Invited review: Effect of subacute ruminal acidosis on gut health of dairy cows.

Subacute ruminal acidosis (SARA) is considered a common disease in high-yielding dairy cows. Despite this, the epidemiological evidence is limited by the lack of survey data. The prevalence of SARA has mainly been determined by measuring the pH of the ruminal fluid. This may not be adequate, because the symptoms of SARA are not solely due to ruminal pH depression, and ruminal pH in addition varies between sites in the rumen, throughout a 24-hour period, and between days.

The impact of SARA has mainly been studied by conducting SARA challenges in cows, sheep, and goats based on a combination of feed restriction and high-grain feeding. The methodologies applied in these studies varied considerably. Variations include differences in the duration and amount of grain feeding, the type of grain, the amount and duration of the feed restriction, the number of experimental cows, and the sensitivity of the cows themselves to the SARA challenges. As a result, the authors cited reviewed the current knowledge to enable distinguishing factors and the effects of grain feeding vs high quality pasture effects in the prevalence of SARA. The review revealed the following.

Grain-based SARA challenges affect gut health as expected. The effects include depressing the pH in, and increasing the toxin content of the digesta. They also include altering the taxonomic composition of the rumen microbiota, reducing the functionality of the epithelia throughout the gastrointestinal tract, and having a moderate inflammatory response. The effects on the epithelia include a reduction in its barrier function, whereas the effects on the microbiota include reductions in their richness and diversity. This may reduce their functionality and reflect dysbiosis. Changes in the taxonomic composition of gut microbiota throughout the gastrointestinal tract are evident at the phylum level, but is less evident and more variable at the genus level. Effects at the phylum level include an increase in the Firmicutes to Bacteroidetes ratio. More studies on the effects of a SARA challenge on the functionality of gut microbiota are however needed.

The inflammatory response resulting from grain-based SARA challenges is innate and moderate and mainly consists of an acute phase response. This response is likely a combination of systemic inflammation and inflammation of the epithelia of the gastrointestinal tract. The systemic inflammation is assumed to be caused by translocation of immunogenic compounds, including bacterial endotoxins and bio-amines, through the epithelia into the interior circulation. This translocation is increased by the increase in concentrations of toxins in the digesta and a reduction of the barrier function of epithelia. More severe SARA can cause rumenitis, but moderate SARA may activate an immune response in the epithelia of the gastrointestinal tract.

Cows grazing highly fermentable pastures with high sugar contents can also have a low ruminal pH indicative of SARA. This is however not accompanied by an inflammatory response but may affect milk production and the gut microbiota. Grain-based SARA in contrast affects several aspects of gut health, but SARA resulting from grazing high-digestible pastures and insufficient coarse fibre less so. Therefore, it needs to be determined which system for inducing SARA is the most representative of particular on-farm conditions.