1a.Objectives (from AD-416)
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. Extension of time is needed to complete the ARISA profiling of the bacterial populations in some of the rumen samples, and to prepare the final report. Additional funds will be used to permit PCR analysis of samples, and do more extensive statistical analysis of data.

1b.Approach (from AD-416)
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.

3.Progress Report
Monitoring the trust agreement was accomplished through periodic e-mail and telephone conversations with a Cooperator’s scientist who collaborated in the work. Eighteen lactating dairy cows were fed a high-starch diet over four periods of 28 d each. During period 1, starch was supplied in the form of corn silage, and some as dry corn. In period 2, starch was provided as both corn silage and ground, high-moisture corn, which ferments more rapidly than other forms of corn. In period 3, the same diet was fed, but with the addition of monensin (13.6 mg/kg dry matter). In period 4, the cows were returned to the same diet without monensin. Milk samples (AM and PM) over the last five days of each period were analyzed for composition, and rumen samples (liquid and solid phases) were collected six hours after feeding on each of the last three days of each period. Rumen liquid samples were analyzed for fermentation products. Solid and liquid samples were analyzed for bacterial populations using Automated Ribosomal Intergenic Spacer Analysis (ARISA), and correspondence analysis was performed to examine bacterial community composition (BCC). Cluster analysis allowed grouping of eight of the cows into four pairs, based on distinctive milk fat responses to the different diets. Monensin (M)-responsive cows displayed milk fat depression on monensin that was not alleviated by monensin withdrawal; these cows also displayed dramatic shifts in BCC upon monensin feeding, and continued to shift even after monensin removal. Starch (S)-responsive cows displayed milk fat depression on starch that did not change upon monensin addition and withdrawal; these cows showed dramatic BCC shifts when switched to the more rapidly fermentable starch, but displayed little shift in response to monensin feeding or withdrawal. Cows that displayed milk fat depression on both treatments (starch-monensin [SM]-responsive) also displayed shifts in BCC, while cows that resisted milk fat depression did not show BCC shifts with either the more rapidly fermented starch or monensin. Of the 203 bacterial taxonomic units detected over the course of the study, 32 displayed significant changes in relative population sizes (either increase or decrease) in the responsive cows relative to the non-responsive cows. One bacterial species displayed relative population sizes of 19- to 46-fold higher in responsive cows (i.e., those susceptible to milk fat depression) than in non-responsive cows. These data provide the first definitive evidence that diet-induced milk fat depression is directly associated with changes in BCC.