Like ground corn before it, the particle size of soybean meal and its effect on digestibility is gaining the attention of university researchers.

A study reported in the Journal of Animal Science by Ohio State University nutritionist Don Mahan and graduate student Nathan Fastinger looked at the effects of reducing the particle size of soybean meal on amino acid and energy digestibility in grow-finish diets.

The treatments included the use of two soybean meal sources and soy protein concentrate. One source was processed through a 60-hp hammer mill to achieve average sizes of 900, 600, 300 or 150 microns. The second source was a common commercial meal averaging 800 microns. The soy protein concentrate had an average particle size of 385 microns.

Particle size is measured in micro-meters, also called microns, and studies have shown an affect on digestibility. Smaller particles have an increased surface area that allows digestive enzymes to more effectively release nutrients.

“As the particle size is reduced, the surface area increases dramatically,” explains Fastinger (see diagram). “The greater the surface area, the more substrate is available to the enzyme. This generally means higher digestibility and better feed efficiency.” Between 600 to 700 microns is typically recommended for most swine diets.

Fastinger points out that commercial soybean meal samples often average a particle size between 800-900 microns, which suggests that amino acid digestibility might be improved with a smaller dimension. Kansas State University researchers determined that a range of 650 to 750 microns is optimal for corn.

When soybean meal is passed once through a hammer mill with a ⅜-in. screen, particle size is reduced to around 450-500 microns. Fastinger calls that the most practical application.

Research Trial

Crossbred barrows in the study were surgically fitted with a T-cannula at the distal ileum of the small intestine and fed treatment diets formulated to 17% crude protein. The ileal digestibility technique is more accurate, since it reduces the confounding effects of microbial populations in the colon and large intestine, according to the Ohio researchers. Those microbes metabolize nitrogen causing an amino acid profile that differs from that exiting the ileum.

The Ohio researchers found that true digestibility of isoleucine, methionine, phenylalanine and valine increased as particle size decreased. When the essential amino acids were averaged, true digestibility increased from 91% to 92.4% as particle size decreased. Energy digestibility was not affected by particle size. Lysine showed a numerical increase in digestibility, but not a statistical difference.

A particle size of 600 microns showed the largest improvement in digestibility when compared to the 900-micron treatment. There were further improvements in amino acid digestibility at the smaller particle sizes, 300 and 150, but these required several passes through a hammer mill.

Fastinger has not pursued on-farm testing of particle size, but it could be adapted into an on-farm grinding procedure, he says. Most producers use soybean meal as it is received from soy oil processing plants, without further grinding, and in most cases it is sufficient in particle size.

Particle size can have a huge economic impact on cost of production, according to the Kansas research. For every 100 microns particle size is greater than the recommended range, the cost to feed efficiency is about 50¢/pig. For example, if particle size has crept up to 1,000 microns, reducing size to 700 microns will save $1.50 for every finishing pig marketed.

Ensuring proper particle size can be accomplished through routine maintenance like changing hammer mill screens or turning hammers. Adjusting the gap between rolls and regrooving rolls in roller mills should also be done regularly.