Influence of particle size and inclusion level of oat hulls on retention and passage in the anterior digestive tract of broilers

1. Six pelleted diets with different ratios of whole oats to dehulled oats, ground either finely (2 mm) or coarsely (6 mm), resulting in 0.05% (LOH), 6.5% (MOH) and 11.4% (HOH) OH inclusion, were fed to broilers to study gizzard functionality and its influence on passage of large oat hull (OH) particles. The hypothesis was that coarse OH would be selectively retained in gizzard for grinding, but higher levels would increasingly result in passage of unground OH.2. In experiment 1, gizzard and small intestinal contents were sampled from 78 birds at 28-29 d of age. Neutral detergent fibre (NDF) of gizzard contents was over three times higher than that of diets, indicating selective retention. An interaction effect of OH and screen size was seen for large particles (>1.6 mm) in the upper ileum, which was higher for MOH and HOH than LOH and only for coarse diets. This indicated that passage of unground coarse large particles from the gizzard increased when it exceeded a maximum retention level.3. In experiment 2, pre-weighed amount of HOH 2 mm and HOH 6 mm diets were fed to 80 birds for 2 h and were sampled on d 30 at 0, 2, 4, 8 and16 h to study OH degradation and flow. Contents were sampled from the crop, gizzard, small intestine, colon/cloaca and excreta. Unrecovered NDF in gastrointestinal tract plus excreta increased with more time and coarseness. An interaction effect between time and screen size was found for large particles in excreta, which was higher for 6 mm diets than 2 mm diets at 16 h.4. The presence of large OH particles in excreta challenged the previous assumption that particles are ground in the gizzard to a certain critical size before passing down further down the digestive tract. Some OH seemed to escape grinding in gizzard and this was higher for coarser particles.