Reducing the particle size of cereal grains and soybean meal in diets fed to pigs improves the digestibility of nutrients and feed efficiency. This is because feed ground into smaller particle sizes has more surface area on which digestive enzymes can work.

Particle size analysis is relatively simple and small increases in digestibility may represent huge economic gains when multiplied by the amount of feed used in a farm. Thus, the particle size of your diets is something you need to “keep an eye on”. But how is particle size measured and what should we aim for? For particle size determination, feed is run through a set of sieves of different sizes using an electric shaker and the proportion of particles retained by each sieve is weighted. The most common measure for particles is the “average particle size” expressed in micro-meters, also called microns.

In general, it is recommended that pig diets are around 600 to 700 microns. However, sometimes the average particle size may be within these limits, but the diet may still have problems with particular ingredients. That is why it is recommended to measure the particle size of the ingredients, where possible, instead of the complete diet.

Maize is the ingredient that most of the work investigating the importance of particle size has been carried out on. It is clear that every decrease in particle size increases feed efficiency.

Energy

Reduction of maize particle size from 1,000 microns to 600 can increase the available energy by more than 5%. However, reducing particle size below 600 microns could slow down milling throughput and increase the energy consumed to too high a level. Very low particle size has also been linked to ulcer problems.

After maize, soybean meal particle size is of next importance. In a trial carried out in the US, soybean meal was used at average sizes of 900, 600, 300 or 150 microns. The digestibility of essential amino acids increased from 91% to 92.4% as the particle size decreased. A particle size of 600 microns showed the largest improvement in digestibility when compared against the 900-micron treatment. There were further improvements at the smaller particle sizes, 300 and 150, but these required several passes through a hammer mill. Researchers calculated that for every 100 microns increase particle size above the recommended range the cost to feed efficiency was about 50c/pig.

Many producers may use soybean meal as it is received, without further grinding and, in some cases, the particle size is fine. However, soybean meal is variable, depending on the source. Ohio State University reports that commercial soybean meal samples often average a particle size between 800 and 900 microns, which suggests that amino acid digestibility, might be improved with a smaller dimension. However, samples recently analysed by Teagasc’s pig development department were mostly around 1,100 microns, which makes grinding even more necessary. If soybean meal is passed once through a hammer mill, it should reduce particle size to below 600 microns. The difference can be seen easily in the pictures taken on an Irish farm. The picture shows soybean meal before (picture one) and after (picture two) passing through the mill.

There is less information on other ingredients such as wheat or barley. In general, barley creates bigger particles than wheat. In a study using barley, reducing the particle size from 1,100 to 900 microns did not show improvements in FCE. However, when it is decreased to 700 microns, the FCE decreased from 3.6 to 3.3, which is a significant decrease.

If ingredients cannot be analysed separately, then it is worth analysing the complete diets. Picture three shows a diet with ideal particle size (700 microns), while picture four shows a coarse diet (1000 microns).

The difference may not be obvious to the eye. However, if we take the coarse diet and separate it into three fractions, we can see in picture five an important fraction that will most probably not be used by the pig. That would change the nutritional value of the diet.

Ensuring that ingredients are of proper particle size can be accomplished through regular analysis of the feed and routine maintenance, such as changing hammer mill screens or turning hammers. Although it takes more time, particle size can also be analysed in pellets by soaking and drying the sample before measuring.