Subacute ruminal acidosis and total mixed ration preference in lactating dairy cows1

Subacute ruminal acidosis (SARA) is a condition where the pH of the rumen becomes abnormally acidic because of increased and altered production of volatile fatty acids. The objective of this experiment was to determine how a SARA challenge affects total mixed ration selection in dairy cows. In this study, 8 multiparous, lactating, ruminally cannulated Holstein cows were given a choice between a long-forage-particle-size diet with slow-fermenting starch (LC) and a short-forage-particle-size diet with fast-fermenting starch in a crossover design. Cows were allowed to adapt to this feeding scheme and were then subjected to a rumen challenge to induce a bout of SARA. The rumen challenge successfully decreased rumen pH and altered rumen volatile fatty acid profiles. Daily average rumen pH decreased from 6.02 to 5.77, and average minimum rumen pH decreased from 5.59 to 5.28. In addition, following the rumen challenge, concentrations of acetate, butyrate, and valerate, and acetate-to-propionate ratio increased. In response to the rumen challenge, intake of LC increased from the baseline level of 18.1% of total daily dry matter intake to 38.3% for that day. During the first recovery day after the rumen challenge, LC intake moderated to 28.0% of total daily dry matter intake. On the second recovery day, LC intake returned to baseline levels at 18.6%. These results indicate that cows are able to alter their diet preference for higher physically effective fiber and slower starch fermentability during a bout of SARA and that they can effectively fully recover from this type of SARA within 72 h when appropriate diets are available.