Particle size is often a neglected part of feed quality control programs, but it can have a significant impact on how efficiently the pig is able to use the diet, enhancing performance while minimizing nutrient excretion and improving handling and mixing characteristics.

Mark Whitney, swine specialist, University of Minnesota Extension, Mankato, Minnesota, USA, points out that reducing particle size increases the overall surface area of the grain allowing digestive enzymes more area to break down nutrients and begin the digestive process. "However, reducing particle size too much can cause bridging and dust problems, increase energy usage and reduces throughput at the feed mill, and can increase the incidence of gastric ulcers," he notes.

Whitney points to research at Kansas State University that summarizes affect of particle size on pig performance in saying that although average micron size is important, so is the variability or range in micron size. Most ground grain samples will have a standard deviation of 1.8 2.4, although upper limits should not exceed 2.25 for hammer mills and 2.0 for roller mills. "Based on interactions observed for particle size and age of the pig, it appears that younger pigs more completely chew feed, and therefore the greatest potential for fine grinding to improve feed efficiency is in finishing pigs," Whitney says.

Improved feed efficiency

The Kansas State University study utilized starter pigs and showed a 4.5 percent improvement in feed efficiency when decreasing average particle size from 877 to 624 microns using a hammer mill, with little effect on growth rate or feed intake. These results are consistent with other studies evaluating nursery and grow-finish pigs, observes Whitney.

Sow lactation studies have also shown improved performance with decreased particle size.

Whitney says a dietary particle size of 700 microns is a good goal and general recommendation for corn-soybean meal based nursery, grow-finish, and sow lactation diets. Every 100 micron reduction in particle size improves feed efficiency by 1 to 1.5 percent, but also increases the incidence of gastric ulcers, reduces feed mill efficiency, and can negatively impact feed bridging.