Grinding of feedstuffs to small particle sizes is a low-cost way to increase their energy and nutrient digestibility. Currently, nutritionists recommend feeding corn ground to an average particle size of 650 to 700 µm. However, it may be advisable to formulate diets containing corn ground to smaller particle sizes due to the greater metabolizable energy (ME) values of these diets. A previous experiment conducted by Rojas and Stein at the University of Illinois demonstrated that when diets are formulated to contain the same amount of metabolizable energy, feeding diets containing corn ground to different sizes to weanling pigs did not have a negative effect on growth performance.

An experiment was conducted to test the hypothesis that when diets are not adjusted to contain equivalent amounts of metabolizable energy, weanling pigs fed diets containing corn ground to smaller particles sizes will have an improved gain to feed ratio relative to pigs fed corn containing corn ground to larger particle sizes.

Four diets based on corn, soybean meal, and 4% fish meal were formulated, using corn ground to a particle size of 339, 485, 677, or 865 µm. The diets were formulated to meet or exceed current nutrient requirements. The diets were not adjusted to achieve uniform ME values, so the diets containing more finely ground corn had greater ME values than the diets containing more coarsely ground corn. A total of 128 pigs with an average initial body weight of 9.41 kg were used. Pigs were weaned at approximately 3 weeks of age and had been fed a common phase 1 diet for 2 weeks before being allotted to diets in this experiment.

Individual pig body weight was recorded at the start of the experiment and at the end of the experiment on day 20.  Daily feed allotments as well as feed left in the feeders were recorded and used to calculate average daily feed intake and gain:feed ratio. Fecal samples were collected on day 20 to measure pH, and blood samples were taken on day 20 to measure plasma urea nitrogen (PUN).

There were no significant differences in average body weight at day 20 or average daily gain among pigs fed diets containing corn ground to different particle sizes (Table 1). As particle size decreased, average daily feed intake was reduced (linear, P < 0.05) and the gain:feed ratio increased (linear, P < 0.01).

The pH in colon contents was reduced (linear and quadratic, P < 0.05) as particle size increased. This indicates that more volatile fatty acids were produced by pigs fed corn ground to large particle sizes compared with pigs fed the diets containing corn ground to smaller particle sizes, likely a result of increased starch fermentation in the colon. Plasma urea nitrogen levels did not differ among pigs fed the different diets, indicating that amino acid utilization was not significantly affected by particle size.

• Feeding diets containing corn ground to different sizes to weanling pigs had no effect on average daily gain. However, as particle size was reduced, the average daily feed intake decreased and the gain:feed ratio increased.
• Grinding corn to smaller particle sizes results in improved feed efficiency, and reduced diet costs.
• Grinding corn to smaller particle sizes does not affect amino acid utilization.

Table 1. Growth performance, colon content pH, and plasma urea nitrogen (PUN) of pigs fed diets containing corn ground to different particle sizes

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