Processing of feed in uniform durable pellets affords significant benefits; the increased bulk density reduces the segregation and dustiness of the feed, thereby reducing losses. Feed conversion efficiency is much improved because it results in less feed spillage during consumption. Pelleting also maintains the integrity of the gastrointestinal tract and the heat treatment associated with pelleting improves feed digestibility by deactivating anti-nutritional factors and hence improves performance.
Part of the beneficial effect of pelleting is due to an increase in metabolisable Energy (ME) value (Table 1) with resulting improvements in weight gain and feed/gain ratio. With wheat bran and shorts, most of the improvement from pelleting can be attributed to the increase in bulk density of the feed with a lesser amount of time and energy being expended for prehension, as well as the increased availability of the contents of the aleurone layer of cells. In the case of wheat middlings, the pelleting caused no significant increase in ME but did result in a marked improvement in feed intake and in performance. The increased feed intake was not due to a change in bulk density but to an alteration in the middlings that prevented beak pasting and necrosis, which occurs with the unprocessed wheat middlings.
There is also an increase of protein utilisation and amino acid absorption with pelleting even when the feed pellets are reground to the consistency of mash (Table 2). Such improvement, however, was noted mainly for wheat bran and wheat germ but not for cereal grains, and can be attributed almost entirely to the destruction of heat-labile toxic factors that impair digestion and utilisation of protein.
Pelleting maintains the normal structure and function of the gastrointestinal tract. The gizzards of birds fed pelleted rations were better developed compared with other birds whose gizzards were atrophied when fed ground rations since they had no hard particles to grind down. The well- developed gizzard can be regarded as a barrier in preventing pathogenic bacteria from entering the distal GIT. Studies have also shown that the small intestine was better integrated with pellet diets compared to the mash diet. Pelleting increases the villous height due to the physical stimulation of the villi and hence increases the absorptive capacity of the small intestine. It also reduces the thickness of the muscular layer lining the small intestines thereby allowing greater contact between the nutrients and villi.
In pellet-fed birds, there is more accessible starch in the caeca from pellet-fed birds which results in increased concentration of VFA and reduced pH. The reduced pH has an antimicrobial effect on pathogenic bacteria entering the distal part of GIT. In one study, there was a reduction of Salmonella in the caeca associated with decreasing caecal pH and increasing amounts of VFA with a reduction in numbers of caecal Enterobacteriaceae in broilers during growth.
Offering pelleted feed to broilers can result in a 67% reduction in the energy required for eating, and hence directing such an amount of energy towards productive purposes. Shown in Table 3 are results of 3 experiments comparing the performance of broiler chicks fed pelleted and ground rations.
These experiments were conducted on the same breed of chickens under similar management conditions. The differences in feed conversion observed within the pelleting treatment could have been linked with differences in pellet quality. It has been estimated that 0.01 in feed conversion is lost with each 10% increase in ‘fines’ in pelleted feeds. Although pellet quality may appear adequate immediately after leaving the feed mill, pellet quality at the time the flock is consuming the feed in the house is what counts. Every effort should, therefore, be directed toward improving the quality of pellets that arrive in the feed troughs for broilers.
Other advantages of pelleted feeds include:
Most of the results of the improved energy and protein values of feeds were obtained with steam-pelleting, but to a lesser extent with dry-pelleting. It was felt that dry pelleting only changes the bulk density for ease of prehension but would not give all of the chemical changes associated with the steam-pelleting. In addition, the enhanced availability of dietary phosphorus brought about by the steam-pelleting over the dry-pelleting could be an additional factor contributing to the improved feed quality and the better performance of the birds when fed the steam-pelleted feeds.
There is a little or no vitamin destruction with pelleting temperatures of up to 80°C. Rations can, therefore, be pelleted at any temperature up to 80°C that will allow for maximum production per hour without any fear of vitamin destruction or decreased ration performance. The same holds true with the energy value of the processed feeds. In one study, the ME value of corn and a soybean mixture was 3.05 and 3.04 Kcal/g at a pelleting temperature of 70°C and 80°C respectively but was reduced to 2.91 Kcal/g at 90°C.
Several feed additives have been shown to produce a marked increase in the firmness of pellets. Among the most widely used are:
While bentonites possess no nutritive value, several reports have indicated that at the level of common usage, which usually does not exceed 2.5% of the diet, bentonites have no detrimental effects and, indeed, may improve growth and/or feed utilisation in chickens. The hemicellulose preparations at levels up to 2.5% may serve as good energy sources since the pentose sugars resulting from hemicellulose hydrolysis are well utilised by chickens up to a level of about 5% of the diet. Lignin, however, has no nutritive value for the chicken.
Moisture addition at the mixer has also been shown to increase pellet durability and decrease pellet mill energy consumption. The increase in pellet durability alone should economically improve broiler production. Generally, a range of 15 to 17% moisture is employed. However, feeds containing liberal quantities of high fibre ingredients will require a higher level of moisture while feed low in fibre will require less moisture. If the moisture addition procedure is followed, then extra drying of the pellets is required so that the mould will not occur in storage.
Recent work has indicated that molasses may be used advantageously as a pellet binder. In one study, the percentages of pellets remaining after tumbling a 100g sample of corn-soybean diet for 10 minutes were 85.6% and 89.8% with molasses levels of 0% and 3%, respectively. Besides aiding in pelleting, molasses would also contribute energy to the diet and thus the addition of 1 to 2% molasses in certain diets should be considered.
Poorly manufactured feed with excess fines results in some of the birds consuming only pellets, leaving the smaller fines for less aggressive birds. Because pellet quality affects the rate of growth, the presence of fines in a feed can affect flock uniformity and impact processing. If fines are fed to poultry, a loss in feed conversion and rate of gain is observed. As indicated above, with each additional 10% fines, a loss of one conversion point will result.
The anatomy of the digestible system is affected by feed particle size, which could impact nutrient absorption. This is especially important considering that the digestive system of broilers selected for rapid growth is less mature as the birds are pushed to market weight faster. Research is limited on the proper pellet sizes required by broilers, and this may need to be addressed as feed manufacturing changes are made. It is likely that a refinement of pellet size to age or body weight can be optimised to improve performance.
Feed in pellet form increases the requirement for lysine in growing birds compared to those fed similar diets in mash form. Because pelleting increases the productive energy of the diet, it is speculated that more lysine is required since the requirement of some nutrients is related to the level of other nutrients available to the bird. If the average increase in feed conversion due to pelleting is a mere 10%, for example, then the theoretical requirement for lysine would be 1.43% compared to 1.30% in mash feed. Many tables of nutrient bioavailability are based on feed in the mash form that has not been processed. Thus, the performance of birds in the field fed poorly manufactured feed may differ from those fed diets with high pellet quality.
References available on request