A few weeks ago, at the Penn State Dairy Nutrition Workshop, I asked the audience what their choice would be between Forage A containing a “fiber degradation rate” (Kd) equal to 2.6% per hour or a Forage B containing a Kd equal to 4.2% per hour. Almost everybody chose Forage B. To make my point, I tricked the audience by picking stems of wheat straw (Forage B) and immature barley stems (Forage A). As I expected, most in the audience was fooled. Knowing this, we must understand now why a higher Kd is not an indicator of a better or more digestible forage.

Fiber in feeds provides a large amount of energy required by dairy cattle. In nutritional terms, the neutral detergent fiber (NDF) concentration represents the fibrous component of a feed ingredient. The NDF in a feed ingredient is divided into an undegradable fraction, known as undegradable NDF (uNDF), and a potentially degradable fraction, known as potentially degradable NDF (pdNDF). Because uNDF cannot be degraded, the degradation rate of uNDF equals 0% per hour. It is worth highlighting that this degradation rate is exclusively linked to the uNDF component. Alternatively, pdNDF can be degraded at a degradation rate greater than 0% per hour. We must understand, however, that this degradation rate greater than 0% per hour is for pdNDF and not NDF.

The figure below depicts the degradation of NDF (Y axis) over ruminal fermentation time (X axis) for wheat straw (blue) and barley stems (red). As mentioned above, the pdNDF of wheat straw has a greater degradation rate than barley stems (4.2% per hour and 2.6% per hour, respectively). However, wheat straw has a much higher proportion of uNDF and a much lower proportion of pdNDF in the total NDF than barley stems. Therefore, there is much less NDF substrate to ferment in wheat straw than in barley stems. In the figure (see the arrows), we observe that 39% of the total NDF has been degraded for the barley stems after 48 hours of fermentation, whereas only 23% of the total NDF has been degraded for the wheat straw.

In conclusion, the ruminal degradation rate of pdNDF or Kd is meaningless as an indicator of forage quality unless we consider the uNDF concentration to provide some perspective.