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Forages are the most important ingredient in a dairy cow’s ration. She is a ruminant and to be a healthy, productive cow she needs a healthy, productive rumen. High quality forages are the key to achieving a healthy rumen and a productive cow. However, because of the importance of forages in maintaining rumen health and cow performance, when deficiencies in the amount or quality of the forage in the ration occur, major health problems are the result. There are many possible causes of forage-related health problems in dairy cattle. Attention is often focused on molds, mycotoxins, and high nitrate levels (especially in drought years). While all of these are important, a majority of the forage-related health problems in the dairy cattle that we deal with as veterinarians can be traced to rumen acidosis. Rumen acidosis commonly results from feeding inadequate levels of "effective" fiber and high levels of highly fermentable carbohydrates (grain). The end product of the rumen fermentation of this type of ration is lactic acid, which drops the normal rumen pH from 6-7 to as low as 4. With such decreases in rumen pH many of the normal microorganisms in the rumen are killed and the rumen may become static. The final results are poor digestion of feed, decreased appetite, poor performance, liver damage, and laminitis. Acidosis can be prevented, however, with adequate levels of effective fiber in the ration. Because of the importance of rumen acidosis to the productivity and profitability of dairy cows, this article will concentrate on how the lack of adequate fiber in the rations of dairy cows can result in rumen acidosis. More specifically, this paper will concentrate on how silage chop length, TMR mixing/feeding problems, and the use of low-lignin corn silage can produce a deficiency in effective fiber.