Dietary carbohydrates can be divided into two basic fractions: fibre and nonfibre carbohydrates (NFC). Fibre in dairy cow rations is essential for animal health, since it is required to support an appropriate rumen function and physiology. Therefore, ruminants require fibre in coarse physical form for a more effective chewing and ruminal activity. Increasing fibre content and forage particle size in diet effectively increases chewing activity resulting in increased saliva flow, rumen pH, acetate-to-propionate ratio, and milk fat levels. Increasing chewing activity and salivary buffer production are believed to be indicators of improving the dietary effect on rumen health and function. However, the physically effective NDF (peNDF) of a feed is related to the physical properties of its fibre (primarily particle size) that stimulates chewing activity and establishes the biphasic stratification of ruminal contents (floating mat of large particles on a pool of liquid and small particles). Thus, objective of this review discussing several parameters, including chewing, ruminal pH, acetate: propionate ratio, and milk fat percentage, have been used as animal responses to assess the effectiveness of NDF in dairy ruminant rations.
