Pelleting and reducing particle size of grains often improve nutrient digestibility by pigs. Pelleting may also reduce particle size of grains, and it is not known if improvements in nutrient digestibility obtained by reducing the particle size of grain and improvements obtained by pelleting are additive or if there are interactions between particle size reduction and pelleting. Therefore, two experiments were conducted to test the hypothesis that particle size reduction and pelleting, separately or in combination, increase the apparent ileal digestibility (AID) of starch, the standardized ileal digestibility (SID) of amino acids (AA), N balance, apparent total tract digestibility (ATTD) of gross energy (GE), fiber, and fat, and net energy (NE) in corn-soybean meal diets fed to growing pigs.

Experimental design

Six corn-soybean meal based diets were used in a 3 × 2 factorial with three particle sizes of corn (i.e., 700, 500, or 300 μm) and two diet forms (i.e., meal or pelleted). All diets contained 0.40% TiO₂ as an index. An N-free diet was also used in Exp. 1 to determine basal endogenous losses of AA. Pigs were allowed ad libitum intake of feed in Exp. 1 and 2.

In Exp. 1, seven pigs (initial weight = 59.30 kg; SD = 2.77) that were equipped with a T-cannula in the distal ileum were allotted to the seven diets using a 7 × 7 Latin square design with seven periods. Each experimental period lasted seven days. The initial five days of each period were considered an adaptation period. Ileal digesta were collected on day 6 and 7 for nine hours using standard procedures. Diets and ileal digesta were analyzed for Ti, starch, and AA to calculate the AID of starch and AA in the diet and the SID of AA was calculated by correcting the AID of AA for the basal endogenous losses of AA.

In Exp. 2, twenty-four pigs (initial weight = 29.52 kg; SD = 1.40) were allotted to the six diets using a 6 × 6 Latin square design with six calorimeter chambers (i.e., four pigs/chamber) and six periods. Each chamber was equipped with a slatted floor, four stainless steel fecal screens, and two urine pans for total, but separate, collection of fecal and urine materials. The temperature and relative humidity inside the chambers were controlled and maintained at the same levels in all chambers. Diets were fed for 13 d, where the initial 7 d were considered the adaptation period to the diet. On d 8, gas analyzers started measuring O₂ consumption and CO₂ and CH₄ production. Fecal and urine samples were also collected from d 8 to d 13. At 0700 h on d 14, pigs were deprived of feed for 36 h. This time was considered a fasting period to determine fasting heat production (FHP). Total heat production (THP) and FHP during the collection period were calculated using concentrations of O₂, CO₂, and CH₄ inside the chambers and N in urine. The ATTD of GE and nutrients were calculated and concentration of NE in diets was calculated using analyzed GE in diets, feces, and urine and measured THP and FHP.

Results

Results from Exp. 1 indicated that there were no interactions in the AID of starch and the SID of Arg, His, Ile, Trp, and total AA (Table 1). The interactions were observed for the SID of Leu, Lys, Met, Phe, Thr, and Val (P < 0.05). Regardless of particle size, values for the AID of starch and the SID of most AA were greater (P < 0.05) in pelleted diets than in meal diets. Regardless of diet form, values for the AID of starch and the SID of most AA were linearly increased (P < 0.05) by reducing corn particle size.

Results from Exp. 2 indicated that no interaction was observed for the ATTD of TDF and N retention (Table 2). The ATTD of GE, N, and AEE, and the concentration of NE were increased (linear; P < 0.05) by reducing the particle size of corn, but the increase was greater in meal diets than in pelleted diets (interaction; P < 0.05). Regardless of particle size of corn, the ATTD of GE, N, and acid hydrolyzed ether extract (AEE), and concentration of NE were greater (P < 0.05) in pelleted diets than in meal diets.

In conclusion, both pelleting and reducing particle size of corn increased nutrient digestibility and NE, but the magnitude of increases was different between meal diets and pelleted diets.

Key points

• There was no interaction between pelleting and reducing particle size of corn in the AID of starch, the SID of total AA, and the ATTD of TDF and N retention.
• The ATTD of GE, N, and AEE, the SID of some AA, and the concentration of NE were increased by reducing particle size of corn, but the magnitude of increases was different between meal diets and pelleted diets.
• Concentration of NE and digestibility of starch, AA, and fat were increased by both pelleting and reducing particle size of corn, but digestibility of fiber and retention of N were not affected by pelleting and particle size of corn.

Table 1. Effects of diet form and particle size of corn on AID of starch and SID of AA in diets fed to growing pigs^1,2

¹Each least squares mean represents 7 observations.

²Values for SID were calculated by correcting AID for basal ileal endogenous losses. Basal ileal endogenous losses were determined (g/kg dry matter intake) as: Arg, 0.65; His, 0.18; Ile, 0.33; Leu, 0.52; Lys, 0.41; Met, 0.08; Phe, 0.30; Thr, 0.53; Trp, 0.09; Val, 0.49; and total AA, 14.43.

³Linear effects of particle size of corn.

Table 2. Effects of diet form and particle size of corn on digestibility of fiber and fat, N balance, and concentration of NE in diets fed to group-housed pigs¹

¹Each least squares mean represents 6 observations.

²Linear effects of particle size of corn.