Field note · 4 min · 2026-09-07
Particle size holds the fiber mat
The mat is a physical structure. If the TMR is too fine, microbes lose their raft and saliva loses its reason to flow.
Physically effective fiber is the fraction of the diet that makes a cow chew and that can float as a mat. Penn State’s work on rumen function treats that mat as the reason long forage is not optional on a high-energy diet.
A mixer wagon can destroy peNDF after the nutritionist left the farm. Over-mixing, a worn knife, or wet fine forage can print the same diet on paper and deliver a slurry. Cows then sort for grain, eat faster, and ferment harder.
Check the bunk face and the refusals, not only the Penn State Particle Separator card in the office. If the refusals are all long stems and the eaten diet is fines, the rumen never saw the ration you formulated.
The ration the cow eats is the ration after mixing, sorting, and delivery — not the sheet.
Sources
- From Feed to Milk: Understanding Rumen Function — Penn State Extension
- Nutrient Requirements of Dairy Cattle, 8th revised edition — National Academies (NASEM, 2021)