In the feed manufacturing industry, particle size reduction (grinding) is second only to extrusion
processes (pelleting, expanders, extruders) in terms of total energy consumption. Any discussion of
grinding then must first deal with the question, "Why grind?"
Why process at all? Of course the answer is ultimately feed efficiency, producing the most milk, eggs,
meat, or fiber at the lowest possible cost. This first step in the feed manufacturing process works
towards the goal of improved feed efficiency by increasing the surface area of the materials being
processed. This increases the amount of materials exposed to the animals digestive system, and
ultimately leads to more complete digestion, thus better feed efficiency.
Because animal needs vary considerably, the degree of processing for various diets also must vary.
Cattle and sheep have rather long, complex digestive tracts and so require a less processed feed
material. Swine have a fairly short, simple digestive system (very much like humans) and therefore
benefit from a more highly processed feed. Poultry have a short but rather complex digestive system,
and depending on the make up of the diet can efficiently utilize feedstuffs less highly processed than
swine. The size and age of the animals also affects the dietary requirements so far as particle size
is concerned. Generally speaking, younger animals require a finer, more highly processed feed than
do older, more developed livestock.
Particle size reduction is also required to prepare the materials for secondary operations such as
mixing, pelleting, or extrusion. In general, a finer ground material or mixture will produce a better quality
pelleted or extruded feed at a lower cost (energy
and maintenance) on the pelleting or extruding
machinery. Of course more energy per ton is
required to achieve the finer grind and often the
capacity of the grinding equipment will be reduced.
The fineness of the grind must be matched to
both the particle size and capacity requirements
of the entire feed manufacturing process.
Due to the wide variety of feed ingredients and
compounds, there is no "ideal" particle size for
each type and age of animal. The chart presented
below is simply an indication of the generally
accepted particle size ranges for all types of
animals and feeds.
How Fine Do You Grind?
Determining and expressing fineness of grind has been the subject of study as long as feed ingredients
have been prepared. While appearances or feel may allow an operator to effectively control a process,
subjective evaluation is inaccurate at best, and makes objective measurement and control virtually
impossible. Descriptive terms such as coarse, medium, and fine are simply not adequate. What is
"fine" in one mill may well be "coarse" in another. Describing the process or equipment is also subject
to wide differences in terms of finished particle size(s) produced. Factors such as moisture content
of the grain, condition of the hammers and/or screens (hammermill) or the condition of the corrugations
(roller mills) can produce widely varying results. In addition, the quality of the grain or other materials
being process can have a dramatic impact on the fineness and quality of the finished ground products.
The best measurement of finished particle sizing will be some form of a sieve analysis, expressed in
terms of mean particle size, or percentage (ranges) on or passing various test sieves. A complete
sieve analysis will not only describe the average particle size, but will also indicate peculiarities in the
distribution such as excessive levels of fine or coarse particles, etc. Typical descriptions that lend
themselves to objective measurement and control might be: corn ground to 750 Microns, not less
than 50% passing 20 mesh, and 85% -10/+40 mesh.