Wheat particle size in low soybean meal diets did not alter broiler growth performance

Reducing soybean meal (SBM) inclusion through the combined use of non‑bound amino acids (NBAA) and alternative protein meals may increase the proportion of fine ingredients in the diet. Including coarser particles or whole grain in wheat‑based, low‑SBM diets may help to restore growth performance. Therefore, this study evaluated the interactive effects of wheat particle size and SBMinclusion level on broiler growth performance and nutrient digestibility. A total of 366 day-old off-sex male Ross308 birds from the female parent line were used in a 4 × 2 factorial designed experiment, including four wheat particle size arrangements (3.2 mm, 6.0 mm, combination of 3.2 mm or 6.0 mm, and whole wheat) and two levels of SBM inclusions (standard or low/ nil). Each dietary treatment had 7 replicates with 6 birds per replicate from day 0 to 35 post-hatch. There was no interaction between wheat particle size and the SBM inclusion level on any growth performance parameters. Across 0-35 days post-hatch, low/nil SBM diets decreased body weight gain (BWG) by 3.3% (P < 0.001), increased feed conversion ratio (FCR) by 2.9 points (P = 0.002) compared to standard-SBM diets, regardless of wheat particle size. However, an interaction was observed for the relative weights of the pectoralis minor muscle (P = 0.021) and gizzard (P < 0.001). Low/nil-SBM diets also decreased pH in the gizzard (P = 0.045), jejunum (P = 0.032), and ileum (P < 0.001), irrespective of wheat particle size. Coarser particle size diets increase apparent protein digestibility coefficients in the distal jejunum (DJ) (P = 0.004) and distal ileum (DI) (P = 0.019) compared to fine-particle diets. In conclusion, wheat particle size did not impact growth performance in broiler chickens offered diets with varying SBM inclusion. However, reducing SBM by 50%, 75%, and 100% in the starter, grower, and finisher phases, respectively, compromised BWG, FCR and breast meat yield.