Feed producers must be concerned about the composition of the cereal grains in the feed and also closely manage the processing steps so that the animal may fully utilise the nutrients. Particle size reduction is an aspect that is of major importance, since it increases the surface area of the grain and thus allows for greater interaction with digestive enzymes. It has been studied for many years now, but still new data become available.
By Dick Ziggers
Particle size analysis is the study of the size distribution of the particles of a powdery material and is closely related to key-processes such as grinding, fractionation and mixing. In addition, size distribution is related to feeding behaviour and digestion of the feed particles by animals.
Since the particles have irregular shapes, size measurement is defined as the diameter of the theoretical sphere whose behaviour is similar to that of the real particle. The distribution is then fitted on theoretical laws. The fineness of the meal is described by the mean diameter and the heterogeneity of the distribution by the standard deviation. Among the main techniques commonly used are sieving (in dry or wet conditions), laser light diffraction and imaging. These last two techniques are non invasive and fast techniques which can be used on-line.
Most of the feed ingredients used in feed manufacturing are subjected to particle size reduction either within the feed plant or prior to receiving. The greatest benefits from particle size reduction in feed manufacturing processes are related to:
1. Greater surface area for faster digestion
2. Improving ease of handling of some ingredients
3. Improving mixing characteristics of ingredients
4. Increasing pelleting efficiency and pellet quality
5. Meeting customer performance for feeds
A lot of energy has been put into particle size research in pigs at Kansas State University (KSU) in the beginning of this century. Here it was found that the young pig does a better job of chewing its feed than growing-finishing pigs. The greatest potential for fine grinding to improve feed efficiency will be for finishing pigs, nonetheless fine grinding or rolling will improve feed efficiency regardless of age. This improved feed efficiency appears to be a result of improved nutrient digestibility. Average daily gain does not seem to be effected by reduced particle size.
Usually feed intake is reduced with reduced particle size. However there seems to be an optimum particle size between 500 and 700 microns. Pigs fed grain ground to 500 microns had a 6% improvement in feed efficiency compared to pigs fed diets containing grain ground to 900 microns. On the other hand production rate in the mill is reduced when particles sizes is decreased from e.g. 700 to 500 microns. The decision on optimum diet particle size needs to include assessment of improvements in feed efficiency versus reduction in milling production. KSU researchers suggest a diet particle size of approximately 700 microns to optimise both pig performance and milling efficiency.
Type of grain counts
Not all grains should be reduced to the same particle size. Studies with high-fibre cereals such as barley indicate that fine grinding of these ingredients may greatly improve their feeding value. Research data indicate grinding of fibrous feed ingredients to approximately 700 microns improves their feeding value and may make them more attractive as substitutes for corn and sorghum.
Wheat needs to be treated slightly differently. Because of its high protein content and propensity to become floury, it presents some unique processing problems. If ground too fine, wheat can reduce feed intake. Therefore in pig diets wheat should be ground coarser than corn or sorghum, between 800 and 900 microns.
One of the known disadvantages of fine grinding is the increased incidence of gastric ulcers. The frequency of ulceration increases when particle size drops below 500 microns. Other disadvantages of fine grinding include bridging problems in bulk bins and feeders as well as increased dustiness of the feed.
Particle size in practice
KSU studies in the US showed that two-thirds of compound feeds sampled were in the recommended 600 to 800 micron range. Similar research was conducted by an Ohio State University (OSU) who found that the average micron size of feed ground in a hammer mill was 916 microns. Roller mills averaged 720 microns. The location of feed manufacture was also significant. On-farm processing yielded an average of 881 microns. Feed mill processed feed averaged 755 microns. On-farm hammer mills produced a wide range of particle sizes. There was also a significant difference in average particle size of feed from on-farm or feed mill operated hammer mills. On-farm grinding averaged 1,029 microns and the feed mills averaged 730 microns.
The economic impact of particle size can be significant. If a 100-sow, farrow-to-finish operation has an average diet cost of $130 per tonne and reduces particle size from 1,100 to 750 microns, this would result in a savings of app. $4,750 per year based on improved feed efficiency.
Researchers in Denmark investigated the effect of particle size (fine and coarse) and feed processing (pelleted vs. non-pelleted) on the villi in the intestines and the adhesion of Salmonella enteric Typhimurium DT12 to the intestines of pigs. They found that the effects of particle size and feed processing on villus height and crypt depth in the small intestine were minor. Feeding coarse diets increased the crypt depth in the colon. Using a pig intestine organ culture model, Salmonella adhered less (60%) to the ileal tissue of pigs fed the non-pelleted diets than to those fed pelleted diets. Their general conclusion was that pigs fed a non-pelleted diet are better protected against Salmonella infections than pigs fed a pelleted diet.
Researchers in Tarragona, Spain looked at feed preferences in pigs and the relationship with feed particle size and texture. They concluded that the texture properties of the feed could explain in part the feed preferences observed in pigs, whereas particle size characteristics had less impact.
