Results and Discussion
It is important to analyze the distribution of particle sizes that make up the average particle size. Average microns in this trial were determined by using a series of screens: 1700, 1000, 710, 500, and 250 microns, respectively. Optimal grinding is described as low variability of particle sizes and the highest percentage of the ground corn particles between 1000 and 500 microns. When graphed, an ideal grind will result in a bell shaped curve that describes the desirable particle size range. In other words, we want a minimum amount of particles over 1700 microns and under 250 microns. See examples below.
Figure 1 illustrates a corn sample that with an average of 1001 microns and a very high percentage of 1700 and 1000 micron size particles. Note the difference in percentages of 1700 and 1000-micron size particles when you compare Figure 1 to Figure 2. Nearly 60% of the ground corn particles for Figure 1 are larger than 1000 microns, an average that is higher than the desired size. Figure 2 shows approximately 18% of the particles larger than 1000 microns and results in a much more desirable distribution of particle sizes. The consistency of particle size in a feed mix helps maintain the integrity of the feed as it is mixed, unloaded, conveyed, and fed down in feeders.
Corn is the major ingredient in most compete feeds and particle size of ground corn has a large impact on average particle sizes in the complete feed. Soybean meal usually makes up the next largest ingredient by weight and can impact micron averages of complete feed. The impact of micron size of soybean meal on feed efficiency is not well documented, but is generally considered not to have an adverse affect on feed efficiency. Soybean meal is a further processed product and regardless of particle size, soybean meal is thought to be highly digestible. Some producers do grind soybean meal to ensure uniform particle size and greater feed integrity.
Soybean meal samples were obtained from 18 producers and feed mills for micron analysis. The results showed a variation in average microns ranging from 746 to 1340 microns. Figure 3 shows the micron range from lowest to highest for the 18 soybean meal samples and reflects an average of 1002 microns.
As particle size decreases, feed efficiency improves and incidence of gastric ulcers increases. This is illustrated above in the conceptual model (Figure 4). The incidence of gastric ulcers determines the lower limit of particle size. Researchers in Kansas reported that incidence of gastric ulcers increased from zero to 50% of examined pigs when particle size was smaller than 600 to 800 microns. Very small particle size also creates problems with feed bridging in bins and feeders.