Undigested NDF Influence on Dry Matter Intake
There are many factors that influence a dairy cow’s dry matter intake (DMI), ranging from environmental factors such as heat stress to metabolic disorders such as milk fever. This article will focus on how undigested NDF (uNDF) can influence DMI.
To begin we need to identify how the rumen can influence DMI. Gut fill is the broad term used to describe several factors. One description is “the retention and accumulation of particles in the rumen”. This raises the question: What affects the retention and accumulation of particles in the rumen? Passage rate, feed particle reduction, and the digestibility of that feed all influence gut fill. Intake amounts and lactation status play a part in passage rate. The influence of feed particle reduction on DMI is simple: For a feed particle to pass through the rumen and into the rest of the GI tract it needs to be reduced in size. On average a critical size of 1.18 mm is required for a particle to pass from the rumen. This estimate was determined by using a dry vertical sieve particle separator.
When the cow consumes feed there are portions of the diet that are rapidly digested and other portions that take longer to digest in the rumen. This goes back to the concept of fast, slow, and indigestible pools of NDF. The longer the feed is retained in the rumen the more time there is for rumen microbes to degrade what is potentially digestible. The fast pool NDF is the portion of the fiber that can be broken down in 24 to 30 hours. The slow and indigestible NDF (iNDF) pools take longer to digest and are retained in the rumen for a longer period of time. This is where the influence on DMI comes into play. Our dairy cow is only able to consume more TMR each day if there is available space, and that space is driven by digestion. For example, a lactating cow in the Miner Institute barn may be consuming 60 lbs of TMR daily. Of that 60 lbs, 30% or 18 lbs is NDF. As that 18 lbs of fiber digests, portions of it will be broken down as either fast, slow, or iNDF pool. For example, assume that 50% of the NDF consumed is in the fast pool. That means that 9 lbs of the fiber will be digested in 24 hours. The remaining 9 lbs consists of both the slow and iNDF pool. These portions will remain in the rumen longer than 24 hours. You may be asking yourself how will she consume another 18 lbs of fiber the next day if she has only digested the fast pool thus far? Keep in mind that the rumen is continually digesting nutrients, and previously consumed slow and iNDF pools will be digesting and passing from the rumen, creating room for more TMR to be consumed.
By changing the uNDF amounts of the diet we can begin to influence DMI. If we were to increase the digestibility of the diet, lowering the %uNDF, we would alter the proportion of the fast, slow and iNDF pools so that more feed would be digested in a 24-hour period. The opposite would occur if we were to decrease the digestibility of the diet. You may be thinking: Why feed any uNDF if it will influence the amount of DMI the cow can consume? The undigestible portions of the feed play a key role in maintaining a healthy rumen, something I'll discuss in a future article.
