Comparing high moisture corn processing equipment – Jon Urness, Vita Plus
A few years ago, Vita Plus set out to determine the correlation between adequate corn silage processing and starch utilization by dairy cows, measured by fecal starch. While a clear relationship was noted, a handful of cases showed adequate corn silage kernel processing and very high fecal starch percentages, upwards of 27 percent. Fecal starch percentage should, ideally, be less than 3 percent, so when fecal starch is measured at 27 percent, more than a quarter of manure dry matter (DM) is undigested starch.
That’s expensive and wasteful. So, if corn silage is processed well, yet fecal starch is still high, where is that undigested starch coming from? In a few cases, we had to look at the grain source to learn why starch was passing through undigested. That’s why it’s important to get all grains, especially high moisture corn, harvested at the right moisture, processed well, and stored properly to prevent spoilage.
Two of the most common types of high moisture corn processing equipment available are hammer mills and roller mills. Either can adequately process corn, but the choice sometimes boils down to availability. Dr. Leland McKinney, Kansas State University feed and grain scientist, has compared the two systems.
Hammer mills generally consist of two or more rotor plates fixed to a main shaft and enclosed in a screened grinding chamber. The hammers may be fixed or free-swinging and attached to the rotor assembly. As it rotates, the hammers impact and shatter the feed. In order to adequately process feed, the tip speed of the hammers must be very high, as much as 17,000 to 25,000 RPM. Wear points include the hammers and screens.
The resulting particle size of material processed through a hammer mill is determined mostly by screen hole size. The screen prevents the ground feed from leaving the grinding chamber until it reaches a given size.
McKinney described roller mills as single, double, or triple pairs of corrugated metallic rolls stacked and enclosed in a steel frame. Feed passes between the rolls and is sheared and compressed to reduce the particle size. The rolls generally turn at approximately 600 RPM, but they are set at differential speeds to increase shear force.
Is one design better?
Both systems will do a great job of processing high moisture corn if they are set up properly and receive regular maintenance. However, McKinney pointed out the following differences and considerations:
- Roller mills do not grind fibrous materials efficiently. Conversely, tub grinders are popular in some areas where long-stemmed hay is processed for inclusion in TMR rations.
- Excessive moisture can result in high heat (think burnt belts) in hammer mill applications.
- Roller mills can handle high moisture a little better, depending on particle size requirements.
- Roller mills produce a more uniform product in size with fewer fines than a hammer mill.
- Roller mills are sometimes considered more energy-efficient for coarse-ground material, but this advantage goes away when the target particle size is 600 microns or less.
- Roller mills are less expensive to purchase than a hammer mill of equal capacity, but that advantage often disappears because of the greater power required to operate a hammer mill.
- Bulk density of roller mill-produced material is typically 5 percent lower than hammer mill feed of similar mean particle size.
- Roller mill grain may not mix with vitamins, minerals and micronutrients as readily as hammer mill material because of the differences in flow characteristics of cubic shaped (roller mill) versus round shaped (hammer mill) particles.