Effects of the digestibility of neutral detergent fiber (NDF) from forage on performance of dairy cows were evaluated statistically using treatment means for 13 sets of forage comparisons reported in the literature. All comparisons reported significant differences in NDF digestibilities of forages in situ or in vitro. Treatment means were blocked by study or by additional factorial treatment within a study to remove variation among experiments. The statistical model included random effect of block, fixed factorial effect of NDF digestibility (high or low), and dietary NDF concentration as a covariate. Enhanced NDF digestibility of forage significantly increased dry matter 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 between 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 better 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. Digestibility of NDF is an important parameter of forage quality.
