Pellet quality: Defining a fine
What constitutes a fine not only by a particle’s physical characteristics, but also based on the potential impact of its size (and possibly its composition) on animal performance?
By Charles Stark, Wilmer Pacheco and Adam Fahrenholz
The feed industry has been measuring pellet quality (percentage of fines in a finished feed) based on information contained in a standard method for over 50 years. ASAE Standard S269.1 was first introduced to the feed industry in 1969, and while it has most commonly been associated with tumble-box testing related to determining the pellet durability index (PDI), it also set the standard definition for what were pellets and fines. The standard described a fine by stating “Fines shall be determined by screening a sample on a wire sieve having openings just smaller than the nominal pellet diameter”. The standard recommended sieves for common sizes of crumbles and pellets, for example a 5/32” (4.0 mm) pellet should be screened over a #6 sieve (3,360 microns) and a 3/16” (4.8 mm) pellet would be screened with a #5 sieve (4,000 microns). Accordingly, the definition of fines for the 5/32” and 3/16” pellets would be anything smaller than 3,360 and 4,000 microns, respectively. The current method ASAE S269.5 (2012) uses the same table as the original method and therefore this has been the basis to define pellets and fines percentage in commercial and integrated operations as well as animal research studies since the standard was introduced.
While the standard was developed to help the feed industry define the physical characteristics of pelleted feed, the development of the method did not take into consideration how the size of a fine affects animal performance. Similarly, the majority of the research conducted on pellet quality in either swine or poultry has used S269.x to define a fine and has not evaluated how animals perform based on different sizes of particles in the feed other than whole pellets. Today, some companies may define pellet fines as particles less than 2,000 or 1,000 microns, which is significantly different than the standard. These differences in the definition of a pellet fine makes it difficult to fully compare the effects of pellet quality on animal performance.
The introduction of the Holmen method in 1980 added another potential definition of a pellet fine, depending on how the test was run in the Holmen Pellet Durability Tester. The modern Holmen test procedure using the NHP series of equipment consists of screening 100 grams of whole pellets into the tester, running the test for a specified time, and then weighing the whole pellets that are removed from the chamber perforated with 1.5 mm holes. Therefore, the Holmen durability procedure defines fines as particles less than 1,500 microns, which could also include large corn particles (particularly noticeable when the effect of corn particle size on animal performance is being evaluated). Adding an additional wrinkle, some feed mills will not only screen the pellets going into the tester, but will also subsequently rescreen the pellets in the basket, which normally lowers the PDI values by 5-8% based on the definition of the fines.
Researchers have reported that the addition of fines to broiler diets reduced performance in all diet phases (Nir et al., 1994; Engberg et al., 2002; Svihus et al., 2004; Corzo et al., 2011; Lemons and Moritz, 2016). Swine researchers have reported similar results in swine performance (Stark, 1994; Wondra et al., 1995; Schell and van Heugten, 1998; Nemechek et al., 2015; and Badger, 2023). However, the definition of what constitutes a fine has not be well defined in these studies. With all of this in mind, a worthwhile challenge for the next generation of feed and nutrition researchers will be to determine what constitutes a fine not only by a particle’s physical characteristics, but also based on the potential impact of its size (and possibly its composition) on animal performance.
