Altering the ruminal microbiome using rumen contents dosing in dairy calves does not influence downstream milk production efficiency

Improving milk production efficiency (MPE) by modulating the rumen microbial community is an important area of active research. Rumen bacterial community composition (BCC) has been implicated in MPE, but the adult rumen microbiota is highly resistant to perturbation. We sought to influence the establishment of the ruminal BCC by early intervention in pre-weaning dairy calves. Two cannulated lactating Holstein cows historically disparate in MPE were selected as donors. Three cohorts of 20 heifer calves each were dosed by oral gavage with rumen inocula sourced from the high-efficiency donor (HE), the low-efficiency donor (LE), or an autoclaved 50:50 mixture from the two donors that served as a microbe-free control (C). Dosings were performed at birth and biweekly through 8 weeks of age. Cows were raised according to standard practices on the farm through the first lactation. Milk production and feed intake were assessed over three 7-day windows in early, middle, and late lactation. A subset of these cows were ruminally cannulated before parturition to permit ruminal bacterial community profiling (HE n = 9, LE n = 9, C n = 6). Samples of solid and liquid-phase rumen contents were collected alongside milk and feed sampling for assessment of BCC by 16S rRNA amplicon sequencing. Inocula derived from the two donors differed in BCC throughout the inoculation period (P < 0.001), and during lactation, ruminal BCC in treatment cows differed by dosing cohort (P < 0.001). However, this difference in BCC did not result in differences in energy corrected milk production, feed intake, feed efficiency, yield of milk components, or rumen VFA profile in treated cows (P > 0.05). We conclude that while biweekly preweaning rumen contents dosing may influence bacterial community development, it is not sufficient to drive downstream MPE in recipients.