Extensive degradation of amino acids in the rumen via microbial deamination decreases the post-ruminal availability of dietary indispensable amino acids. Together with the normal decrease in voluntary dry matter intake (DMI) around parturition in dairy cows, microbial metabolism contributes to a markedly negative balance of indispensable amino acids, including methionine which may be the first-limiting for milk production. The main objective of the current study was to profile changes in major bacterial species with key functions in cellulose and hemicellulose digestion, xylan breakdown, proteolytic action, propionic acid production, lactate utilization and ruminal biohydrogenation in cows supplemented with rumen-protected Methionine (SM; Smartamine M, Adisseo NA, Alpharetta, GA, USA) from -23 through 30 d relative to parturition. Because ~90% of the methionine in SM bypasses the rumen, ~10% of the methionine is released into the rumen and can be utilized by microbes.
As expected, there was an increase in overall DMI after parturition (Day,P < 0.05) during which cows consumed on average 19.6 kg/d versus 13.9 kg/d in the prepartum period. The postpartum diet contained greater concentrations of lipid and highly-fermentable carbohydrate from corn grain, which likely explains the increases in the relative abundance ofAnaerovibrio lipolytica,Megasphaera elsdenii,Prevotella bryantii,Selenomonas ruminantium,Streptococcus bovis, andSuccinimonas amylolytica. Despite similar DMI prepartum, cows fed SM had greater (Treatment × Day,P< 0.05) abundance prepartum ofFibrobacter succinogenes, Succinimonas amylolytica, andSuccinivibrio dextrinosolvens. However, the greater DMI in cows fed SM after parturition (19.6 kg/d versus 13.9 kg/d) was associated with lower abundance ofFibrobacter succinogenes(2.13 × 10-3versus 2.25 × 10-4) andSelenomonas ruminantium(2.98 × 10-1versus 4.10 × 10-1). A lower abundance (Day,P < 0.05) was detected on d 20 compared with d -10 forFibrobacter succinogenesandSuccinivibrio dextrinosolvens. The relative abundance ofButyrivibrio proteoclasticusandEubacterium ruminantiumwas stable across treatment and time.
In diets with proper balance of rumen-degradable protein and fermentable carbohydrate, the small fraction of Methionine released from the rumen-protected supplement did not seem to compromise growth of major bacterial species in the rumen. In fact, it had a positive effect on 3 major species prepartum when DMI was similar between groups. Because the actual requirements of Methionine (and Lysine, for example) by the cow during the transition period are unknown, it appears warranted to study the rumen microbiome as it relates to supply of rumen-protected amino acids.