What is the primary energy source produced by rumen microbes and absorbed in a cow's bloodstream?

• A. Glucose.
• B. Volatile fatty acids: acetate, propionate, and butyrate.
• C. Amino acids.
• D. Fatty acids from dietary fat.

Answer: (B)

Volatile fatty acids produced by rumen microbes supply most of a cow’s daily energy. In the rumen, microbes ferment carbohydrates from feed and yield acetate, propionate, and butyrate. These small molecules are absorbed directly through the rumen wall into the bloodstream and become the primary energy source for the animal. Propionate is the main gluconeogenic precursor, supplying liver-made glucose, while acetate and butyrate are used for energy production and other metabolic needs. Glucose itself isn’t directly absorbed from the rumen; it’s largely produced in the liver from propionate. Amino acids come from microbial and dietary protein, not the primary energy currency, and fatty acids from dietary fat are absorbed later in the digestive tract rather than being the main energy output of rumen fermentation.