Opinions vary regarding how much distiller’s grains (DG) can be used effectively in lactation rations. In order to better understand conflicting reports and recommendations about the effect of DG on milk yield we examined 23 studies about DG in the dairy science literature. Inclusion of DG in treatment diets ranged from 4 to 42% of dietary DM across all studies. Our analysis of the entire database, shows that inclusion rate of DG per se did not affect FCMY, MY, MF%, or MP%. However, when the fermentability of rations is considered there were significant changes in milk yield responses to inclusion of DG.