Pork producers from across Iowa submitted representative samples of processed feed ingredients (corn, soybean meal, distiller’s dried grains with solubles, etc.) or complete diets (gestation, lactation, nursery, grow-finish) for an evaluation of fineness of grind.

As a part of an undergraduate honors project, animal science student Mat Vaughn collected 63 samples that were identified by farm and county, and whether the feedstuffs were processed commercially or on the farm using a hammer mill (screen size and age) or roller mill (corrugations/inch). Producers were also asked to submit an estimate of feed efficiency in the barn where the diets were fed.

Average particle size was determined using a three-sieve system developed by Kansas State University, which includes a receiving pan, a 300-micron sieve with one ball and two caruncles, a 600-micron sieve with one ball and one caruncle, and a 1,700-micron sieve. The balls and caruncles serve to break up feed and prevent it from clogging the sieves as they decrease in size. Particle size analysis procedures are available at: www.asi.ksu.edu/DesktopDefault.aspx?tabid=1225.

The mean particle size of the 63 samples was 786 microns. Optimal particle size for grow-finish diets is 650-750 microns. For every 100 microns over the recommended micron-size window, feed costs increase about 65 cents/pig, Vaughn notes.

“The finer the feed is ground, the greater the surface area where digestive enzymes can actively begin the digestion process. Conversely, as particle size increases, greater amounts of undigested feed pass through the digestive tract, thereby increasing nutrient excretion levels and creating environmental, as well as economic concerns,” Vaughn adds. “Alternatively, when the grind of the feed becomes too fine, the stomach can become too fluid, which results in the pigs becoming more susceptible to gastric lesions or ulcers.” Furthermore, bridging in feeders and storage bins can become a challenge if feed is too finely ground.

Of the total, 52 samples were complete feeds and the particle size averaged 803 microns. Seven samples were pure corn used in complete feeds with an average particle size of 731 microns.

When milling processes were compared, there were no statistical differences between on-farm and commercial processing or whether feeds were made using a hammer mill or a roller mill.

Thirty of the 52 samples of complete feeds (57%) exceeded the upper level of the acceptable 750-micron particle size range. Those samples averaged 901 microns.

“The feed samples that had greater than ideal feed particle size represent opportunities for producers to improve production and economic efficiency,” Vaughn notes.

If the samples were representative of all swine diets fed to grow-finish pigs in Iowa, Vaughn estimates that producers are losing $14 million through inadequate grinding. To extrapolate this loss, if 57% of the state’s pigs are fed diets exceeding 750 microns, slightly more than 11 million pigs are eating a diet averaging 900 microns, which is 200 microns above optimal. Therefore, for each 100 microns exceeding recommended levels, that’s 65 cents x 2 = $1.30/pig × 11 million pigs, or roughly $14 million.

If your feed is not being ground to optimal micron size, Vaughn suggests turning dull hammers, checking screen size, replacing dull or worn rollers, adjusting roller speeds, or replacing rollers if corrugations are getting large.