By Rick Kleyn, SPESFEED (Pty) Ltd

Particle Size has long been an area of interest, in both the feed and poultry industries. It has become more topical of late, as we continue to fine-tune our production systems. Particle size encompasses both the size of the various feed ingredients used in poultry diets as well as the consistency of the particle size. Ingredient texture impacts on two areas of the poultry industry. Firstly, particle size impacts directly on the bird it’s self and the manner in which it utilises the nutrients in it’s diet, both in the case of laying hens and broilers. The issue is further complicated by the fact that the manner in which materials are digested differs between different ingredients. Secondly, particle size impacts on the manner in which ingredients are handled and processed in the mill although as will be seen the information in this regard is scant. This paper will deal with both of these aspects.

Definition of Particle Size

Before any discussion on feed particle size can be entered into, it is important that we define exactly what it is geometric mean diameter (GMD) that we are dealing with. Particle size is established by the geometric mean diameter (GMD). However, the complete information on particle size must include a measure of dispersion. This measure is the geometric standard deviation (GSD), which establishes the range of variation among the different particle sizes (Nir et al. (1994). Both of these measures are described by the ASEA (1983), but sadly they are seldom reported in the literature as they independently affect broiler growth and performance as shall be seen. Briefly, a sample of grain is passed through a series of sieves and the amount of grain retained on each screen size is determined. The average particle size of the sample is then determined by standard formulas and given as geometric mean diameter (GMD, expressed as microns or µ). Particle size uniformity is described by geometric standard deviation (GSD), a small GSD representing higher uniformity.

Feed particles are what a bird actually sees and touches in its diet. Broilers and layers eat using their own sensory perception of the food, ignoring in the short term all of the work done by the nutritionist (Picard et al., 2002). The sensory perception of chickens is the crucial link between food technology and behavioral responses, a direct connection between the feed mill and the farm. The feed cues perceived by chickens take a number of forms. These include visual, tactile, olfactory stimuli and to a minor degree taste.

Young chickens tend to eat brightly coloured feed particles first and they will always show preference for larger feed particles, relatively independently of the composition of the particle. It is of interest that they will consume minimal quantities of a "new" food at first, but there is a progressive switch of preferences according to sensory and nutritional testing of the food.

The manner in which ingredients are ground and the coarseness of that grind has a direct impact on the physiology of the birds. Nir et al. (1994) asserted that nutrient digestibility decreases when small particles are used because they cause gizzard atrophy and discrete intestinal hypertrophy caused by bacterial fermentation. It was further suggested that particle breakdown in the proximal small intestine is slower when particles are larger. This causes an increase in peristalsis, leading to a better nutrient utilization. Cumming (1994) suggested that when fine diets are fed to either broilers or laying hens that gizzard acts as a "transit" rather than a grinding organ. As a result of this the feed is not retained in the gizzard for any significant period and is therefore not exposed to the digestive enzymes of the proventriculus at a low pH. The role of these poorly digested feed particles in the upper intestinal tract is unknown, however they may play a role in aberrant bacterial populations such as E Coli. There is also evidence to suggest that an active, normal gizzard plays a role in the chickens’ resistance to coccidiosis.

Carré (2000) makes the point that it seems that coarse grinding should be positive for reducing water losses, and also, in some cases, for protein digestibility. The latter effects would be explained by a better control of the intestine transit time by the gizzard emptying when using coarse ground feeds. Krabbe (2000) verified this by showing that finely ground diets (561 m m) compromised nutrient metabolism, with particle size affecting metabolisable energy, nitrogen retention and dry matter retention (Table 1).

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Table 1. Effect of MGD on metabolic responses of 7-day-old broilers.

Adapted from Krabbe (2000).

Magro & Penz (1998), working with diets containing increasing particle sizes, found the best production results with the highest MGD feeds. They were also able to illustrate the impact that particle size has on the bird’s digestive system.

Adapted from Magro & Penz, 1998.

The problem of particle size is not restricted to grains or to the diet as a whole. Brugalli (1996) showed that particle size is also an important variable when evaluating meat meal quality (20% and 40% substitution). Working with 15 to 41-day-old broilers, he verified that the meat meal with a MGD of 420 m m had a higher metabolisable energy content as compared to the same meat meal with MGD's of 510 and 590 m m, independent of the level of substitution (Table 3).

Adapted from Brugalli, 1996.

