The source of carbohydrates in the diet influences the amount and ratio of VFA produced in the rumen. The microbial population converts the fermented carbohydrates to about 65% acetic acid, 20% propionic acid and 15% butyric acid when rations contain a large proportion of forages. In this case, the supply of acetate may be adequate to maximize milk fat production, but the amount of propionate produced in the rumen may limit the amount of milk produced because of limited supply of glucose (especially in early lactation).

The non-fibrous carbohydrates (present in many concentrates) promote the production of propionic acid whereas the fibrous carbohydrates (present primarily in forages) stimulate the production of acetic acid in the rumen. In addition, the nonfibrous carbohydrates yield more VFA (i.e.,more energy) because they are fermented faster and more completely.

Thus, feeding concentrates usually result in an increased VFA production and an increased percentage of propionate at the expense of acetate (Figure 2). When large amounts of concentrates are fed (or when forages are finely ground), the percentage of acetic acid may drop below 40%, while the percentage of propionic acid may increase above 40%. Milk production may be increased because of the increased supply of glucose coming from propionate, but acetic acid for fat synthesis may be in short supply. In general, this shortage of acetic acid is associated