Evaluation of the importance of the digestibility of neutral detergent fiber from forage: Effects on dry matter intake and milk yield of dairy cows

Effects of the digestibility of neutral detergent fiber (NDF) from forage o n performance of dairy cows were evaluated statistically using treatment me ans for 13 sets of forage comparisons reported in the literature. All compa risons reported significant differences in NDF digestibilities of forages i n situ or in vitro. Treatment means were blocked by study or by additional factorial treatment within a study to remove variation among experiments. T he statistical model included random effect of block, fixed factorial effec t of NDF digestibility (high or low), and dietary NDF concentration as a co variate. Enhanced NDF digestibility of forage significantly increased dry m atter intake (DMI) and milk yield. A one-unit increase in NDF digestibility in vitro or in situ was associated with a 0.17-kg increase in DMI and a 0. 25-kg increase in 4% fat-corrected milk. Differences in NDF digestibility b etween treatments were greater when measured in vitro or in situ than when measured in vivo. Digestibility of NDF in vitro or in situ might be a bette r indicator of DMI than NDF digestibility in vivo because forages with high in vitro or in situ NDF digestibilities might have shorter rumen retention times, allowing greater DMI at the expense of NDF digestibility in vivo. D igestibility of NDF is an important parameter of forage quality.