Research has shown that grinding cereal grains in diets fed to pigs into smaller particle sizes improves growth performance. Feed ground to smaller particle sizes has more surface area on which digestive enzymes can work, so digestibility of energy and nutrients that are enzymatically digested may also be improved. Previous research at the University of Illinois indicated that particle size had no effect on the digestibility of phosphorus, but that decreasing particle size increased energy digestibility in corn in the range of 338.5 μm to 864.5 μm.

However, it is not known if reduced particle size also increases amino acid digestibility. Therefore, an experiment was conducted to determine the standardized ileal digestibility (SID) of amino acids in corn that was ground to different particle sizes and fed to growing pigs.

A total of 10 growing barrows with an average initial body weight of 29.2 kg were surgically fitted with a T-cannula in the distal ileum. The pigs were fed one of five experimental diets. Four of the  diets consisted of 97.5% corn, with vitamins and minerals added to meet the pigs' requirements. The corn in each diet was ground to a mean particle size of 338.0 μm, 485.0 μm, 676.5 μm, or 864.5 μm. All amino acids in the diets were supplied by corn. The last diet was a protein-free diet that was used to determine the basal endogenous losses of amino acids from the pigs. The 10 pigs were allotted to the five diets in such a way that each diet was fed to two pigs in each of five 7-day periods. The initial five days of each period was considered an adaptation period to the diet and ileal digesta were collected on days 6 and 7 of each period and analyzed for crude protein and amino acids.

No significant differences in SID of crude protein and most amino acids was observed among experimental diets (Table 1). The exception was that for tryptophan, a quadratic response was observed and the SID was greatest (P < 0.05) when corn was ground to a particle size of 676.5 μm. But other than that, no influence of particle size on amino acid digestibility was observed. This response is in contrast to the digestibility of energy in corn, and indicates that particle size has no influence on the digestibility of amino acids in corn if the mean particle size is at least 865 μm.

• Particle size in the range of 338.5 μm to 864.5 μm had no effect on the standardized ileal digestibility of crude protein or most amino acids in corn
• Amino acid digestibility cannot be improved by grinding corn to a particle size that is finer than 865 μm.
• As the particle size of corn is reduced, the risk of pigs developing gastric ulcers is increased, which also needs to be taken into account when deciding on the particle size of corn.

Table 1. Standardized ileal digestibility (SID) of crude protein and amino acids in different particle sizes of corn by growing pigs

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