Abstract
Simple SummaryThe rumen protozoa have been demonstrated to enhance methanogenesis and impact intraruminal recycling of microbial protein. However, they are also known to contribute to fiber degradation and the stabilization of ruminal pH changes. The apparent metabolic impact of ciliated protozoa in the rumen may contribute to the variation in feed efficiency. This study examined the relationship between the rumen protozoa and feed efficiency in beef steers. We monitored feed intake and body weight for a 70-day feed efficiency trial. Following the trial, rumen content was collected, protozoal DNA was extracted from the content, and the relationship between protozoa community diversity and species abundance with feed efficiency was examined. The ciliated protozoal community diversity differed between low- and high-feed efficient steers. Greater abundances of unidentified protozoa genera were detected in the low-feed efficient steers. These data suggest that unidentified protozoa and ciliated protozoal community diversity influence feed efficiency in beef steers. 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 α ≤ 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.
Highlights
The rumen microbiome is key to effective feed degradation and utilization by the host animal, affecting its nutritional status
At the end of the feeding trial, the steers were ranked by residual feed intake (RFI; the difference between actual dry matter intake (DMI) and expected DMI) as a measure of feed efficiency, based on performance and feed intake measured from day 0 to day 70 [13]
The rumen microbiome contributes to the variation in feed efficiency phenotypes [3]; much of the research examining feed-efficiency-associated rumen microbial communities is related to bacteria and archaea, likely due to their functional and genetic diversity, high abundance, and/or contribution to greenhouse gas production
Summary
The rumen microbiome is key to effective feed degradation and utilization by the host animal, affecting its nutritional status. Ruminal bacteria provide many critical functions in support of ruminant nutrition and health, such as proteolytic and fibrolytic activities and vitamin synthesis Due to these important roles, rumen microbiome research has historically focused on bacterial communities heavily [1,2,3]. 65% of the dietary N is converted to microbial protein, providing high-quality protein to the host [6] This breakdown of microbial protein results in the production of oligopeptides and free amino acids, which are rapidly fermented to short-chain fatty acids and ammonia, resulting in the intraruminal nitrogen cycling commonly known to cause low nitrogen use efficiency in ruminants [4,5,6]. This starch hoarding by rumen protozoa has been hypothesized to help stabilize the rumen ecosystem and improve its resilience to pH fluctuations
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