Effects of Specific Gravity and Particle Size of Passage Marker on Particulate Rumen Turnover in Holestine Dairy Cattle

This experiment was carried out to evaluate the effect of specific gravity (SG) and particle size (PS) of Chromium-mordanated alfalfa neutral detergent fiber (NDF) markers on estimation of particulate rumen turnover. Thirty-two multiparous, mid lactation Holstein dairy cows (body weight=654±24 kg) were allot-ted to a completely randomized design experiment with two replications in a 4×4 factorial method. The experiment was accomplished over 21 d (adaptation, 14 d; sample collection, 7 d). Cows were ad libitum fed twice daily at 09:00 and 21:00 with similar total mixed rations. To prepare the marker, alfalfa in the individual bales was chopped for theoretical cut length 19, 10 and 5 mm. The fine alfalfa was prepared by grinding the 5 mm cut alfalfa through a 2 mm screen. The marker preparation followed a modified proce-dure of Uden et al. (1980) by different PS (8.45, 4.38, 3.00 and 1.10 mm) and K2Cr2O7 concentration in mordanting solution (50, 300, 500 and 900 g/kg K2Cr2O7 instead of 300 g/kg). Fecal grab samples were taken at 0, 6, 12, 16, 20, 24, 30, 36, 48, 60, 72, 96, 120 and 144 h after dosing. Reduction of PS and en-hancement of K2Cr2O7 concentration increased SG of marker, ruminal passage rate, but decreased ruminal mean retention time, time delay and total mean retention time. The coefficient of correlation between SG and PS of marker, ruminal passage rate, time delay and ruminal mean retention time were -0.62, -0.70 and 0.65; 0.45, 0.75 and 0.12, respectively. The SG of themarker was the most influential factor affecting ru-minal passage rate and mean retention time.