Particle size refers to the average diameter of individual particles of feed, or simply the feed’s “fineness of grind.” Cereal grains provide the primary energy source for swine diets; therefore, you must be concerned not only with the composition of the grain but also with how it is processed. Because feed costs represent about 80 percent of the cash cost of producing swine, wise feed use is critical for your economic profitability. If you mix feed on the farm, you may improve whole-herd feed conversion by making minor adjustments in feed processing such as particle size reduction or uniform feed mixing.
Reducing Particle Size
The benefits of providing a well-balanced diet can be lost if adequate care is not taken in the manufacturing of feed. Decreasing particle size has a great impact on the efficiency of feed use. Particle size reduction provides several benefits:
• improves digestibility of protein and other nutrients by increasing surface area
• improves feed conversion as a result
• influences how uniformly feed is mixed
• reduces amount of segregation when feed is delivered to animal
Fine particle size also has some negative associations, so these concerns must be considered before determining feed particle size:
• implication as a cause of increased ulcers in swine
• reduction in feed intake and bridging of feed in feeders and bins
• increased respiratory problems from higher dust levels
• increased costs of feed processing
The type of grain being fed influences the ideal particle size for swine diets.
Feed efficiency is improved with corn diets when average particle size is reduced to 460 microns. An improvement of 6 to 10 percent results when corn is ground to 460 microns when compared to corn that is coarse ground at 1,107 microns. Expect little or no improvement in average daily gain. The incidence of ulcers in finishing pigs may be increased at the 460-micron particle size in corn, however. Follow research recommendations for grinding corn to a particle size of 700 to 800 microns for most swine diets.
GRAIN SORGHUM AND WHEAT
Reducing grain sorghum particle size may not improve feed efficiency as much as corn-based diets. Research indicates that reducing particle size from 1,217 to 539 microns results in a 9 percent improvement in efficiency. Because grain sorghum and wheat tend to shatter more than corn, the proportion of extremely fine, dust-like articles may be greater with these small grains, even when using the same feed processing equipment. To produce similar particle sizes in small grains (grain sorghum and wheat) as in corn, different milling equipment may be necessary. Small grains processed through a roller mill usually have greater particle size uniformity and fewer fines or dust particles.
Feed Milling Equipment
Quality swine feed can be manufactured on the farm with many different types of equipment; however, the mill operator must fully understand the limitations of the feed processing equipment. Common methods of on-farm particle size reduction involve two types of mills: hammer mills and roller mills.
Hammer mills have the following characteristics when compared to roller mills:
• easier maintenance
• higher horsepower requirement
• wider variety of materials for processing
• greater range of particle sizes
• higher noise level
• increased dust
• greater capacity per size
Hammer mills reduce particle size by
• exploding from the impact of the hammers
• cutting from the hammers and screen
• rubbing action or attrition
Advantages of hammer mills:
• ability to handle any combination of grains
• low maintenance costs
Factors that Change Particle Size
If a hammer mill is used to process grain, several factors may be changed to increase or decrease particle size. The size of the openings in the hammer mill screen greatly determines the size of the particles that are produced. In general terms, feedstuffs that have passed through a 1/8- to 3/16-inch screen have an average particle size of 600 to 800 microns. Screens of one-fourth to three-eighths of an inch normally produce particles that are 800 to 900 microns. However, it is difficult to relate a screen size to specific microns because of equipment variations such as tip speed, wear, moisture content of the grain, and other factors.
Reducing the RPMs generally produces a lower percentage of fines, but it increases the time required to grind. The number and condition of the hammers affect fineness. Moisture content of the grain also determines particle size. Low-moisture grain shatters more, creating more fines than grain with a normal moisture content of 10 to 12 percent. Increasing the flow rate of grain through the hammer mill normally increases particle size and results in greater variation of particle size.
Roller mills have the advantage of creating a more uniform particle size compared with a hammer mill. Roller mills have a higher initial purchase cost, but they are cheaper to operate. Other considerations for using roller mills are as follows:
• regrooving of rollers required for maintenance
• lower horsepower
• used for small grains
• narrow range of particle sizes produced
• quieter than hammer mills
• less dust from milling
Controlling Particle Size
Particle size may be controlled in a roller mill by the space setting between the rollers, by corrugations, by spiral roller versus nonspiral roller, and by the speed differential of the rollers. Roller mills are limited to nonfibrous products. Most roller mills use a saw tooth or Dawson configuration. To get a particle size of 800 microns, it is necessary to have rollers with 10 to 12 grooves per inch. Most manufacturers also recommend a differential drive of 10 to 25 percent, with rolls turned so that the sharp edge of each roll meets the grain. Roll speed ranges from 350 to 600 RPM, based upon surface speed. This low speed creates less dust and wear. Roller mills can process grain using half the energy of a hammer mill and can give uniform particle size.
Mixing efficiency refers to how well a batch of feed is mixed. Larger particle size or coarse-ground grain may increase the mixing time and increase the amount of ingredient separation.
Various types of mixers require different amounts of time to thoroughly mix feed. Vertical mixers are used most often on the farm. Portable grinder mixers normally are a single-screw vertical mixer; a vertical mixer requires about 15 minutes after the last ingredient has been added to adequately mix a batch of feed. For proper mixing, horizontal mixers require 5 to 10 minutes after the last ingredient has been added.
Factors that influence feed uniformity and increase mixing time:
• particle size and density
• moisture content of the grain
• equipment wear
• RPM of the mixer
• overfill of the mixer
• buildup of fats and oils
Wise feed use is critical for economic profitability of a swine operation. Particle size has a great influence on the efficiency of feed use. By decreasing the particle size of feed, the surface area is increased, allowing for more efficient action of the enzymes that improve the digestibility of nutrients. Improving nutrient digestibility should improve feed conversion. Also, particle size reduction can influence feed uniformity when mixed and can reduce the amount of ingredient segregation that occurs between the mixing and the animals’ feed consumption.
Follow the recommended average diameter of particles in the swine diet of 500 to 800 microns. Increase the geometric, mean diameter of particles in the diet as the animals age.
By Mark Crenshaw, Extension Animal Scientist
Information Sheet 1633
Mississippi State University Extension Service