Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2009
Publication Date: 12/30/2009
Citation: Palmonari, A., Stevenson, D.M., Mertens, D.R., Cruywagen, C.W., Weimer, P.J. 2010. pH Dynamics and Bacterial Community Composition in the Rumen of Lactating Dairy Cows. Journal of Dairy Science. 93:279-287.
Interpretive Summary: In ruminants such as dairy cattle, the rumen contains bacteria and other microbes that help digest feed and provide an excellent source of protein for the animal. The degree of acidity in the rumen is generally regarded as a major factor controlling microbial digestion of feeds, especially of fiber, in cattle. In an effort to learn how acid conditions affect bacterial populations in the rumen, we continuously monitored acid level in the rumen of eight dairy cows as it changed over three feeding cycles; then we compared their rumen bacterial populations. Most cows had similar bacterial profiles despite substantial difference in acid level profiles. Two of the cows had bacterial profiles quite different from the other six, despite having acid level profiles that were intermediate between those of the other cows. These two cows also produced milk of very low fat content; and further analysis of the bacterial populations revealed one bacterial species whose population was elevated and another whose population was depressed relative to those in the other cows. The elevated species has been reported to produce compounds that inhibit milk fat production when grown in laboratory culture, but has not previously been shown to be elevated in fat-depressed cows. The data suggest that controlling populations of these two species in the rumen may provide a means for preventing milk fat depression in dairy cattle (a serious economic problem in the dairy industry), thereby bringing dairy farmers a higher price for their milk and helping dairy processors get more product, such as cheese or butter, from that milk.
Technical Abstract: The influence of pH dynamics on ruminal bacterial community composition was studied in 8 ruminally cannulated Holstein cows fitted with indwelling electrodes that recorded pH at 10-min intervals over a 3-d period. Cows were fed a silage-based TMR supplemented with monensin. Ruminal samples were collected each day just before feeding, and at 3 and 6 h after feeding. Solid and liquid phases were separated at collection, and extracted DNA was subjected to PCR amplification followed by automated ribosomal intergenic spacer analysis (ARISA). Although cows displayed widely different pH profiles (mean pH= 6.11 to 6.51, diurnal pH range = 0.45 to 1.39), correspondence analysis of the ARISA profiles revealed that 6 of the 8 cows showed very similar bacterial community compositions. The 2 cows having substantially different community compositions from the other 6 had intermediate mean pH values (6.30 and 6.33) and intermediate diurnal pH ranges (averaging 0.89 and 0.81 pH units). Fortuitously, these two cows alone also displayed milk fat depression, along with markedly higher ruminal populations of one bacterial operational taxonomic unit (OTU) and reduced populations of two other OTUs. Cloning and sequencing of the elevated OTU revealed phylogenetic similarity to Megasphaera elsdenii, a species known to produce trans-10, cis-12 conjugated linoleic acid (CLA), a known regulator of mammary lipogenesis. The higher populations of both M. elsdenii and OTU246 in these 2 cows were confirmed using real-time PCR with species-specific primers, and the population sizes of these two taxa were highly correlated (r2=0.99, p<0.001). By contrast, the relative population sizes of Streptococcus bovis and genus Ruminococcus, two taxa considered to respond to ruminal pH, did not differ among cows (mean = <0.01% and 10.6%, respectively, of rRNA gene copy number, determined by real time-PCR). The results indicate that cows with widely differing pH profiles can have rather similar ruminal bacterial community compositions, and that milk fat depression can occur at intermediate ruminal pH. The results support recent reports that milk fat depression is associated with shifts in bacterial community composition in rumine, and are specifically related to the relative population size of Megasphaera elsdenii.