The compression test methods more generally employed in the routine evaluation of
pellet attrition more or less reflect a mixture of measuring fines produced by fragmentation
and abrasion rather than what is normally distinguished by the terms ‘hardness’ and
‘durability’, respectively. There may be a relationship between hardness and durability
(Wood, 1987) and between the results obtained by different devices for durability
(McKee, 1990). However, these relations may only hold for a given feed composition.
The relationship is strongly influenced by the diet ingredient composition of the pellet
and the pelleting conditions used. With all devices applied, pellets are subjected to a
change in particle size distribution. Therefore, the addressed question should not be
“what is the hardness or durability of feed pellets” but from a scientific point of view
“which combination of fragmentation or abrasion reflects at best the way feed pellets
are handled, conveyed or stored”. The subsequent choice for a type of device also
depends on the outline of the factory, whether mechanical or pneumatic transport is
used, the form and shape of storage bins and, last but not least, the ultimate consumer’s
demand (whether man or animal) for a certain physical quality of pellets. For instance,
Skoch et al. (1983) found differences in feeding preference of pigs, when feeds in
different forms were given in a ‘choice-feeding’ experiment. Pigs preferred pellets over
meal but softer pellets over harder pellets.