The metabolizable energy content of corn ground to smaller particle sizes is greater than that of corn ground to larger particle sizes, because the reduced particle size provides more surface area for digestive enzymes to act on. This results in more starch being digested in the small intestine with a subsequent absorption of glucose.

Currently, nutritionists recommend feeding corn ground to an average particle size of 650 to 700 µm. However, it may be advisable to formulate diets containing corn ground to smaller particle sizes due to the greater ME in these diets. If diets are formulated to a constant ME, the inclusion of added fat can be reduced if corn ground to a smaller particle size is used.

In a previous experiment, growth performance did not differ among growing-finishing pigs (average initial body weight: 32 kg) fed diets containing corn ground to particle sizes ranging from 339 to 865 µm if diets were formulated to the same ME by reducing the concentration of added fat as corn particle size was reduced. The experiment discussed in this report was conducted to test the hypothesis that added fat can be reduced in diets fed to weanling pigs if corn ground to a smaller particle size is used.

Four diets based on corn, soybean meal, and 4% fish meal were formulated, using corn ground to a particle size of 339, 485, 677, or 865 µm. The diets were formulated to contain the same amount of metabolizable energy by adding soybean oil in varying amounts, with more oil added as corn particle size increased to compensate for the reduced concentration of metabolizable energy in corn ground to larger particle sizes. A total of 128 pigs with an average initial body weight of 9.95 kg were used. Pigs were weaned at approximately 3  weeks of age and had been fed a common phase 1 diet for 2 weeks before being allotted to diets in this experiment.

Individual pig body weight was recorded at the start of the experiment and at the end of the experiment on day 20.  Daily feed allotments as well as feed left in the feeders were recorded and used to calculate average daily feed intake and gain:feed ratio. Fecal samples were collected on day 20 to measure pH.

There were no significant differences in average body weight at day 20, average daily gain, or average daily feed intake among pigs fed diets containing corn ground to different particle sizes (Table 1). However, a linear increase (P = 0.05) in gain:feed ratio, from 0.617 to 0.654, was observed as particle size decreased. This observation indicates that the increase in ME of diets due to reduced particle size of corn may be greater in weanling pigs than in growing pigs. The values for ME for the four different particle sizes of corn that were used in diet formulations were obtained in growing pigs and not in weanling pigs. By reducing the amount of added soybean oil as the particle size of corn was reduced, it was expected that G:F would be constant among diets. However, the fact that G:F increased in pigs fed diets containing corn ground to the smaller particle sizes, indicate  that the ME of corn with the reduced particle size was greater than the values used in diet formulations.

Colon pH increased linearly (P < 0.01), from 5.82 to 6.19, as particle size decreased, which indicates that as particle size increased, microbial fermentation increased. This is likely a result of more undigested starch entering the hindgut in pigs fed diets containing corn ground to larger particle sizes.

• When diets are formulated to contain the same amount of metabolizable energy, feeding diets containing corn ground to different sizes to weanling pigs had no effect on average daily gain or average daily feed intake.
• There is a possibility that diet costs can be reduced by decreasing the amount of fat added to diets containing finely ground corn.
• The reduction in pH of colon contents that was observed for pigs fed the corn with increased particle sizes indicates that more volatile fatty acids were produced by these pigs compared with pigs fed the diets containing corn ground to  a smaller particle size. This is likely a result of increased starch digestion in pigs fed corn ground to a smaller particle size.

Table 1. Growth performance and colon content pH of pigs fed diets containing different corn particle sizes

This report is based on unpublished data by O. J. Rojas and H. H. Stein.