The effect of expander processing and screen size on nutrient digestibility, growth performance, selected faecal microbial populations and faecal volatile fatty acid concentrations in grower–finisher pigs

A 2 × 2 factorial experiment with grower–finisher pigs was conducted to investigate the effect of screen size and expander processing (yes and no) on digestibility, pig performance, selected faecal microbial populations and faecal volatile fatty acid concentrations. The feed was ground at two levels (using a 3 or 6 mm hammermill screen) to obtain a fine or coarse feed and given to the pigs as either raw or expanded feed. The diets were expander processed at 105 °C for 5 s at 35-bar pressure. Fine grinding increased the coefficient of total tract apparent digestibility (CTTAD) of gross energy of the diets (P<0.05) and reduced faecal dry matter concentration (P<0.05). Pigs offered the 3 mm diets had a lower (P<0.05) feed intake (2.47 kg/day versus 2.55 kg/day, S.E.M. 0.029) and a better (P<0.01) feed conversion ratio (2.61 kg/kg versus 2.80 kg/kg, S.E.M. 0.034) than the pigs offered the 6 mm diets. Expander processing had no effect on CTTAD or growth performance. There was a significant interaction (P<0.05) between screen size and expander processing on the faecal Lactobacilli and Enterobacteria population. Coarse grinding increased (P<0.05) the faecal Lactobacilli population and decreased (P<0.05) faecal Enterobacteria population when the diets were not heat treated. However, screen size had no effect on Lactobacilli and Enterobacteria population when the diets were expanded. Coarse grinding increased (P<0.05) the total faecal volatile fatty acid concentration and molar proportions of propionic acid and butyric acid while decreasing (P<0.05) the faecal proportions of acetic acid compared to fine grinding. In conclusion, fine grinding of feed improved feed intake and feed conversion ratio. However, it did increase faecal Enterobacteria populations. Expander processing had no effect on pig performance.