RUMEN CONTENTS AND RUMINAL AND FECAL PARTICLE-SIZE DISTRIBUTION IN STEERS FED A MIXED DIET AT 3 AMOUNTS OF INTAKE

This study was conducted to investigate the effects of intake of a mix ed diet by cattle and time post-feeding on rumen contents and on parti cle size distribution of ruminal digesta. Six ruminally cannulated ste ers received a mixed diet consisting on an average of (% of dry matter ) 43 perennial ryegrass silage, 25 maize silage, 30 concentrate and 2 mineral-vitamin mix. The experimental design was a repeated 3 x 3 Lati n square with 21 day periods. The diet was offered twice daily (07:00 and 19:00 h) at approximately 1, 1.5 and 2 times of estimated maintena nce energy requirements (denoted low, medium and high intake, respecti vely). The rumens were evacuated manually at 3 and 7.5 h post-feeding and total ruminal contents separated into mat and bailable liquids. Dr y matter weight distribution of total recovered particles was determin ed by a wet-sieving procedure and used to partition ruminal mat and ba ilable liquids among percentages of large (greater than or equal to 4. 0 mm), medium (< 4.0 mm and greater than or equal to 1.0 mm), and smal l (< 1.0 mm and greater than or equal to 0.063 mm) particles. Intake d id not influence markedly the distribution of different particle fract ions, whereas time post-feeding had a pronounced effect. With increasi ng time after feeding, percentage of particles greater than or equal t o 1 mm decreased, whereas the percentage of particles < 1 mm increased , illustrating intensive particle breakdown in the reticulo-rumen. Dry matter contents and the proportion of particulate dry matter in the r umen increased as intake increased, i.e. ruminal mat increased at the expense of bailable liquids. From this study it can be concluded that feeding a mixed ration to steers at intakes of 1 to 2 times maintenanc e energy requirements influences the structure of rumen contents witho ut changing the processes of particle breakdown.