High grain diets primarily increase the proportion of which volatile fatty acid in the rumen?

High grain diets lead to an increase in the proportion of propionic acid in the rumen. This happens because grains are high in fermentable carbohydrates, which are readily metabolized by rumen microbes. When these microbes ferment carbohydrates, they produce volatile fatty acids (VFAs) as their primary energy source.

Propionic acid is particularly favored in high grain diets because the increased carbohydrate availability stimulates the production of microbes that primarily produce this specific VFA during fermentation. Propionic acid serves as a substrate for gluconeogenesis in ruminants, playing a crucial role in energy metabolism and influencing milk production in lactating animals.

In contrast, a diet that is higher in forage tends to promote the production of acetic acid and butyric acid. While butyric and acetic acids are also important, they are produced in larger quantities from the fermentation of fibrous materials rather than grains. This differentiation in VFA profiles based on diet composition is essential for understanding ruminant nutrition and metabolism.