effects of vitamin E supplementation. Colonization of grass vs hay by rumen microbes

Increasing the efficiency of forage degradation and nutrient utilization are key targets for ruminant science. This study investigated the successional microbial colonization of ryegrass (GRA) vs ryegrass hay (HAY) in the presence of zero or 50 IU/d supplementary vitamin E using a rumen simulation technique. A holistic approach was used to link the dynamics of rumen fermentation and feed disappearance with the structure of the liquid- (LAB) and solid-associated bacteria (SAB) based on 16S rRNA (cDNA) sequencing. Results showed that forage colonization by SAB was a tri-phasic process which was highly affected by the forage conservation method: Early colonization (0 - 2h) by rumen microbes was twice as fast for GRA than HAY diets and dominated by Lactobacillus and Prevotella which promoted increased levels of lactate (+56%) and ammonia (+18%). HAY was slowly colonized by a bacterial community dominated by Streptococcus leading to lower DM disappearance (-72%). During secondary colonization (4-8h) the solid associated community increased in size and decreased in diversity as the secondary colonizers took over (i.e. Pseudobutyrivibrio). Secondary colonization was three times slower for HAY than GRA diets, but this delay was compensated by a greater community complexity in terms of diversity (+197 OTUs) and network density (+21 nodes, +51 edges) resulting in similar feed disappearances during this period. Tertiary colonization occurred later than 8h after feeding and consisted of a slowdown in the colonization process and simplification of the bacterial network. This slowdown was less evident for HAY diets which had higher levels of tertiary colonizers (i.e. Butyrivibrio and Ruminococcus) leading to higher DM disappearance (+52%) during the 8 - 48h interval. The LAB community was dominated by Prevotella and was particularly active during the early fermentation of GRA diets and during the late fermentation for HAY diets indicating that the availability of nutrients in the liquid phase reflects the dynamics of feed disappearance of each feed. Vitamin E supplementation had minor effects on the rumen function but promoted a slight acceleration in the SAB colonization sequence and a simplification of the LAB community leading to increased DM disappearance (+19%) during secondary colonization. Our findings suggest that when possible, grass should be fed instead of hay, in order to accelerate feed utilization by rumen microbes, otherwise new strategies should be developed to improve the physical-chemical structure of conserved forages and/or the cellulolytic capacity of the rumen microbiota.