The objective of this experiment was to evaluate the effects of different dietetic fiber physical effectiveness on the retention time and apparent digestibility of lactating dairy cows. Ten Holstein cows (average 550 kg of live weight) were allotted to a switch back experimental design in three successive periods. The animals were individually housed in a tie stall and fed with total mixed rations constituted by Tifton hay (50.4%) and concentrated (49.6%). The Tifton hay was ground in commercial miller with sieves with mesh of 3,2; 4,8; 7,9; 15,9 and 25,4 mm. Samples from each grinding were submitted to the Particles Separator Model Penn State for determination of profile distribution of the fiber particles. Five treatments were constituted: T1-100% of particles below 8 mm; T2-76% below 8 mm and 24% from 8 to 19 mm; T3-36.7% below 8 mm, 26.6% from 8 to 19 mm and 36.7% above 19 mm; T4-32% below 8 mm, 28% from 8 to 19 mm and 40% above 19 mm; T5-26% below 8 mm, 28% from 8 to 19 mm and 46% above 19 mm. The fiber particle profile exercised effect on the retention time of the diets, being observed that those with larger particles size spend more time in the gastrointestinal tract. Dry matter and organic matter digestibility coefficients were higher for diets with smaller fiber particle size when compared to higher fiber particle size. There was not effect on the crude protein and nonfiber carbohydrate digestibilties. The digestibility of fiber was higher in the diets with finer fiber particle size. The results suggest that the fiber particle size of the diet could be decisive on the retention time and on the digestibility of the diets.
effective fiber; granulosity; rate of passage
