This work was carried out to evaluate the physical characteristics of the digesta throughout the gastrointestinal tract and to exam the conception of critical particle size that escape from the reticulum-rumen (RR), in the dairy bull calves fed diets with different concentrate levels. Thirty two dairy bull calves, one half straightbred Holstein and half crossbreed Holstein-Zebu, slaughtered with average six months of age and 190 kg LW, after fasting for 16 hours. The diets fed libitum were constituted of coast-cross hay, and of different concentrate levels (ground corn, soybean meal and mineral salt): 45.0, 60.0, 75.0, and 90.0 of concentrade in diet. The content of RR, omasum (OM), abomasum (ABO), small (SI) and large intestine (LI) were sampled to evaluate the average particle size (APS), modulus of fineness (MF) and average relative resistance (ARR) to escape, and to exam the critical particle size conception that escaped from the RR. The intake of diet with crescent levels of concentrate did not influence the APS and the MF, as the particles already escaped the RR. Consequently, the particle size of the LI could represent the particle size that escape RR. In this study, more than 80% of the particles presented in the RR were lower than the critic particle size threshold to escape, showing that the reduction of particle size, although it was a prerequisite for escape, it was not the rate-limiting step regulating the RR material clearance. The relative resistance of the digesta to escape from the RR increased as the particles was larger, although the ARR decreased with the increase of the concentrate levels in the diet.
bull calves; concentrate level; particle size gastrointestinal tract
