Ruminal protozoal populations of Angus steers differing in feed efficiency

Authors: CLEMMONS, BROOKE - University Of Tennessee; Shin, Sung; Smith, Timothy - Tim; EMBREE, MALLORY - Native Microbials; VOY, BRYNN - University Of Tennessee; SCHNEIDER, LIESEL - University Of Tennessee; DONOHOE, DALLAS - University Of Tennessee; MCLEAN, KYLE - University Of Tennessee; MYER, PHILLIP - University Of Tennessee

Submitted to: Animals
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2021
Publication Date: 5/27/2021
Citation: Clemmons, B.A., Shin, S.B., Smith, T.P.L., Embree, M.M., Voy, B.H., Schneider, L.G., Donohoe, D.R., McLean, K.J., Myer, P. 2021. Ruminal protozoal populations of Angus steers differing in feed efficiency. Animals. 11(6). Article 1561. https://doi.org/10.3390/ani11061561.
DOI: https://doi.org/10.3390/ani11061561

Interpretive Summary: The rumen of cattle are specialized to break down the components of plants into digestible materials for the nutrition of the animal. A key aspect of rumen function is the contribution of the microbial component which acts to perform many of the degradation steps to provide the animal with a variety of nutrients. Numerous studies have examined the bacterial species present in rumen and identified associations of microbial community composition with the efficiency of converting feed to growth in the animal. However, another non-bacterial type of organism called protists make up as much as 50% of the total microbial mass of the rumen, but are not characterized by the methods used for bacteria. The present study examines the protists, or "protozoal" content of the rumen in animals that have been characterized for feed efficiency and other measures of rumen function. The study found that there were specific protozoal species associated with higher feed efficiency, but that those species have not previously been identified, indicating a need for further experiments to characterize these potentially important microbes.

Technical Abstract: Feed accounts for as much as 70% of beef production costs, and improvement of the efficiency with which animals convert feed to product has the potential to have substantial financial impact on the beef industry. The rumen microbiome plays a key role in determining feed efficiency; however, previous studies of rumen microbiota have not focused on protozoal communities despite the estimation that these organisms represent approximately 50% of rumen content biomass. Protozoal communities participate in the regulation of bacterial populations and nitrogen cycling—key aspects of microbiome dynamics. The present study focused on identifying potential associations of protozoal community profiles with feed efficiency. Weaned steers (n = 50) 7 months of age weighing approximately 260 kg were adapted to a growing ration and GrowSafe for 2 weeks prior to a 70-day feed efficiency trial. The GrowSafe system is a feeding system that monitors feed intake in real time. Body weights were collected on the first day and then every 7 days of the feed efficiency trial, and on the final day, approximately 50 mL of rumen content were collected via orogastric tubing and frozen at -80 °C. Body weight and feed intake were used to calculate residual feed intake (RFI) as a measure of feed efficiency, and steers were categorized as high (n = 14) or low (n = 10) RFI based on ±0.5 standard deviations about the mean RFI. Microbial DNA was extracted, and the eukaryotic component profiled by amplification and sequencing of 18S genes using degenerate primers that can amplify this locus across a range of protists. The taxonomy of protozoal sequences was assigned using QIIME 1.9 and analyzed using QIIME and SAS 9.4 with significance determined at a = 0.05. Greater abundances of unassigned taxa were associated with high-RFI steers (p = 0.03), indicating a need for further study to identify component protozoal species. Differences were observed between low- and high-RFI steers in protozoal community phylogenetic diversity, including weighted beta-diversity (p = 0.04), Faith’s phylogenetic diversity (p = 0.03), and observed Operational taxonomic unit (OTU) (p = 0.03). The unassigned taxa and differences in phylogenetic diversity of protozoal communities may contribute to divergences observed in feed efficiency phenotypes in beef steers.