Objective:
1) To identify cows that display strong, intermediate, & weak degrees of milk fat depression when switched from a high-fiber to high-starch diet. 2) To determine production response & ruminal chemistry of these cows fed a high-starch diet before & after Rumensin feeding, & after Rumensin withdrawal. 3) To identify (using molecular techniques) the specific uncultured bacterial species whose ruminal populations change in response to Rumensin feeding and withdrawal.

Approach:
We will identify & group dairy cows by degree of milk fat depression on high-starch diets. We will use culture-independent molecular techniques to characterize rumen bacterial community structure in cows on a high-starch diet before & after Rumensin feeding, and after Rumensin withdrawal. Changes in milk production & composition will be associated with changes in specific bacterial subpopulations.