Water is a fiber plasticizer

When we talk peNDF we often fixate on chop length. Moisture changes what happens next. peNDF is still estimated from particle size × NDF on the diet the cow ate — usually particles retained on the ≥4 mm Penn State screens times NDF. Dry matter does not rewrite that definition, but it changes mixing behavior and how the separator reads an as-fed sample.

Wet forage tends to tear and smear under augers more readily than a drier crop at the same theoretical chop. What looked adequate at the cutterhead can arrive at the bunk finer after over-aggressive mix time. A PSPS shake on fresh TMR is the check: if forage DM drops, re-shake after mixing, not only at harvest. Compare as-fed and refusals the same way.

Moisture can also change bunk behavior. Wetter mixes may hold fines better against sorting; overly dry fluffy long hay can float and invite selection. Those are operational delivery claims — watch the bunk face and the refusal map — not a claim that water "collapses the mat" by plasticizing cell walls into mush.

When forage dry matter drops, re-check particle size after mixing, watch bunk sorting and rumination, and treat mix aggressiveness as a peNDF lever before rewriting fiber on paper.