Characteristics of dairy cows with a greater or lower risk of subacute ruminal acidosis: Volatile fatty acid absorption, rumen digestion, and expression of genes in rumen epithelial cells.

The objective of this study was to examine whether lactating dairy cows with a greater or lower risk of subacute ruminal acidosis (SARA) have differences in volatile fatty acid (VFA) absorption rate, expression of genes involved in VFA metabolism and intracellular pH regulation in rumen epithelial cells, and in situ carbohydrate digestibility in the rumen. We fed 14 ruminally cannulated mid-lactating dairy cows (119±47.2d in milk; body weight 640±47.9kg) a high-grain diet consisting of 30% forage ad libitum, with an 18-d diet adaptation and a 7-d sample and data collection period. Eight cows with the lowest acidosis index [area below pH 5.8 normalized for dry matter intake (DMI); 0.10±0.16 pH × min/kg of DMI] and 5 with the highest acidosis index (3.72±0.19 pH × min/kg of DMI) were classified as animals with lower risk (LS) and higher risk (HS) of SARA, respectively. Minimum (5.75 vs. 5.33) and mean rumen pH (6.33 vs. 5.98) were higher for LS than for HS cows. In addition, the duration and area of rumen pH below 5.8 was lower in LS cows (24.9 vs. 481min/d; 2.94 vs. 102 pH × min/d). Although DMI, milk yield, and milk component yields did not differ, milk fat concentration tended to be higher for LS cows than for HS cows (3.36 vs. 2.93%). However, we observed no difference in VFA absorption rate between LS and HS cows. In situ starch and neutral detergent fiber digestibility were not different between LS and HS cows, but the relative mRNA abundance of lanosterol synthase (LSS) was higher for LS cows than for HS cows. In addition, the mRNA abundance of hydroxy-3-methylglutaryl-CoA synthase 1 (HMGCS1) tended to be higher for LS cows than for HS cows. These results suggested that VFA absorption rate might not explain the difference in rumen pH between LS and HS cows in the current study, even though expression of some genes related to VFA metabolism in rumen epithelium may be associated with variation in the risk of SARA among lactating cows. This variation in the risk of SARA may not be attributed to differences in the capacity of rumen microbes to ferment carbohydrates, because in situ carbohydrate digestibility in the rumen was not different between cows with higher and lower risk of SARA.