The top sieve is not the goal
Long stems look like fiber and often become refusals.
Knowledge level in this topic: Intermediate. Barn-side working knowledge in this topic: checks, pairing, and what to do next.
3 min read · 2026-09-07
In this note
- 2–8%
- 19 mm
- 8 mm
- 4 mm
The Penn State Particle Separator top sieve catches the longest stems. On a high-producing TMR, that 19 mm fraction is meant to stay small — typically about 2–8%.
Broomstick stems look like fiber on the clipboard and get sorted out of the bunk.
Most of the peNDF mass that supports chewing lives on the 8 mm and 4 mm sieves together — particles long enough to help hold a mat without inviting refusals.
The 4 mm screen is the peNDF cutoff used to estimate physically effective fiber; it is not the main scratch layer by itself.
The 2013 update that added the 4 mm sieve changed how peNDF is estimated on farm; the companion guideline is an eatable-length TMR — enough length to chew, not so much that cows refuse it.
If the top sieve runs high and the refusals are stem-heavy, you built a sorting diet, not a safer rumen.
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Overmixing Steals the Mat
Your ration might look perfect on paper, but if the mixer grinds your forage too fine, you lose the physically effective fiber required to build a healthy rumen mat.
Knowledge level in this topic: Intermediate. Barn-side working knowledge in this topic: checks, pairing, and what to do next.Sister note · 3 min
Home plate: Rumen · Physically effective fiber · Particle size
- Particle sizeWhat actually stimulatesPlaced here
- Water is a fiber plasticizerForage dry matter changes how the TMR mixes and how the particle box reads — even when chop length matches a drier day.Same focus
- Chop length is not peNDFChop length is a harvester setting; peNDF is the fiber structure that actually reaches the rumen after moisture, processing, mixing, and sorting.Same sheet
- Heat stressHeat load is temperature plus humidity, not the office thermostat. Mild heat stress starts near THI 68: shade, air, then water.Bridge → Barn
Mark size is the kind. Color is the atlas area.
- Sheet
- Focus
- Field note
- Rumen
- Barn
Sources
- Penn State Particle Separator — Penn State Extension
- Methods Used to Measure Forage and Ration Particle Size — Penn State Extension