Gastrointestinal tract morphology, nutrient digestibility, and digesta mean retention time in laying hens of two different breeds fed with fine or coarse oat hulls

The inclusion of fiber-rich ingredients in poultry diets is expected to increase due to societal-, ecologic-, and economic developments. Particle size of dietary fiber sources, such as oat hulls (OH), has been shown to play a key-role in nutrient digestibility and digestion process, but the response may depend on the genetic background of the bird. This study aimed to test the hypothesis that laying hens varying in genetic background respond differently to the particle size of OH regarding gastrointestinal tract development, apparent ileal (AID) and total tract digestibility (ATTD), and digesta mean retention time (MRT). A total of 224, 29-wk-old Dekalb White (DW) and Bovans Black (BB) laying hens were assigned to 1 of 2 dietary treatments (fine vs. coarse OH). The diets were formulated to contain 150 g/kg OH as the main dietary fiber source. Titanium dioxide (3 g/kg) and cobalt-EDTA (2 g/kg) were added as inert tracers. Bovans Black hens had greater relative weights of the gizzard (+3.9% of BW, P = 0.015), ceca (+14.3% of BW, P < 0.001), and colon (+9.1% of BW, P < 0.001) compared with DW hens. Moreover, AID of nitrogen (+3.5%; P < 0.001) was greater in BB vs. DW hens, whereas AID of starch was only greater in BB when fed the fine OH diet (breed × diet, P = 0.037). In contrast, ATTD of NSP (-11.2%, P = 0.035) was lower in BB vs. DW hens and digesta MRT was longer in proventriculus + gizzard (solids: +25 min, P < 0.001; liquids: +5 min, P < 0.001) and ileum (solids: +53 min, P = 0.001; liquids: +38 min, P < 0.001). Birds fed coarse OH only had a greater relative weight of the gizzard (+3.9%, P < 0.001) compared with those fed fine OH. In conclusion, our study indicates that gastrointestinal tract traits, nutrient digestibility and digesta transit behavior differed between breeds, regardless of the particle size of OH in the diet.