Carré (2000) highlights the impact that grinding will have on different types of plant material. Coarse grinding may result in low availability’s for some of the intracellular components of dicotyledon whole seeds as observed for soybeans, rape seeds, faba beans and peas. In contrast, cereals seem to be much less sensitive to particle size variations. In dicotyledon seeds, the intracellular components do not show the same sensitivity: the highest sensitivities to particle size variations are generally observed for lipids and starch. In the case of starch, this difference may be as high as 30%. Protein on the other hand remains little affected by particle size. However, if a pelleting process is applied after grinding, the effects of the grinding intensity are considerably reduced with digestibility values approaching the maximum.

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Grinding of ingredients is a routine part of feed manufacture. Traditionally hammer mills have been used for finer diets while roller milling has been applied mainly to produce courser feeds. Roller mils can however produce fine grinds of similar uniformity to those produced by hammer mill. In either case a range of particle sizes is produced depending on a number of factors including the type of grain used, the speed of grinding and the screen size used (McCracken, 2002).

Beyer (2003) comments that as feed mills have become larger and throughput has increased, it seems that feed quality (the physical quality) has been less emphasised. This is surprising considering that properly formed feed could result in saving more feed conversion points than many of the other things we work so hard to implement at so great a cost. He goes on to say that particle size is another manufacturing parameter that has received little attention in previous research, which is surprising in the light of some of the information that has already been covered in this paper.

It is believed that smaller particle size leads to improved pellet quality (hardness) and efficiencies of pelleting presses. However, once the pellet is exposed to the gastro intestinal tract of the bird (the crop in particular), the feed dissolves and we are in effect feeding a finely ground mash diet. Nir et al., (1994) make the comment that it appears that the size of the particles in the mash maintains its effect on performance, even after subsequent granulation by pelleting. One can’t help but wonder if this aspect plays a role in the outcome of the many trials that are conducted to measure the effect of pelleting and pellet quality whereby pellets are reground and the fed to broilers, with seemingly inconsistent results.

The work of Reece (1986) shows that particle size may not have the impact on pellet quality that we think it does, and this is confirmed by Louw (2003), who contends that the particle size only needs to be smaller than the size of the die in the pellet mill.

What is known is that the finer feed particles are milled the more energy that is required to mill them as was shown by Dritz & Hancock (2001). In addition it has been shown that the energy reduction through the use of a roller mill as opposed to a hammer mill is in the order of 28% (Nir et al., 1994). Halvorsen and Kipfer (2002) acknowledge that a roller mill will produce particles with a better GSD, but raise concerns over their reduced capacity and high capital and maintenance costs.

Scharlach (1999) conducted a survey of the fineness of grind of maize samples collected mostly from pig farmers in South Africa. This information is relevant because the same sorts of equipment would be used on both pig and poultry farms in this country. Of the 21 maize samples tested, none had an average particle size of less than 800 microns while 75% of the samples had an average particle size of more than 1000 microns.

As feed manufactures we need to match the chickens nutritional and physiological requirements as closely as possible, while bearing in mind that the machinery and systems that we use have certain limitations. This is complicated by the fact that many companies have a single production line that is often difficult to adapt to different diets. Mills are available in which the screens (size) can be changed automatically and in which the speed of rotation can be altered. By using a combination of these two factors, a wide range of particle sizes can be created by a single machine (Louw, 2003)

i) Layer diets: In practical terms, the particle size of layer diets is of importance although little literature exists in this regard. Many common issues such as mixing difficulties, wrong particle size and ingredient separation can be overcome by relatively fine milling of raw materials. However, feed that has been ground too fine can greatly reduce consumption. In hot climates a correct feed particle size will help to reduce the underfeeding observed in the summer months. That is why it is recommended that at least 80% of the particles should be between 0.5 and 3.2 mm. Using this type of feed has the added advantage that milling throughput is faster (Anon, 1998).

Cabrera, (1994) introduces another aspect when evaluating particle size in layer diets. Clearly laying hens perform better on coarse maize, but the reverse is true for sorghum (Table 6). This work did not show an increase in feed intake with courser maize, but it did show an improvement in feed conversion

ii) Broiler Diets: The ideal particle size that should be used in broiler diets is possibly more complex than the situation that exists in laying hens. As has already been shown, different classes of feed ingredient should probably be milled to different sizes. In addition, Nir et al. (1994) have shown that the lower the GSD, the better the performance. They showed that when the GSD was near 2, independent of the MGD, broiler performance was impaired (P<0.01).

