Particle size reduction in cereal grains often results in an improved digestibility of starch due to increased surface area of grains, which subsequently increases the interaction with digestive enzymes. Improvement in the apparent total tract digestibility (ATTD) of gross energy (GE) upon particle size reduction has also been demonstrated in corn and a number of other ingredients when fed to weanling or growing-finishing pigs.

Extrusion results in improved digestibility of GE, which is primarily due to increased gelatinization of starch that occurs when cereal grains are processed in the presence of heat. The effects of extrusion may be different depending on the particle size of feed ingredients, but it is not known if improvements in nutrient digestibility obtained by extruding and reducing particle size of grain are additive or if there are interactions between them. Therefore, the objective of this experiment was to test the null hypothesis that there are no interactive effects of particle size reduction and extrusion on the ATTD of GE and concentrations of digestible energy (DE) and metabolizable energy (ME) in corn fed to young pigs.

Experimental design

One batch of yellow dent corn was procured and divided into 2 separate batches. One batch was extruded and then ground to 3 particle sizes using a hammermill (i.e., 700, 500, and 300 μm). The other batch was ground to the 3 particle sizes without being extruded. There was, therefore, a total of 6 batches of corn. These 6 batches were used as the only source of energy in six diets. Vitamins and minerals were included in all diets to meet or exceed the estimated requirements for growing pigs (NRC, 2012).

A total of 48 growing pigs (initial body weight: approximately 12 kg) were allotted to a randomized complete block design with 2 blocks of 36 and 12 pigs per block, respectively, for a total of 8 replicate pigs per diet. Pigs were individually housed in metabolism crates that were equipped with self-feeders, nipple waterers, and slatted floors. A screen and a urine pan were placed under the slatted floor to allow for the total, but separate, collection of urine and fecal materials. The daily feed allowance was limited to 3.0 times the energy requirement for maintenance (i.e., 197 kcal/kg × weight^0.60; NRC, 2012), which was provided each day in 2 equal meals at 0730 and 1530 h. Throughout the experiment, pigs had free access to water. Fecal and urine samples were quantitatively collected from d 6 to d 9 after 5 days of adaptation.

After sample preparation, diets, fecal, and urine samples were analyzed for GE to calculate the ATTD of GE and concentrations of DE and ME. Because corn was the sole source of energy in all diets, the ATTD of GE in diets was considered the ATTD of GE in corn; concentrations of DE and ME were calculated by dividing the DE and ME in the diets by the inclusion rate of corn (i.e., 96.76%). The statistical model included diet as fixed effect and replicate and block as random effects and contrast coefficients were used to compare the corn forms (i.e., non-extruded vs. extruded), linear effects of reducing particle size of corn, and the interaction between the form and linear effects of reducing particle size. Results were considered significant at P < 0.05 and considered a trend at P < 0.10.

Results

Results demonstrated that there were no interaction between particle size and extrusion for feed intake, and feed intake was not impacted by neither particle size or extrusion (Table 1). The ATTD of DM and concentrations of DE and ME in diets and the ATTD of GE and concentrations of DE and ME in corn were increased by reducing particle size of corn if corn was not extruded but that was not the case for extruded corn (interaction; P < 0.001). However, regardless of particle size, the ATTD of GE and concentrations of DE and ME in corn were increased (P < 0.001) by extrusion.

In conclusion, reducing particle size of corn increased energy digestibility in non-extruded corn, and extrusion of corn increased energy digestibility and concentrations of DE and ME regardless of particle size.

Key points

• The ATTD of GE and concentrations of DE and ME in non-extruded corn increased with reduced particle size.
• Extrusion of corn increased ATTD of GE and concentrations of DE and ME.

¹Each least squares mean represents 8 observations.

²Linear effects of particle size of corn.

³Interactions between extrusion and linear effects of reducing particle size.