In the 8th edition of Nutrient Requirements of Dairy Cattle, the National Academies of Sciences, Engineering, and Medicine (also known as NASEM) put more emphasis on forage neutral detergent fiber (NDF) than on total NDF when formulating diets for lactating dairy cows. Specifically, NASEM recommends including at least 15% to 19% forage NDF in the diets, although no distinction exists concerning the quality of NDF. Conceptually, this means that any forage can be included regardless of the quality of the forage.

Let’s imagine a diet formulated to contain 30% corn silage with 38% NDF and an unknown inclusion of alfalfa hay containing 40% NDF. If the goal is to formulate a diet containing 19% forage NDF, then the ration should include 19% alfalfa hay. Mathematically, this works out as: 30 x 38% NDF + 19 x 40% NDF = 19% NDF.

Alternatively, let’s imagine a diet formulated to contain 30% corn silage with 38% NDF and an unknown inclusion of grass hay containing 63% NDF. If we keep the same goal of 19% forage NDF, then the ration should include 12% grass hay (30 x 38% NDF + 12 x 63% NDF = 19% NDF). In both cases, we have fulfilled the same goal with two different forages. However, the quality of the fiber is a missing part in this analysis, right? Then, how does fiber quality fit in these recommendations?

In a study from Michigan State University, for example, lactating dairy cows were fed diets containing either alfalfa silage or orchardgrass silage as the only forage source in the diets. The alfalfa silage contained 42.3% NDF, of which 54.5% was undegraded NDF (uNDF), whereas the orchardgrass silage contained 58.2% NDF of which 27.7% was uNDF. While the alfalfa diet included roughly 60% alfalfa silage (DM basis) and the grass diet included about 43% orchardgrass silage, both diets contained 25% forage NDF. Intuitively, one might expect the orchardgrass diet to be a better diet based on the lower uNDF of the silage. However, milk yield, dry matter intake, and NDF intake did not differ between diets. These results highlight that there is more to learn about fiber and fiber metabolism beyond the simple composition of the fiber (such as uNDF).

In a recent study from our laboratory, we observed that cows consuming diets containing alfalfa hay had a faster ruminal passage rate and a shorter mean retention time of uNDF than cows consuming diets containing orchardgrass hay, and this occurred despite the greater concentrations of dietary uNDF in the alfalfa-based diet. This aligns quite well with the study from Michigan.

Overall, these findings suggest that the kinetics of ruminal digestion and passage influence NDF utilization in ways that go beyond uNDF concentration or forage quality. Evidently, we have a lot to explore in the world of forage quality and fiber utilization.