High pelleting temperatures reduce broiler performance
Six separate studies are reviewed in which a relationship between pelleting temperatures and broiler performance was measured. The studies included both wheat and corn-based diets, but were predominantly with wheat diets. They cover pelleting temperatures from 65 to 105 deg C. In all cases a relationship was established, such that the higher pelleting temperatures were associated with poorer broiler performance, in terms of weight gain and feed:gain, and in one study, mortality. Specifically, it appears that pelleting temperatures over 85 C should be avoided. The reasons for this relationship are not clear, but presumably are due to losses of some heat labile nutrients, which may include vitamins, and binding of lysine and perhaps formation of indigestible complexes of starch with protein.
In the first research, a mash compound grower feed, based on corn, wheat, oats and soybean meal was subjected to steam conditioning and pelleting at three different temperatures (69, 78 and 86 C), measured as the core temperature at the outlet of the conditioner. It was concluded that the temperature during conditioning and pelleting of the feed influenced technical quality of the pellets as well as the nutritive quality for young broiler chickens. High temperature improved the physical quality of the pellets, but caused lower levels of metabolisable and poorer growth performance.
The second involves a trial in corn-based diets tested three enzymes at pelleting temperatures of either 85 or 93 C. The higher temperature significantly depressed weight gains and feed: gain.
It has been observed that technological treatment may cause the formation of starch that is not susceptible to enzymatic hydrolysis (resistant starch). It has been shown that there is considerable variation in the rate and extent of starch digestion in the chicken, and that this variation affects energy utilisation.
In the case of diets based on viscous grains (wheat and barley), increased viscosity due to greater fibre disruption may occur at higher pelleting temperatures. This impedes both nutrient absorption and the readsorption of endogenous excretion products in the small intestine, thereby increasing the amount of substrate available for bacterial growth in the hind gut. An excessive microbial growth disturbs the gut environment and cause problems such as sticky droppings or necrotic enteritis.
Since higher processing temperatures are linked to higher intestinal viscosity, it is possible that in an attempt to sterilise the feed we could unwittingly be putting the bird at higher risk of infection from other sources.
Normal pelleting temperatures are generally not thought sufficient to solubilise dietary fibres or to form resistant starch. However, conditions within the feedmill conditioner, which involve combinations of temperature, moisture, pressure and time may be such as to cause damage to nutrient availability.
For corn (and sorghum) -based diets there may be loss of lysine and arginine due to maillard complexing with sugars and loss of available energy due to a kind of retrogradation of starch into resistant complexes following high temperature pelleting.
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Article made possible through the contribution of the Australian Poultry Science Symposium (APSS) 2006.