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Sub-acute ruminal acidosis (SARA): A tale of two microbiomes
Most mechanistic studies of sub-acute ruminal acidosis (SARA) in cattle involve the experimental induction of SARA by dietary starch or legume. Our aim was to determine how these observations relate to on-farm conditions in NE Scotland. In six beef farms, management practices, feed composition/particle size and animal activity using motion sensor collars were monitored using 20 animals per farm. At slaughter, rumen wall condition was assessed under four categories: color, papillae integrity, papillae shape, and post-washing blackness. Ruminal fluid and caecum contents were collected for VFA and soluble lipopolysaccharide (LPS) analysis, and also for microbial community analysis. 86 ss rRNA amplicon libraries were generated by PCR, which were subsequently sequenced in equimolar concentrations using Illumina MiSeq.
Close examination of the feed indicated that the process of mixing grain with forage was critical. Rumen wall damage did not appear to be correlated with the particle size of the total mixed ration, but rather with the dustiness of the barley component of the feed. Motion sensor data showed that the rate of change of movement appeared to be correlated with the condition of the rumen wall across farms. Ruminal LPS concentrations were significantly different (P<0.001) between farms, and caecal LPS concentrations were significantly higher (up to 27-fold) than corresponding ruminal LPS concentrations, and exhibited a stronger relationship with the rumen damage scores. We therefore postulate that the hindgut has a greater effect on ruminal health than previously thought.
Microbial community and subsequent PCA analysis revealed significant (P <0.001) clustering of ruminal fluid and caecal content communities. Within each sample type, there was also significant clustering by farm; however, the clustering was weaker when grouped by damage score. Bacteroidetes, Firmicutes and Proteobacteria were the predominant phyla in the ruminal digesta. In the caecal content, Proteobacteria were barely detected and comprised mainly the Moraxellaceae family (known pathogens, “pink eye” in cattle). This was surprising as LPS from Enterobacteriaceae has been implicated with SARA. It is possible that growth of Proteobacteria is not supported in the caecum and the bacteria therefore lyse, causing the high LPS concentrations.
In conclusion, the particle size of the barley component of the feed appears to have a relationship with SARA related pathology. Furthermore, this study has highlighted the role of the hindgut in the pathology of SARA, which warrants further investigation.
Keywords: caecum, rumen, sub-acute ruminal acidosis (SARA)