How fine do you grind?

To produce pellets or extruded feeds of acceptable quality the particle size of the ground materials must be correct.
Finer grinding will result in a better-quality pellet or extruded feed increases the capacity of the pellet mill or extruder, and reduces wear of the pellet mill or extruder working parts such as dies, rollers, and worms.

Because animal needs vary considerably, the degree of processing for various diets also must be different.

Ruminant animals such as cattle and sheep have rather long, complex digestive tracts and so require a less processed feed material. On the other hand, many of the ingredients used in ruminant feed pellets consist of low protein, high fibre material so fine grinding may be required to achieve a reasonable pellet quality.
Swine have a short, simple digestive system (much like humans) and therefore benefit from a more highly processed feed, while poultry has a short but rather complex digestive system and, depending on the makeup of the diet, can efficiently utilize feedstuffs less highly processed than swine.
Although it has been postulated that finer grinding increases substrate availability for enzymatic digestion, there is evidence that coarser grinding to a more uniform particle size improves the performance of birds maintained on mash diets (and in lower but still significant way in pellet diets). This counter-intuitive effect may result from the positive effect of feed particle size on gizzard and gut development. A more developed gizzard is associated with an increased grinding activity, resulting in increased gut motility and greater digestion of nutrients.

The size and the age of the animals also affect the dietary requirements so far as particle size is concerned. Younger animals require a finer, more highly processed feed than do older, more developed livestock.
Anyway, 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, because 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 sizes produced.
Factors such as moisture content of the grain, a 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 processed can have a dramatic impact on the fineness and quality of the finished ground products.