Particle size relevance
Guillou and Landeau from French cooperative Ucaab, compiled a quantitative database with data from 23 scientific papers and internal reports, in order to derive response laws to particle size variation for nutritional value. General conclusions from their survey: in growing swine (weaned piglets and growing-finishing pigs) energy faecal digestibility is reduced by 0.6 unit and nitrogen faecal digestibility by 0.8 unit when particle size increases by 100 μm (micron). Among technical parameters, only feed conversion (FCR) of piglets is related to particle size: +100 μm in average particle size increased FCR with 0.03 unit. These ranges are low compared with other sources of variation in digestive or metabolic use of diets.
Poultry prefer larger particles
Day-old chicks learn to associate nutritional effects with the sensorial characteristics of feed particles, thanks to a precise visual observation of details and specific tactile capabilities of their beak. Selection of feed particles is fast and accurate. However this selection may vary according to the sensorial experience of the bird concerning the feed. Eating rate depends on the size and hardness of the pecked particles.
In poultry diets the effects of particle size appear to be puzzled with complexity of the diet as well as further processing such as pelleting or crumbling. The response to reduced particle size (600 to 500 microns) in broiler chicks appears to be greatest when fed simple (corn-soybean) diets in meal form. Feeding a complex diet in crumbled form did not appear to require particle size below 1,000 microns. Studies with laying hens suggest that there is no advantage in reducing particle size below 800 microns.
In New Zealand Amerah et al (2007) studied the influence of feed particle size and feed form in broilers. Bird performance was superior in birds fed pelleted diets compared with those on mash diets.
Pelleting evened out the differences in particle size distribution between treatments and, as a result, wheat particle size had no effects on the performance of broilers fed pelleted diets. In mash diets, coarse grinding of wheat improved weight gain and feed:gain compared with medium grinding. Pelleting had a negative effect on AME (n). Overall, the results showed that feed form had a greater influence on the different measured parameters than did particle size.
In an additional study Amerah et. al (2008) found that differences in particle size distribution still existed between diets (corn or wheat-based) after pelleting especially in the proportion of coarse particles (1 mm and over). In corn-based diets, coarse grinding improved weight gains compared with fine grinding, but this particle size effect was not observed in wheat-based diets. In both diets, coarse grinding lowered feed:gain of broilers compared with fine grinding. In wheat-based diets, coarse grinding improved AME (n) compared with fine grinding. Heavier gizzard weights were observed in birds fed coarse corn-based diets. This effect was not present in birds fed coarse wheat-based diets. Overall the researchers concluded that the effect of feed particle size varies depending on the grain type.
Although it is assumed that fine grinding increases particle surface and thus increases availability for enzymatic digestion, there is evidence that coarser grinding to a more uniform particle size improves the performance of birds maintained on mash diets. This counter-intuitive effect may result from the positive effect of feed particle size on gizzard development. A more developed gizzard is associated with increased grinding activity, resulting in increased gut motility and greater digestion of nutrients.
Although grinding to fine particle size is thought to improve pellet quality, it will markedly increase energy consumption during milling. Systematic investigations on the relationships of feed particle size and diet uniformity with bird performance, gut health and pellet quality are warranted if efficiency is to be optimised in respect of the energy expenditure of grinding.
Indian research at CCS Haryana Agricultural University investigated the effects of five different particle sizes (2 – 6 mm hammer mill screens) in broiler feeds. General conclusion was that feed particle size resulted from the 5mm screen (868 micron) hammer mill was most efficiently utilised by the broilers, as it resulted in improved FCR, reduced electricity consumption for grinding and better body weight gains in birds.
Laying hen studies
Little work has been carried out in laying hens regarding particle sizes in the diet. A recent study conducted by Dr Marian Scott at the University of Belfast, North Ireland, has evaluated the effect of particle size and feed form on egg production and egg quality parameters. The treatments included a whole wheat ration plus balancer, wheat ground through three particle sizes (2, 5 and 8mm) and formulated into diets offered in three forms (pellets, crumbs and mash) to laying hens. Performance of the hens is summarised in Table 1. The effect of feed form is not reflected in this table, but this had no significant effect on egg production or egg quality, but crumbed diets reduced feed cost per 100 eggs. The finer particle size (2mm) resulted in better performance than coarser particle sizes.
However, overall optimum performance was achieved when the whole wheat plus balancer ration was given to hens. Offering feeds in such a form would also reduce diet costs per 100 eggs produced and in addition, reduce the energy required for diet production. However, this would need extra investments in the feeding system for feeding the whole wheat. Research at the Polytechnical University of Madrid, Spain, investigated the effect of the main cereal of the diet and particle size in young brown layers. The only significant effect detected was that feed intake was greater for hens fed coarse-ground cereals (10-mm screen) than for hens fed medium and fine-ground cereals (8 or 6-mm screen). None of the egg quality parameters studied were influenced by dietary treatment. Neither type of cereal nor particle size affected productive performance or egg quality of young brown hens.
Studying the influence of particle size is complicated by the integration of indirect effects on nutritional value, such as mix stability, or by interactions with elementary steps of the process other than grinding (such as pelleting). Moreover, optimal particle size selection depends on factors other than nutritional-like flowing properties of the meal: feed restriction due to poor flowing properties is still a real concern. Technical constraints nowadays still override nutritional benefits in the decision making process for high quality meal.
Feed Tech Vol. 13 No. 5, 2009