Effect of particle size of wheat on nutrient digestibility, growth performance, and gut microbiota in growing pigs

• As particle size of wheat grain reduced, CP digestibility increased linearly in vitro.
• A quadratic effect exit between apparent N digestibility and particle size of wheat.
• Particle size of wheat from 430 μm to 470 μm enriched beneficial fecal microflora.
• Particle size of wheat from 430 μm to 470 μm suppressed bacterial pathogens in fecal.
• Particle size of wheat from 430 μm to 470 μm increased pig growth performance.

This research was conducted to determine the effect of particle size of wheat on digestibility (in vitro and in vivo), growth performance, and gut microbiota in growing pigs. The pig were fed wheat–soybean meal-based diets containing 70.85% wheat ground to mean particle sizes of 330, 430, 450, 470, 580, and 670 μm. Before blended into diet, the ground wheat were used to measure dry matter (DM) and crud protein (CP) in vitro digestibility by the two-step method (pepsin and trypsin). Then, a total of 30 barrows with an average bodyweight (BW) of 21.2±1.2 kg were fed 6 diets containing different particle sizes of wheat (330, 430, 450, 470, 580, and 670 μm) to determine apparent digestibility of DM, gross energy (GE), and N in vivo. Finally, the effect of the particle size of wheat on growth performance and microbiota was assessed by feeding the same diets for 28 d with 6 pens per diet and 10 pigs (initial BW: 10.4±0.9 kg) per pen. Reducing particle size from 670 to 330 μm increased the DM digestibility from 17% to 26% (linear, P<0.05) and CP digestibility from 55% to 66% (linear, P<0.05). Digestibility experiment in growing pigs demonstrated that particle size exerted no effect on DM and GE digestibility in vivo. In contrast with in vitro digestibility, as the particle size of wheat reduced, apparent N in vivo digestibility increased first and then decreased, with the particle size of 430 μm supporting the greatest N digestibility (quadratic, P=0.028). Average daily feed intake (ADFI) and gain/feed were unaffected by particle sizes. However, the diarrhea incidence increased (P<0.05) with the particle size reduced. A quadratic effect (P=0.038) of the particle size and average daily gain (ADG) was also observed, with the greatest ADG occurring with 430–470 μm. Moreover, the increase in the particle size of wheat from 430 to 470 μm increased (P<0.05) the number of beneficial bacteria (Bifidobacterium sp. and Lactobacillus sp.), but also suppressed (P<0.05) bacterial pathogens (Eschericha coli), which effectively promoted (P=0.02) the diversity of gut microflora. The results in the study indicated that the particle size of wheat from 430 to 470 μm was acceptable in diets for growing pig.