The objective of this experiment was to evaluatethe effects of different dietetic fiber physical effectiveness on the retentiontime and apparent digestibility of lactating dairy cows. Ten Holstein cows (average550 kg of live weight) were allotted to a switch back experimental design inthree successive periods. The animals were individually housed in a tie stalland 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 meshof 3,2; 4,8; 7,9; 15,9 and 25,4 mm. Samples from each grinding were submittedto the Particles Separator Model Penn State for determination of profile distributionof the fiber particles. Five treatments were constituted: T1100% of particlesbelow 8 mm; T276% below 8 mm and 24% from 8 to 19 mm; T336.7% below8 mm, 26.6% from 8 to 19 mm and 36.7% above 19 mm; T432% below 8 mm, 28%from 8 to 19 mm and 40% above 19 mm; T526% below 8 mm, 28% from 8 to 19mm and 46% above 19 mm. The fiber particle profile exercised effect on the retentiontime of the diets, being observed that those with larger particles size spendmore time in the gastrointestinal tract. Dry matter and organic matter digestibilitycoefficients were higher for diets with smaller fiber particle size when comparedto higher fiber particle size. There was not effect on the crude protein andnonfiber carbohydrate digestibilties. The digestibility of fiber was higherin the diets with finer fiber particle size. The results suggest that the fiberparticle size of the diet could be decisive on the retention time and on thedigestibility of the diets.
