PASSAGE OF SAND PARTICLES THROUGH THE GASTROINTESTINAL TRACT OF DAIRY COWS R. G. Patzer, W. W. Sutton, and G. D. Potter U.S. Environmental Protection Agency Environmental Monitoring and Support Laboratory, Las Vegas ABSTRACT A study was performed to determine whether the passage rate of particles through the bovine gastrointestinal system is related to particle size. Silica sand of four graded size ranges was obtained, and each size range was labeled with a different gamma-emitting radioisotope. The nominal diameters for the sand particle batches were 20, 80, 200, and 450 um. A dose of each sand size was orally administered to each of four lactating Holstein dairy cows maintained in metabolism stalls. Fecal material from each cow was collected and analyzed by gammma-ray spectroscopy to determine the excretion rates for each particle size. The data show that the passage time for soil particles through the gastrointes- tinal tract of dairy cows varies significantly with particle size. The smallest particles were passed rapidly by all cows; half of these were excreted within 1.7 days and 90 percent within 2.8 days of dosing. The three larger size particle groups were passed more slowly and with large variations among cows. Up to 8 days (average about 4 days) were required to excrete 50 percent of these larger particles and up to 12 days (average about 9 days) to excrete 90 percent of the particles in feces. INTRODUCTION The objective of this study was to determine whether the rates of passage or residence times for particles in the ruminant digestive tract are related to particle size. 151