Metaphylaxis or treating the entire population of cattle at arrival with an antimicrobial has been studied extensively in the cattle industry; however, little information is available on the impacts of treating only a proportion of the population with antimicrobials at arrival. The study objective was to determine potential associations between the proportion of animals in a pen treated with antimicrobial therapy with pen performance and nasopharyngeal microbiome. Yearling steers (n = 160) were randomly allocated to study pens (n = 40) and pens were systematically randomized to one of two antimicrobial treatments (META: all four head received tulathromycin; MIXED: two of four head randomly selected to receive tulathromycin). The study was conducted in conjunction with an essential oil feeding trial. Deep nasal pharyngeal (DNP) swabs were collected from every steer at Days 0, 14, 28, and 56. All DNP swabs were individually cultured for Pasteurella multocida and Mannheimia haemolytica. Samples of DNA were extracted from DNP swabs, pooled by pen, and analyzed by metagenomic shotgun sequencing to compare nasopharyngeal microbiome composition and quantity of resistance genes between test groups. Neither antimicrobial nor essential oil treatment groups had any significant associations with performance or DNP microbiome. Sampling day was significantly associated with alpha and beta diversity at the species level. Shannon's diversity and Inverse Simpson diversity were significantly lower on Day 14 versus both Day 0 and Day 56. These data indicated a shift in microbial populations across study days; however, the microbiome diversity and relative abundance were not significantly different between antimicrobial treatment groups.
Keywords: bovine respiratory disease; growth performance; nasal microbiota; tulathromycin.