The overall goal for this experiment was to determine the impact of average particle size of corn, wheat and corn DDGS with two different grinding technologies on the digestion of energy and nutrients in growing pigs and finishing pigs. Corn and wheat were ground to three different particle sizes (300, 500, and 700 microns), using either a roller mill or a hammermill for each particle category (making 6 diets per each ingredient). For corn DDGS there were 3 diets, each diet included 51% of corn ground at 500 microns with a roller mill, and then mixed with 45% corn DDGS ground to 450 microns using either a hammer mill or a roller mill, or not further ground (unprocessed particle size was 650 microns). Samples from middle sieve fractions were taken for chemical analysis. A total of 120 growing barrows housed individually and fed the experimental diets for each weight category (Growers = 55 kg and Finishers = 110 kg). Digestibility of dry matter, energy, crude protein, fat and neutral detergent fiber were determined on all samples. Results from corn showed that digestion of dry matter, energy, and crude protein was greater in the finisher compared to the grower pigs. Surprisingly, digestibility of fat was greater in grower as compared to finisher pigs. Lowering mean particle size increased digestibility of energy and nutrients in corn ground with a roller mill but not with a hammer mill. Part of the reason for this interaction may be related to physical differences of the final ground product. Digestion of neutral detergent fiber was influenced by the interaction of body weight, particle size and grinding method. Basically, for growing pigs, lowering particle size of corn with a hammer mill resulted in lower fiber digestibility. However, in finishing pigs, NDF digestibility was similar across particle sizes and processing method. On the other hand, in finishing pigs, lowering particle size of corn using a roller mill resulted in greater fiber digestion when particle size was reduced from 700 to 500 microns; there was no further benefit to reducing particle size to 300 microns. Results from wheat showed that there was an interaction among the three factors evaluated. Growing pigs had improved digestion of dry matter, energy, crude protein, fat and fiber when particle size was lowered from 700 to 500 microns using either a hammer mill or a roller mill. There was no further improvement when particle size was further reduced from 500 to 300 microns. In finishing pigs, digestion of dry matter, energy, crude protein, fat and fiber was improved by lowering mean particle size with a hammer mill from 700 to 500 microns but not to 300 microns. Using a roller mill, digestion of dry matter and fiber decreased by lowering mean particle size from 700 to 300 microns, while digestion of GE was decreased only by lowering mean PS from 700 to 500 microns, with no further benefit to grinding to 300 microns. In corn DDGS, digestion of dry matter, energy and crude protein was greater in finisher pigs compared to growing pigs. Lower particle size improved digestion of dry matter, energy, fat and crude protein irrespective of milling method. Digestion of fiber in DDGS was not affected by particle size or body weight. In conclusion, mean particle size, grinding method and BW can influence the digestibility of corn, wheat and corn DDGS differently; while smaller particle size was found to be generally better, this did not hold true in all instances. If ingredients can efficiently be ground to different particle sizes, profits can be maximized at the lowest possible processing cost.
• Overall, reducing mean particle size improved feed utilization in pigs through increased digestibility; however, this did not hold true in all instances.
• Digestibility of corn progressively increased as particle size was reduced (700 to 300 microns) with a roller mill; in contrast, with a hammermill, there was little difference in digestibility across all particle sizes.
• Digestibility of wheat was influenced in 55 kg pigs by reducing particle size only from 700 to 500 microns irrespective of the grinding method used. In contrast, 110 kg pigs showed specific optimal particle size for best digestibility percentages (700 microns for roller mill and 500 microns for hammermill).
• Reducing the mean particle size of corn DDGS either with a hammermill or a roller mill (from 650 to 450 microns) provided only a modest advantage in terms of improved digestibility.