Forage passage rates: What we know – Pat Hoffman, Vita Plus

By Pat Hoffman, VIta Plus dairy technical specialist

Have you heard – or perhaps even said – something along these lines when walking cows?  Anyone who has worked with lactating dairy cow diets has likely made some accusatory or ancillary comment about the passage rate of the diet or forages in the diet.  Why?  Maybe it’s a great crutch to explain why cow performance is not meeting expectations.  Perhaps stories of forage passage rate implications have become so engrained in our conversations we simply would be remiss not to mention the effects – even if we don’t exactly know what the effects are.

So, what do we really know about forage passage rates and how can understanding forage passage rates help us formulate better diets for our lactating dairy cows?  Thanks to some recent academic research, our understanding of forage passage rates is improving to the point we might consider the effects in actual ration formulation.

uNDF passes faster
This statement is true – undigested neutral detergent fiber (uNDF) passes faster than potentially digestible (pd) NDF.  While that may hurt our heads a bit, research has clearly demonstrated that uNDF passes out of the rumen faster than pdNDF.  In a recent review of 19 studies, researchers at the University of Wisconsin-Madison observed the average passage rate of uNDF was approximately 3.0 percent per hour, while the passage rate of pdNDF was 2.4 percent per hour.  The differences in passage rates between uNDF and pdNDF are subtle, but they have important implications.

Compared to grasses, legume forages have greater uNDF and less pdNDF.  Because uNDF passes faster than pdNDF, feeding lactating dairy cows a diet containing legume forages will generally increase the passage rate, which may support greater dry matter (DM) intake.  The downside of this relationship is legume forages pass faster and contain less pdNDF, which can decrease the potential caloric supply from NDF.  In contrast, grasses contain more pdNDF and less uNDF.  As a result, grass forages have slower passage rates compared to legume forages.  Because the greater pdNDF fraction passes slower than the lesser uNDF fraction, feeding grass forages most likely will increase gut fill and/or slightly impede DM intake.

An ingenious design
The business of uNDF leaving the rumen faster than pdNDF hurts our human brains because something tells us that uNDF takes up extra space – right?  But think about it from a cow design standpoint.  Why would a dairy cow, who is dependent upon bacteria for NDF digestion, be designed to retain NDF that is totally indigestible?

Luckily, she is not designed that way and is designed to preferentially retain pdNDF in the rumen so it can be more thoroughly digested.  Bacteria can readily colonize pdNDF and, while fiber digestion is occurring, these bacteria produce gas to float the less dense pdNDF to the top of the rumen (the forage mat), where it has a longer retention time.  In contrast, uNDF is more dense and less colonized by bacteria. Thus, uNDF tends to sink and leave the rumen at a faster rate.

It’s really an ingenious design when we think it through and we are pursuing ways to consider forage passage rates in ration formulation.  Stay tuned.