Subacute rumen acidosis (SARA) is frequently encountered in ruminants on high-concentrate rations and characterized by mild to moderate rumen pH depression. Although the measurement of rumen pH is considered as a gold standard approach to diagnose SARA, rumen fluid collection is conceived as a cumbersome and invasive procedure. In the present study, the suitability of transabdominal ultrasonography to identify structural changes of the rumen mucosa associated with SARA was explored. Five adult rumen canulated bulls previously adjusted to a roughage-based ration were switched in 10-day intervals to rations with increasing concentrate content. Seven rations with a concentrate content ranging from 5% to 95% were fed. The rumen pH and rumen mucosal thickness were measured at the end of each 10-d feeding period. The left flank was divided into 12 acoustic windows and the mean rumen mucosal thickness for each acoustic window determined for each ration. Repeated measure ANOVA and stepwise regression analyses were conducted. Increasing the dietary concentrate content resulted in thickening of rumen mucosa which was most pronounced in the upper part of the ventral rumen sac and the least pronounced in the uppermost and lowermost parts of the rumen. The intersection of a horizontal line going through the costochondral junction and a vertical line coming from the third lumbar vertebra was found to be most suitable to identify animals with rumen pH < 5.5 at 4 h post feeding. We conclude that transabdominal ultrasonography of the rumen mucosa has the potential to be a suitable, non-invasive diagnostic tool to identify adult ruminants with SARA.

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