Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 19, 2009
Publication Date: December 30, 2009
Citation: Weimer, P.J., Stevenson, D.M., Mertens, D.R. 2010. Shifts in Bacterial Community Composition in the Rumen of Lactating Dairy Cows Under Conditions of Milk Fat Depression. Journal of Dairy Science. 93:265-278. Interpretive Summary: Milk fat depression (MFD), a reduction in the fat content of milk, is a serious economic problem in the dairy industry. It often results from feeding diets high in starch or in certain feed additives. In an effort to see if there is a relationship between MFD and the population of bacteria in a cow’s rumen*, we subjected 18 lactating dairy cows to diets that can induce MFD. We then selected four pairs of cows that displayed diet-dependent milk fat responses for examination of their rumen bacterial profiles. Cows that displayed MFD showed diet-dependent shifts in bacterial population profiles that differed from nonresponsive (non-MFD) cows. In particular, the population of one species (known for its ability to produce chemicals in laboratory culture that inhibit milk fat synthesis) was elevated 20- to 50-fold in MFD cows upon induction of MFD; but it was barely detectable in these cows prior to induction or in non-MFD cows. Controlling populations of this species in the rumen may provide an opportunity to reduce milk fat depression in dairy cattle, 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. * (In ruminants such as dairy cattle, the rumen contains bacteria and other microbes that help digest feed and that provide an excellent source of protein for the animal.)
Technical Abstract: Eighteen ruminally cannulated dairy cattle were fed a series of diets (in 28-d periods) designed to elicit different degrees of milk fat depression (MFD) for the purpose of relating MFD to ruminal bacterial populations. Cows were fed a TMR containing 25% starch (DM basis), supplied as corn silage, a slowly fermented starch (SFS treatment, period 1), then switched to a TMR containing 27% starch, much of it supplied as ground high-moisture corn, a rapidly fermented starch (RFS treatment, period 2). In period 3, the RFS diet was amended with 13.6 mg monensin (Mon)/kg DM (RFS/Mon treatment), and in period 4, the cows were returned to the RFS diet without monensin (RFS/Post treatment). Effect of both starch source and monensin on milk fat percentage varied by cow, and cluster analysis identified 4 pairs of cows having distinct milk fat patterns: S-responsive cows displayed MFD on starch, but no additional MFD upon monensin feeding and a fat rebound upon monensin withdrawal. M-responsive cows showed no MFD upon switch from the SFS to the RFS diet, but displayed strong MFD upon monensin feeding, and no recovery after monensin withdrawal. SM-responsive cows displayed reduction of milk fat on both RFS and RFS/Mon diets, and fat returned to the levels in the RFS diet upon monensin withdrawal. Non-responsive cows did not display changes in milk fat percentage with dietary treatment. Archived (-80 degrees C) ruminal liquor and solids from the 4 pairs were processed to isolate bacterial DNA, which was subjected to ARISA (automated ribosomal intergenic spacer analysis), followed by correspondence analysis to visualize bacterial community composition (BCC). S-responsive cows displayed major shifts upon a switch from SFS to RFS diets, but little change upon monensin feeding and withdrawal. M-responsive cows displayed major shifts in BCC upon monensin feeding, and continued shifts upon monensin withdrawal. SM-responsive cows showed a more gradual shift in BCC in response to both starch source and monensin. Non-responsive cows showed only minor shifts in BCC with dietary treatment. Several of the “operational taxonomic units” (OTU) corresponding (with some qualification) to different bacterial species in the ARISA data matrix showed marked differences between MFD-susceptible cows (groups S, M and SM) and milk-fat non-responsive cows (group N). Overall, this study is the first to associate MFD with shifts of microbial communities in the rumen.