The proper size of particles in the feed product affect animal feed efficiency, gut health, and ration manufacturing cost. Both the average size of particles as well as uniformity of size can be important, depending on the species, animal production targets, and milling capability. In manufacturing both protein concentrates and complete feed products for pigs and poultry, the optimum feed particle size is a balancing act.
The balancing begins with the formula, which may specify mean particle size and uniformity of grind or particle size distribution. Feed plants of post-grind design may have the capability to apply grind uniformity criteria across most of the components of the ration. In the case of pre-grind plants, attention to particle size applies mainly to the macro ingredients, for example maize and soya. Most research in connection with particle size and gut health focuses on the grain component as the major part of the ration for monogastrics.
Much of the research on feed particle size, feed efficiency, and gut health involves pigs where problems can emerge over the longer grow-out period. Early research from the USA and Europe points to how feed particle size affects digestion, such that finer particles improve nutrient absorption but can encourage keratinisation or erosion of digestive tract tissues, possibly leading to ulceration.
American and Danish research
The main American work comes from Kansas State University (KSU) over the past two decades, showing that for corn-based pig diets there is a 1.0-1.5% improvement in feed efficiency for every 100-micron reduction in average particle size over the particle size range of 1200-400 microns. Other work shows that the risk of tissue keratinisation and ulceration can increase with finer particle sizes. Researchers still need to determine the importance of average particle size versus particle size distribution and whether small amounts of coarse material can overcome the detrimental gut effects of fine particles. In any case, grinding more finely is costly, so KSU recommends an optimum particle size for grow-finish pigs of 700 microns, with the advisory that narrower particle size distribution or greater uniformity in particle size is preferable.
More recent work by Drs Knud Erik Bach Knudsen, Lene Lind Mikkelsen, and others at the Danish Institute of Agricultural Sciences at Tjele suggests that feed structure, which relates to both particle size and type of plant material, has significant effects on pathogenic bacteria in the pig gut, particularly salmonella species. Some of the Danish research suggests that feeding a coarsely ground meal to pigs changes the physico-chemical and microbial properties of stomach contents, which decreases the survival of pathogenic micro organisms such as salmonella. Other Danish research suggests pelleted rations for pigs may cause secretion of mucins that can bind pathogenic salmonella organisms and promote colonisation and disease. By contrast, the work suggests, coarsely ground, non-pelleted feed may provide some protection against salmonella infections.
The trend favouring home-mixed feeds for pigs in the 1990s, especially in Denmark and Germany, applied the theory that pelleting, which enables the agglomeration of fine particles, may cause or promote ulcers. Recent investigations, however, have pointed to possible multiple factorsnot just fine particle size. For example, Brazilian researchers have found Helicobacter heilmannii in a group of heavily ulcerated pigs. A related species of bacteria, Helicobacter pylori, has been implicated in contributing to ulcers in humans.
Besides feed efficiency and gut health, particle size characteristics play a role in nutrient excretion, especially from pigs. At a recent London Swine Conference in Ontario, Canada, Drs Janice Murphy (Ontario Ministry of Agriculture and Food) and Kees de Lange (University of Guelph) noted: "Proper processing of feeds represents a very practical means to positively impact nutrient excretion through improvements in feed digestibility. Particle size is an area where producers can significantly improve feed efficiency."
Canadian research had shown a rise in average daily gain from 0.79 kg to 0.96 kg when particle size was reduced from 950 microns to 750 microns. These data tended to support the KSU research which found that feed not ground fine enough could result in feed efficiency losses of 3-8%.
In a survey of Ontario swine farms, more than 90 farms feeding home-mixed rations based on dried corn averaged 880 microns per farm, with a range from 690 microns to 1467 microns. This survey was a joint project with Kenpal Farm Products and the University of Guelph, exploring the relationship between corn particle size and ulcers in pigs. A previous Ontario survey in 1990 had found that the average particle size of feed mixed on-farm was approximately 1000 microns.
While the Ontario pig producers had reduced feed particle size somewhat, they had not been convinced to cut particle size to an average of 700 microns and to monitor it carefully.
The Canadians also noted work at Prairie Swine Centre in Manitoba which indicated that reducing particle size from 900 microns to 600 microns was effective in reducing faecal nitrogen (N) by 11% but not total N excretion. "This is likely," Drs. Murphy and de Lange commented, "because feed intake was not reduced with decreasing particle size and excess digestible protein intake was broken down after absorption and excreted in urine."
Too-large feed particles may lead to other problems, too, suggests Vigortone nutritionist Dr Dennis Wilson, based in the US state of Iowa. Pigs tend to separate large particles in the feeder, which then accumulate in the trough, becoming stale. If the particles become wet and mould, the resulting mycotoxins also can reduce feed intake and cause poor growth. But feed particle size that is smaller than 600 microns can pack in feed bins, feed lines, and feeders, Dr Wilson cautions, causing bridging or binding which can interfere with feed intake.
Even though home-mixing producers may intend to grind at the optimal particle size, there are many other variables on-farm that could increase particle size variability. Canadian researchers have suggested that something as simple as moisture content of the corn, for example, might explain how particle size could vary as much as 300-400 microns between farms using the same size hammer mill screens. Others have pointed out that there is no universal screen size that will produce uniform particle size in every situation. In any case, as hammer mill hammers and screens wear, they yield increasingly variable particles. Feed mill operators, who work with the grinders every day, are in a better position to monitor and maintain the hammer mill.
Research into the optimal balance of particle size, disease avoidance, and milling efficiency continues and may be spurred by a rise in coarse-texture grinding in North America, as has occurred in some parts of Europe.