Recent research at Miner Institute has focused on the relationship between undigested and physically effective NDF (abbreviated as uNDF and peNDF). Physically effective NDF is commonly measured using the 4-mm screen in the Penn State Particle Separator and uNDF is measured as the undigested NDF following 240 hours of in vitro fermentation. Both of these measures are highly useful in the field because they allow us to do a much better job of predicting the cow’s response to NDF. How dietary fiber particle size and digestibility interact to affect the chewing, intake, and productive responses of the cow is a hot topic among nutritionists.