Promoting a higher rumen microbial diversity favours the energy but not the protein metabolism in goats fed forage diets
Background: Ruminants have evolved to have a complex and highly diverse rumen microbiota. This diverse rumen microbiota allows ruminants to digest complex carbohydrates and produce microbial protein, as a result it provides a competitive advantage to wild ruminants fed low quality and high botanically diverse diets. However, a high microbial diversity also has some drawbacks derived from the low efficiency of N utilization and rumen methanogenesis. This study explores if promoting a complex rumen microbiota represents an advantage or a handicap in the current dairy production systems in which ruminants are artificially reared in absence of contact with adult animals and fed preserved and less diverse forage diets. Results: Inoculation of artificially reared goat kids with fresh rumen fluid from adult goats adapted to forage (RFF) or concentrated (RFC) allowed to increase the rumen microbial diversity later in life. As a result, inoculated animals had higher bacterial, and methanogens diversity and the presence of a complex rumen protozoal community, whereas the control (CTL, non-inoculated) animals remained protozoa-free. The inoculation with autoclaved rumen fluid (AUT) had minor long-term effects than using fresh inoculum. A higher microbial complexity had beneficial effects on the energy metabolism allowing a faster adaptation to the diet, a higher forage digestion and an energetically favourable shift from acetate to butyrate production in the rumen. These changes were associated to the presence of a certain rumen bacterial taxa and a diverse protozoal community. On the contrary, the presence of rumen protozoa (mostly Entodinium) had a negative impact on the N metabolism leading to a higher bacterial protein breakdown in the rumen and lower microbial protein flow to the host.  Conclusions: Promoting a high rumen microbial diversity represents a desirable attribute when animals are fed forages in which the N supply does not represent a limiting factor.