Grinding feedstuffs increases their energy and nutrient digestibility, because the reduced particle size provides more surface area for digestive enzymes and microbes to act on. Currently, nutritionists recommend feeding corn ground to an average particle size of 650 to 700 µm. However, research has shown that corn ground to smaller particle sizes contains more metabolizable energy than corn ground to larger particle sizes, which leads to greater feed efficiency.

An experiment was conducted to test the hypothesis that diets containing corn ground to reduced particle size can be formulated with less fat than diets containing corn ground to a greater particle size without compromising growth performance or carcass characteristics.

A total of 72 growing pigs with an average initial body weight of 32.0kg were used in the experiment. Pigs were fed on a three-phase program. Within each phase, four corn-soybean meal based diets were formulated using corn ground to 339, 485, 677, or 865 µm. The diets were formulated to contain the same amount of metabolizable energy by adding soybean oil in varying amounts, with more oil added as corn particle size increased to compensate for the reduced concentration of metabolizable energy in diets containing the corn ground to the larger particle size.

Body weight was recorded at the start of the experiment, on days 29 and 58, and at the end of the experiment on day 93.  Daily feed allotments and orts were recorded and used to calculate average daily feed intake and gain:feed ratio. Live body weight prior to slaughter, hot carcass weight, back fat thickness, loin eye area, and loin pH were measured.

At day 29, day 58, day 93, and for the period from days 0-93, no differences were observed in body weight, average daily gain, average daily feed intake, or gain:feed ratio among pigs fed the experimental diets (Table 1).

No differences in fat free lean percentage, back fat thickness, loin eye area, or loin pH were observed among pigs fed the experimental diets (Table 2). There was a tendency (P = 0.06) for greater dressing percentage in pigs fed corn ground to the smallest size compared with other sizes, in part because viscera weight was less in the pigs fed corn ground to the smallest size.

• When diets are formulated to contain the same amount of metabolizable energy, feeding diets containing corn ground to different sizes has no effect on growth performance.
• Particle size of corn in the diets does not affect most carcass characteristics, including fat free lean percentage, back fat thickness, loin eye area, and loin pH.
• Diets containing corn ground to smaller particle sizes can contain less fat, and may therefore reduce feed costs compared with diets containing corn ground to larger particle sizes.

Table 1. Growth performance of pigs fed diets containing corn that was ground to different particle sizes

Table 2.Carcass characteristics of finishing pigs fed diets containing corn that was ground to different particle sizes

^a-bMeans within a row lacking a common superscript letter differ (P < 0.05).

This report is based on unpublished data by O. J. Rojas and H. H. Stein.