What does the cows stomach bacteria do?

There are several types of bacteria in the rumen of a bovine that have different jobs to do. But basically, their main job is to break down the fibrous material found in the coarse plant material that a cow so commonly eats so that she can obtain energy, protein, VFAs (volatile fatty acids), vitamins and minerals to feed the very large population of microflora in the rumen and reticulum, which in turn feed her.