In vitro NDF digestibility of forage is positively correlated with DM intake. Several years ago, Michigan State researchers found that on average a 1 unit increase in NDF digestibility increased DM intake 0.3 lb. This relationship was only valid within a forage class such as legumes, grass or corn silage and not across forage classes. However, even high NDF digestible forages can limit DM intake when forage NDF in a diet is above 25% of the DM.
The range in 30-hour NDF digestibility (percent of NDF) within forage classes is quite high: corn silage 50% to 60%, alfalfa 35% to 55% and grasses 30% to 70%. Some recent research from Cornell indicates total NDF is not a very good indicator of NDF digestibility.
Seven corn silage samples with only a 1% range in total NDF (42% to 43%) had 30-hour NDF digestibilities ranging from 35% to 57%. This information suggests we should strive to achieve a high NDF digestibility in our forages, but must also use all of the forage and NDF parameters described above in formulating and evaluating our lactating cow diets.