Fiber is an essential component of diets for dairy cattle. In high producing dairy cows, about a quarter of the energy for milk production comes from digested fiber. Neutral detergent fiber (NDF) is a laboratory assay done on forages to measure the total content of fiber in a feed. The NDF is a ‘bulky’, slow to digest feed component, which can restrict feed intake and milk production. Forages are analyzed for NDF content and diets for lactating dairy cows are typically formulated to contain 28% to 35% NDF on a dry matter (DM) basis depending primarily on level of milk production and the feed ingredients being used in the diet.
The digestibility of NDF also profoundly affects feed intake and milk production. Fiber digestibility can have a much greater impact on milk production than the digestibility of any other feed component. The digestibility of the fiber in 38%-NDF alfalfa can vary from 30% to over 60% of the NDF. In a dairy ration containing 15 lb. DM of this alfalfa, a doubling of fiber digestibility would increase the intake of digestible energy enough to support up to 8 to 10 lb. more milk per cow per day.
A new in vitro lab assay has been developed by UW-Madison Dairy Scientists that predicts total-tract NDF digestion in ruminants. The in vitro total-tract NDF digestibility (TTNDFD) test predicts NDF digestion for alfalfa, corn silage, grass forages and byproduct feeds. UW-Madison researchers have validated the accuracy of the in vitro TTNDFD test against directly measured NDF digestibility in lactating dairy cattle. The University of Wisconsin recently was awarded a patent for the in vitro digestibility procedure that is used in calculation of TTNDFD. In the UW-Extension Team Forage Focus on Forage paper Using In Vitro Total-Tract NDF Digestibility in Forage Evaluation, UW-Madison Dairy Scientist David Combs reviews the TTNDFD test and discuss its use in forage evaluation.