Is particle size important for swine diets?

Cereal grains provide the primary energy source for swine diets. Producers must be concerned not only with the composition of the grain, but also how it is processed. Because feed cost represents approximately 80 percent of the cash cost of producing swine, feed utilization is critical for economic profitability.

Producers mixing feed on the farm may be able to improve whole-herd feed conversion by making minor adjustments in feed processing such as particle size reduction, or uniformly mixing feed. Particle size refers to the average mean diameter of individual particles of feed, or simply “the fineness of grind” of the feed. Particle size can be determined by passing the feedstuff through a series of sieves with progressively smaller openings.

Particle size reduction has a great impact on the efficiency of feed utilization. Decreasing particle size improves digestibility of nutrients by increasing surface area, which allows digestive enzymes in the pig’s digestive system to digest the nutrients in the feed. Digestibility of protein, energy, and other nutrients is generally improved with smaller particle size. Improving digestibility should result in improved feed conversion. In addition, particle size reduction can influence how uniformly feed is mixed and reduce the amount of segregation, which occurs delivering feed to the animal. It should be noted that the benefits of providing a well balanced diet could be lost if adequate care is not taken in manufacturing of the feed.

Although decreasing particle size can improve feed conversion, there are other factors to consider before determining what particle size the feed should be: 1) extremely fine particle size has been implicated as a cause of increased ulcers in swine; 2) dusty feeds may cause reduced feed intake, bridging of feed in feeders and bins; 3) associated with increased respiratory problems; and 4) higher feed processing costs.

Grain Type

The type of grain being fed will influence ideal particle size for swine diets. It appears that feed efficiency is improved with corn diets when average mean particle size is reduced to 460 microns. An improvement of 6–10 percent has been shown for corn ground to 460 microns versus coarse grain at 1107 microns. Little or no improvement in average daily gain should be expected. However, the incidence of ulcers in finishing pigs may be increased at the 460 micron particle size in corn. Research at Kansas State indicates that corn should be ground to a particle size of 700 to 800 microns for most pigs.

Grain sorghum may not improve feed efficiency by reducing particle size as much as corn based diets. Reducing particle size from 1217 to 539 microns resulted in 9 percent improvement in efficiency based upon research conducted at Kansas. Since grain sorghum and wheat tend to shatter more than corn, the proportion of extremely fine, dust like particles may be greater using the same feed processing equipment. To produce similar particle size of the small grains (grain sorghum and wheat) to that of corn, different milling equipment may be necessary. Small grains processed through a roller mill tend to have greater particle size uniformity and less fines or dust.

A Kansas State University survey of particle size analysis of feed samples over a five year period (1578 samples, 1986 to 1990) indicated that the majority of producers are possibly losing 3 to 8 percent feed conversion due to coarsely ground feed (Table 1). Only about 20 percent of the samples analyzed fell within the recommended particle size of 700-800 microns.

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 relative to roller mills:

  • easier maintenance
  • higher horsepower
  • process wider variety of materials
  • greater range of particle size
  • nosier
  • more dust
  • greater capacity per size

Hammer mills reduce particle size by 1) explosion from the impact of the hammers; 2) cutting by the edge of the hammers and screen; and 3) rubbing action or attrition. Advantages of hammer mills include their ability to handle any combination of grains and they have a low maintenance cost

If a hammer mill is used to process grain, there are several factors, which may be changed to increase or decrease particle size. The size of the openings in the hammermill screen greatly determines the size of particles which are produced. In general terms, feedstuffs that have passed through a 1/8 to 3/16-in. screen will have a mean particle size from 600 to 800 microns. Screens of 1/4 to 3/8 in. normally produce particle size from 800 to 900 microns. However, it is difficult to relate a screen size to specific microns due to the variations of equipment such as tip speed, wear, moisture content of the grain, etc.

Reducing the RPM’s generally produces a lower percentage of fines, although the time required to grind is increased. The number and condition of the hammers will affect fineness. Moisture content of the grain determines particle size. Low moisture grain will shatter more, creating more fines than normal moisture content of 10 – 12 percent. Increasing the flow rate of grain through the hammer mill will tend to increase particle size, but it may result in more variation of particle size.

Roller mills have the advantage of creating a more uniform particle size than a hammer mill. Roller mills normally have a higher initial purchase cost but are less expensive to operate. Other considerations are listed below:

  • maintenance requires re-grooving of rollers
  • low horsepower
  • used for small grains
  • narrow range of particle sizes produced
  • quiet
  • little dust produced when milling

Particle size may be controlled in a roller mill by the setting of the rollers, corrugations, spiral roller versus non-spiral and speed differential of the rollers. Roller mills are limited to non-fibrous products. Most roller mills will 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 will also recommend a differential drive of 10-25 percent with rolls turned so that the sharp edge of each roll meets the grain. Roll speed ranges from 350 to 600 RPM depending upon surface speed. This low speed will create less dust and wear. Roller mills can process grain with half the energy of a hammer mill and give equal particle size.

