In a previous article, I mentioned that undegraded neutral detergent fiber (uNDF) describes the potential utilization of NDF. However, to fully understand NDF utilization, we still need to integrate uNDF with the rate of digestion (kd). In easy terms, uNDF says very little if we ignore kd. Let me explain.

The figure below depicts the NDF digestion kinetics of four plant leaves from a study funded by the National Forage Testing Association (NFTA). The green and red lines represent the percentage of NDF degraded for barley and pearl millet, respectively. The kd for barley is 4.74% per hour, whereas the kd for pearl millet is 2.05% per hour. Because these two forages have similar proportions of potentially digestible NDF (pdNDF; roughly 83%) and uNDF (approximately 17%), it is clear that NDF from barley degrades much faster than the NDF from pearl millet.

Similarly, the purple and blue lines represent the percentage of NDF degraded for alfalfa A and alfalfa B, respectively. The kd for alfalfa A is 10.41% per hour, whereas the kd for alfalfa B is 6.02% per hour. Again, because these two forages have similar proportions of pdNDF (about 62.8%) and uNDF (around 37.2%), it is clear that the NDF from barley degrades much faster than the NDF from pearl millet.

So far, we can conclude that greater kd is better at similar uNDF concentrations. Let’s interpret kd when uNDF differs substantially.

As said above, the purple line represents alfalfa A, with a kd equal to 10.41% per hour, and the green line represents barley, with a kd equal to 4.74% per hour. At first sight, we might interpret these values as if alfalfa NDF degrades much faster than barley NDF. However, that is not true because the pool of pdNDF is greater for barley than for alfalfa A. Therefore, there is more to degrade in barley than in alfalfa A. From another perspective, if you pay close attention to the green and purple lines, approximately 60% of the NDF has been degraded by 24 hours in both cases. Even though the percentage of pdNDF degraded by 24 hours is less for barley than for alfalfa A, the absolute amount of degraded NDF is similar.

In conclusion, kd is necessary for understanding fiber utilization through fiber digestion kinetics. However, kd is meaningless without some context, and uNDF provides that context. In easy terms, kd says very little if we ignore uNDF.