Ruminal acidosis is a persisting digestive disorder in modern ruminant production; however, the susceptibility of cows to ruminal acidosis differs among cows fed a common diet. The overall objective of this research was to evaluate factors affecting the susceptibility of cows to ruminal acidosis. This research demonstrated that feeding sucrose in replacement for corn grain to Holstein cows in early lactation does not increase the risk for ruminal acidosis and may actually increase ruminal pH. However, regardless of dietary treatment, cows in early lactation were at risk for ruminal acidosis. In Study 2, diets differing in the forage-to-concentrate ratio were fed to Holstein cows to evaluate changes in the in vivo rate of short-chain fatty acid absorption (SCFA) and the expression of genes coding for transporters and enzymes involved in the absorption and metabolism of SCFA in ruminal tissue. Contrary to the hypothesis, the fractional rate of absorption and expression of genes involved in SCFA absorption and metabolism were not affected by the forage-to-concentrate ratio. Considerable variation among individual cows for the severity of ruminal acidosis was detected for cows on the diet containing the low forage-to-concentrate ratio. To determine the cause of this variation, a ruminal pH measurement system was developed to accurately and precisely measure ruminal pH in non-cannulated small ruminants. Sheep were then subjected to a ruminal acidosis challenge model in vivo, and the absorption of acetate and butyrate across the isolated ruminal epithelia was measured in vitro in Ussing chambers. The results of this study demonstrated that differences in the severity of ruminal pH depression among animals could largely iv be accounted for by differences in the absorptive capability of the ruminal epithelium. In summary, although ruminal acidosis is a common digestive disorder in dairy production systems, variation in the susceptibility to ruminal acidosis is common. The cause for much of this variation is due to differences in the absorptive capacity of the ruminal epithelia. / Animal Science

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