A grain-based subacute ruminal acidosis challenge causes translocation of lipopolysaccharide and triggers inflammation

The effects of a grain-based subacute ruminal acidosis (SARA) challenge on translocation of lipopolysaccharide (LPS) into the peripheral circulation, acute phase proteins in blood and milk, feed intake, milk production and composition, and blood metabolites were determined in 8 lactating Holstein cows. Between wk 1 and 5 of 2 successive 6-wk periods, cows received a total mixed ration ad libitum with a forage to concentrate (F:C) ratio of 50:50. In wk 6 of both periods, the SARA challenge was conducted by replacing 21% of the dry matter of the total mixed ration with pellets containing 50% wheat and 50% barley. Rumen pH was monitored continuously using indwelling pH probes in 4 rumen cannulated cows. Rumen fluid samples were collected 15min before feed delivery and at 2, 4, 6, 12, 14, 16, 18, and 24h after feed delivery for 2 d during wk 5 (control) and wk 6 (SARA). Peripheral blood samples were collected using jugular catheters 15min before feeding and at 6 and 12h after feeding at the same days of the rumen fluid collections. The SARA challenge significantly reduced average daily pH from 6.17 to 5.97 and increased the duration of rumen pH below pH 5.6 from 118 to 279 min/d. The challenge reduced dry matter intake (16.5 vs. 19kg/d), milk yield (28.3 vs. 31.6kg/d), and milk fat (2.93 vs. 3.30%, 0.85 vs. 0.97kg/d), and tended to increase milk protein percentage (3.42 vs. 3.29%), without affecting milk protein yield (1.00 vs. 0.98kg/d). The challenge also increased the concentration of free LPS in rumen fluid from 28,184 to 107,152 endotoxin units (EU)/mL. This was accompanied by an increase in LPS in peripheral blood plasma (0.52 vs.