Lott et al. (1992) conducted two trials in which corn was ground to different particle sizes by hammer mill grinding and fed to chicks in either mash (Experiment 1) or crumbled (Experiment 2) form, from 1 to 21 days (Table 7). In the first experiment, hammer mill screen sizes of 3.18 mm or 9.59 mm were used producing corn with GMD of 716 µ and 1,196 µ, respectively. Body weight was significantly higher and feed conversion significantly improved when chicks were fed the diets incorporating the corn ground on the 3.18 mm screen. In a second trial, corn was ground using a series of hammer mill screens ranging from 3.18 mm to 7.94 mm, resulting in corn with GMD ranging from 690 µ to 974 µ. Chicks fed crumbled diets incorporating corn with the various grind sizes did not differ in body weight or feed utilisation.

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Table 7. Effect of hammer mill screen size for grinding corn used in crumbled diets on performance of broiler chickens from 1 to 21 d (Lott et al., 1992)

These findings would at first appear to be at odds with the work of Krabbe (2000) presented in Table 1., where it was clearly shown that coarsely ground feed is more efficiently utilised. However, uniformity of the diet has long been assumed to be important for optimum performance of broilers, especially those raised in close confinement using automated feeding equipment. McCoy et al. (1994) have demonstrated the importance of a low coefficient of variation on average daily gain and feed conversion by growing chicks. Therefore, the possible implication of particle size on diet uniformity is of importance when considering fineness of grind. This is likely to be exacerbated under local conditions where the primary grain source is yellow maize, as broilers have been shown to prefer yellow coloured feed components (Weeks et al., 1997). It should be remembered that in young birds the beak dimensions mitigate against prehension of large particles and that the gizzard is less well developed than in older birds McCracken, (2002).

This author has had practical experience of the problems associated with feeding a course mash as a broiler starter diet. Ingredient selection resulted in uniformity problems in the birds.

As broilers grow, so the impact of selective feeding is removed from the equation. This is clearly illustrated by the work of Magro and Penz (1998) as shown in Table 2.

From the work presented above it is clear that while we know something about the impact of particle size on poultry production, there is much still to know. Fortunately, most of the work that has been carried out has had to do with our major ingredient, maize. How we should handle the other ingredients in the diet has yet to be determined, but the work of Carré (2000) and Brugalli (1996) would suggest that perhaps we should be milling our proteinatious materials finer than our grain sources.

For laying hens it would appear is that coarsely ground maize is the probably the best manner in which they should be fed. The South African data to hand would indicate that most integrators are probably doing the correct thing in this regard, but many of the mash diets being produced by larger feed mills tend to be a little on the fine side.

In the case of broilers the issue is more complex. It is true that more coarsely ground maize leads to improved nutrient utilization by the birds. This remains the case even after pelleting. If young birds, during the so-called starter phase, are being fed a mash diet, it would seem that the advantages of a larger particle size are outweighed by the disadvantages of selective eating. Feed millers are of the opinion that a finer grind leads to improved pellet quality but this particular standpoint is probably open to debate. Penz (2002) is of the opinion that he would sacrifice some pellet quality if it meant that the maize particle size was greater.

In short, feed millers will need to have better control over both the particle size and the uniformity of those particles than they do at present. It needs to be born in mind that mills often manufacture feed for other classes of animal as well, each with their own unique requirements. Nutritionists will need to build up a clearer picture of what feed quality is to be expected for each class of bird. Only by working together on this issue will improve real improvement be made in this regard.

References

Anon (1998), The Importance Of Particle Size In Layer Feed. The Poultry Times of India. (9) 1.

ASAE. 1983. Method of determining and expressing fineness of feed materials by sieving. Am. Soc. Agric. Eng. Standard S319.2. Yearbook of Standards.

Beyer, R. S. (2003). Effects of feed processing and texture on bird performance. Proceedings 50th Maryland Nutrition Conference for Feed Manufacturers. University of Maryland.

Brugalli I., (1996). Efeito da granulometria na biodisponibilidade de fósforo e nos valores energéticos da farinha de carne e ossos e exigência nutricional de fósforo de pintos de corte. Universidade Federal de Viçosa. 83p. Diss. Mestr. Zootecnia (as cited by M Penz, 2002)

Cabrera, M. R. (1994). Effects of sorghum genotype and particle size on milling characteristics and performance of finishing pigs, broiler chicks, and laying hens. Kansas State University. M. S. Thesis. (as cited by Dritz & Hancock, 2001)