Particle size holds the fiber mat
If the TMR is too fine, the forage raft thins — less trapping, less re-chewing, less bicarbonate-rich saliva.
3 min read · 2026-09-07
The rumen mat is a floating raft of longer forage particles. It traps smaller feed, slows passage, and gives the cow something to re-chew — which pumps bicarbonate-rich saliva back into the vat.
Chemical NDF on the sheet does not guarantee that structure. Over-mixing, an aggressive knife setup or over-processing, a fine chop, or cows sorting against long stems can shrink the raft while the formulation still looks fine.
Measure the ration after mixing and delivery with the Penn State Particle Separator, then shake the refusals the same way. A jump in long particles left behind is sorting against physically effective fiber. Pair the sieve with the barn walk: resting cows chewing, manure that looks loose or grainy, and a bunk that is not just empty.
peNDF is the physical bridge that supports chewing and mat formation; uNDF240 is a different fill constraint from undegradable fiber after long fermentation. NASEM frames related ideas through physically adjusted NDF (paNDF), while Tri-State and Miner work treat peNDF and uNDF240 as linked field measures — useful together, not interchangeable.
Fix the particle the cow actually eats before you rewrite the chemistry.
Sources
- Penn State Particle Separator — Penn State Extension
- From Feed to Milk: Understanding Rumen Function — Penn State Extension
- Nutrient Requirements of Dairy Cattle, 8th revised edition (paNDF framing) — NASEM (2021)
- A Tale of Two Fibers: Physically Effective and Undegradable Neutral Detergent Fiber and Their Interaction with Starch (Grant & Cotanch) — Tri-State Dairy Nutrition Conference / Miner Institute
- Physically Effective Fiber: How Much Is Enough? — William H. Miner Agricultural Research Institute