Feed Mixing

Mixing efficiency refers to how well a batch of feed is mixed. Various types of mixers will require different amounts of time to thoroughly mix feed. Vertical mixers are used most often on the farm. Portable grinder mixers are normally a single screw vertical mixer and will require about 15 minutes after the last ingredient has been added to adequately mix a batch of feed. Horizontal mixers require 5 to 10 minutes after the last ingredient has been added for proper mixing.

Items that will influence feed uniformity and increase mixing time include: particle size and density, moisture content of the grain, equipment wear, RPM of the mixer, overfill of the mixer, and buildup of fats and oils. Larger particle size or coarse ground grain may increase the mixing time and increase the amount of ingredient separation.

Animal Performance

Nursery Pigs

A total of 240 weanling pigs (avg initial wt of 11.7 lb) were used to determine the effects of particle size of corn and two sorghum genotypes on growth performance. Treatments were pelleted diets of corn, hard endosperm sorghum, and soft endosperm sorghum ground to particle sizes of 900, 700, 500, and 300 microns.

Reducing particle size increased electrical energy required for milling and decreased production rate. However, there were differences among the grain sources for energy required for milling and production rates. Grinding the sorghums to 500 microns required less energy than grinding corn to 900 microns (3.4 v 4.8 kwh/ton, 2.6 vs 1.9 ton/h, respectively). The most efficient gains were achieved at 300 microns for d 0-7, 300 to 500 microns for d 0-14 and 500 microns for d 0-35. Overall, pigs fed diets containing corn grew faster, consumed more feed, and were more efficient than those fed sorghum. Starter pigs fed corn had 15-20 percent greater ADG and 4 percent greater efficiency of gain than pigs fed the sorghums. This study suggested that as the pig gets older, optimum particle size may increase. Considering energy required for milling, production rate, and growth performance, milling sorghum and corn to 500 microns is recommended in pelleted diets for newly weaned pigs.

Growing Finishing Pigs

Seventy pigs (avg wt 119 lbs) were used to determine the effects of sorghum genotype on milling characteristics, growth performance, nutrient digestibility, and stomach morphology in finishing pigs. Treatments consisted of a control diet using corn ground to 600 microns, hard endosperm sorghum, and soft endosperm sorghum milled to mean particle sizes of 800, 600 and 400 microns and substituted for the corn in the control diet on a weight per weight basis.

The sorghums required less energy to grind, had greater production rates, and produced less noise during milling than the corn. Pigs fed the diets with hard and soft endosperm sorghum had average daily gain, average daily feed intake, and feed/gain similar to those fed corn. Pigs fed hard sorghum grew faster, but pigs fed soft sorghum were more efficient.

As particle size was decreased, energy required for grinding increased and production rate slowed. Efficiency of gain and nutrient digestibility was maximized and excretion of nutrients as feces was minimized at 400 microns for both hard and soft endosperm sorghum. Considering the positive effects of fine grinding on efficiency of gain and nutrient digestibility, but the negative effects on energy required for milling, production rate and stomach morphology, an acceptable compromise for particle size of soft and hard endosperm sorghum in pelleted diets for finishing pigs is recommended less than 600 microns.

Lactating Sows

One hundred primiparous sows were used to determine the effects of corn particle size in lactation diets on sow and litter performance. Treatments included corn diets in which the corn was ground to mean particle sizes of 1,200, 900, 600, and 400 microns. Particle size of corn had no influence on sow weight or backfat loss, or pig survivability. However, feed intake and digestibility’s of dry matter, nitrogen, and gross energy were increased (6, 5, 7, and 7 percent, respectively) as particle size was reduced from 1,200 to 400 microns. The combination of increased feed intake and improved digestibility’s resulted in increased intake of digestible nutrients. Digestible energy intake was increased 14 percent (13.72 to 15.60 Mcal/d) as corn particle size was reduced from 1,200 to 400 microns. Intakes of digestible dry matter and nitrogen were also increased (11 and 14 percent, respectively). The increased intake of digestible nutrients resulted in a 11 percent increase in litter weight gain. Reducing particle size increased severity of keratinization and lesions in the esophageal region of the stomach (although all treatment averages were low to moderate) and the change was not associated with reduced sow performance. Based upon the data in this study, nutrient intake of sows and litter weight gains can be increased by grinding corn for lactation diets to particle sizes of 600 to 400 microns.


Feed utilization is critical for economic profitability of a swine operation. Particle size reduction has a great influence on the efficiency of feed utilization. By decreasing particle size of feed the surface area is increased, which allows for more efficient action of the digestive enzymes to improve digestibility of nutrients. Improving nutrient digestibility should improve feed conversion. Also, particle size reduction can influence feed uniformity when mixed and reduce the amount of segregation occurring from mixing to the animal.

Extreme reduction of particle size has been implicated as a cause of increased ulcers in swine, reduced feed intake, bridging of feed in feeders and bins, associated with increased respiratory problems, and higher feed processing costs. Recommended geometric mean diameter of swine diets should include a range of 500 – 800 microns increasing the geometric mean diameter as animal age increases. Although, it should be noted that most on-farm manufactured feed will use only one particle size for all diets.

TABLE 1. Summary of On-Farm Particle Size Reduction

TABLE 1. Summary of On-Farm Particle Size Reduction
Particle size, microns
% of
3.2 20.3 41.2 28.1 6.5 .7