Effects of ruminal dosing of Holstein cows with Megasphaera elsdenii on milk fat production, ruminal chemistry, and bacterial strain persistence1

Megasphaera elsdenii is a lactate-utilizing bacterium whose ruminal abundance has been shown to be greatly elevated during milk fat depression (MFD). To further examine this association, a total of 23 cannulated multiparous Holstein cows were examined in 3 experiments in which strains of M. elsdenii were directly dosed into the rumen (~2×1012 cells/dose); control cows were dosed with sterile lactate-free culture medium. Cows were fed a total mixed ration (292g of starch/kg of dry matter) that contained primarily corn silage, alfalfa silage, finely ground high-moisture corn, supplemental protein, and corn oil (3g/kg of dry matter). Experiments differed in stage of lactation of the cows (early or late), dosing events (single dose, or 4 doses over a 5-d period), timing of dose (prefeed or 4h postfeed), and M. elsdenii strain (laboratory strain YI9 or 3 strains isolated from cows in the same herd). Dry matter intake and milk yield and composition were measured from 5 to 0d before dosing and 1 to 7d after first dosing, plus later time points that varied by experiment. Milk yield and composition were not affected by dosing. Megasphaera elsdenii was quantified in the liquid phase of ruminal contents by automated ribosomal intergenic spacer analysis, or by PCR with relative quantification (M. elsdenii 16S rRNA gene copy number as a percentage of total bacterial 16S rRNA gene copies). Neither the M. elsdenii-dosed or control cows displayed MFD after dosing, and in almost all cases M. elsdenii populations returned to low baseline levels (<0.02% of 16S rRNA gene copy number) within 24h of dosing. This rapid decline in M. elsdenii also occurred in several cows that were dosed with a strain of M. elsdenii that had been isolated from that particular cow during a previous bout of MFD. Ruminal pH and total millimolar volatile fatty acids and lactate did not differ between dosed and control cows, although acetate-to-propionate ratio declined in both groups and butyrate increased after dosing with M. elsdenii. The results confirm that establishing exogenously added bacterial strains in the rumen is difficult, even for strains previously isolated from the recipient cow. The potential role of M. elsdenii as an agent of MFD remains unclear in the absence of successful establishment of the dosed strains.