The minimum amount of dietary NDF needed by a cow is largely based on ruminal and animal health (NRC, 2001). On average, NDF is less digestible than non-fibre carbohydrates (NFC, NRC, 2001). Therefore, NDF concentration is negatively correlated with energy concentration. The NDF is related to ruminal pH, fat percentage, and chewing activity (NRC, 2001). Fat is commonly added to diets of high yielding dairy cows to increase energy density. However, the addition of fat can interfere with rumen fermentation which can result in decreased fibre digestion. Fibre characteristics of the diet may influence the extent to which fats interfere with rumen fermentation. Therefore, the type of fibre influenced the degree of negative effects caused by ruminally active fats (Tackett et al, 1996). The main aim of the present research was to evaluate the effect of supplemental fat and NDF on fibre digestibility, rumen pH and chewing activity in lactating dairy cows.

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