Glossary · Feed
Physically Effective Fiber (peNDF)
The portion of fiber that is long enough to stimulate chewing and rumen function. Measured with the Penn State Particle Separator.
Physically effective fiber (peNDF) is the portion of fiber in a ration that is long enough to stimulate cud chewing, saliva production, and proper rumen function.
Not all fiber is equal. Fiber that has been chopped too fine slips past the rumen mat and behaves more like grain — fast-fermenting, acid-producing, no chewing required. peNDF specifically measures the long-fiber portion that does the work fiber is supposed to do.
peNDF is measured in the lab with the Penn State Particle Separator (PSPS) — a stack of sieves that sorts the TMR by particle length. Industry guidelines:
- >19 mm screen: 6–10% of TMR for lactating cow rations
- 8–19 mm screen: 30–50%
- Pan (under 4 mm): under 50%
When peNDF runs low, cows develop sub-acute ruminal acidosis (SARA). Symptoms include depressed butterfat, reduced DMI, and irregular cud chewing. The fix is typically forage particle size at chop or hay supplementation.
PSPS data is one of the few quick lab outputs that nutritionists can act on the same day. Most well-managed dairies run PSPS weekly.