uNDF240 Sets the Fill Limit

A forage report prints NDF as one number, but the rumen treats it as at least two. Part of the cell wall ferments, and part never does. That second part, measured as uNDF240, can only leave by being chewed small and passed, and until then it occupies space. This note is about that fill. Particle size and peNDF are covered in particle size holds the fiber mat and chop length is not peNDF, the left-flank fill picture in the four compartments, and passage in what leaves the rumen.

NDF is the sum of potentially digestible NDF and undigested NDF. To measure the undigested part, the lab ferments a forage sample with rumen fluid for 240 hours in vitro, then reports the fiber left over as uNDF240. Research papers and many lab reports give it ash-corrected (uNDF240om), on the same aNDFom basis as NDF. There is a useful line here: uNDF is a lab measurement at a stated time point, while indigestible NDF (iNDF) is a theoretical pool that could only be reached with infinite time. The 240-hour end point is chosen because the residue changes little with more fermentation. Forage NDF can be described as three pools: a fast-digesting pool, a slow-digesting pool, and the undigested fraction. Earlier data show the undigested fraction is not a fixed coefficient; it varies with plant maturity and growing conditions. That helps explain why two corn silages with similar NDF can behave differently at the bunk.

A 2000 review of intake regulation notes that distension of the reticulo-rumen often limits dry matter intake in high-producing cows and in cows fed high-forage diets. One research group treats the uNDF240 held in the rumen as a baseline of fill, although in its alfalfa hay study intake tracked how fast the digestible fiber fermented more than uNDF240 intake. Sugars, starch and digestible fiber ferment and disappear from the rumen. The undigested fraction does not. It stays until rumination breaks it down far enough to pass, so it builds up as rumen mass. The size of that mass is larger than daily intake suggests. In summarized research trials, the rumen held about 1.6 times the uNDF240 the cow ate in a day across four corn silage–based diets. On one of those diets, cows eating about 2.2 kg of uNDF240 a day carried about 3.5 kg of it in the rumen. The authors noted that the ratio still needs testing with other forages and stages of lactation. Too little can also be a problem. Low-uNDF240 diets are linked with a higher risk of subacute ruminal acidosis, which is why they call for close attention to peNDF.

Reference points, not targets: Intake as % of body weight. In those summarized trials, cows consumed roughly 0.30 to 0.40% of body weight as uNDF240 per day, and the authors put the maximum near 0.40% for those corn silage and haycrop diets. In the same summary, cows in another trial fed finely chopped alfalfa hay reached 0.48%, so the ceiling depends on the forage. Particle size moves the ceiling. In a 2018 study, diets held 8.9% or 11.5% uNDF240 in ration DM, each fed with shorter or longer hay. The high-uNDF240 diet with long hay limited intake. Chopping the hay finer in that diet raised intake by 2.5 kg/day, and uNDF240 intake reached 0.45% of body weight. Dietary uNDF240 was changed by changing forage content, so the two were confounded, as the authors note. Ration level. For corn silage–based diets, the same researchers suggest intake may fall when ration uNDF240 exceeds about 10 to 11% of DM, and that below about 7% the risk of acidosis may rise. These are ration values from one institute's research diets, not forage cut-offs. Corn silage. Extension guidance gives 7 to 11% of DM as typical for corn silage uNDF240 and lists under 11% as optimal. Hybrids in the 2025 Pennsylvania (PDMP) corn silage trial averaged 9.9%. In one study, brown midrib silage ran under 8% and conventional silage about 11%. A corn silage below about 10% beats that trial average, but it is not a pass mark, and values depend on the lab and method.

uNDF240 is a chemistry-and-fill number. peNDF is a physical-form-and-chewing number. Brown midrib silage can be low in uNDF240 and still supply adequate peNDF if it is chopped long enough. Finely chopped hay or straw can be high in uNDF240 but low in peNDF. NDFD30 tells you how much fiber ferments in 30 hours; uNDF240 tells you what is still left after 240 hours. The two matter most together when the ration is highly fermentable. In one study where every diet was low in uNDF240 (about 7% of DM), even moderate rumen-fermentable starch (about 19% of DM) lowered milk fat. The authors advise against chopping low-uNDF240 forage too finely because cows still need physically effective fiber. The acid side of that story is in SARA is not grain overload, and starch access from the kernel is in kernel processing unlocks the starch.