標題: Usage of fat in broiler nutrition [打印本頁] 作者: qiaoyongniu 時間: 2009-12-15 09:15 標題: Usage of fat in broiler nutrition 本帖最后由 qiaoyongniu 于 2009-12-15 09:20 編輯
Usage of fat in broiler nutritionPUBLICATION DATE: 13/06/2007
RATING
AUTHOR: Bekezela Dube (Courtesy of AFMA Matrix)
Poultry production is a business, which like any other enterprise seeks to generate profit. This is done by keeping production costs as low as possible.
Wilson & Bayer (2000) found that feed cost is about 60 -70% of the total cost of production, and energy alone contributes about 70% of the feed cost (Saleh et al. 2004). Poultry were observed to eat the amount of feed that approximately meets their energy requirements (Rose, 2001). In addition, they consume a daily amount of food that is approximately 5% of their body weight.
This suggests that in order to minimize feed costs one must use the cheapest form of energy, in other words the energy source that produces the greatest growth rate per unit cost. Energy should be used to balance poultry rations.
Figure 1: World Production estimates of oil crops for 2003/2004 (AFMA Symposium, 2004).
The amount of energy fed to broilers is affected by the age of the bird (Wiseman, 2003). The implication of this is that the stage of growth dictates which physiological processes are important. Furthermore, the ability of the bird, given the size and capability of the gut to utilize certain nutrients, also determines the level of nutrient inclusion.
Table 1: Fat sources commonly used in poultry feeds with their respective energy contributions.
The fat included in the diet increases the amount energy provided to the bird.
Besides providing energy to the bird, adding fat into poultry diets helps reduce dust, which may have adverse effects on bird health.
An antioxidant should be added to diets to prevent the fat in the feed from becoming rancid, especially during hot weather (Wilson & Bayer, 2000), because high temperatures accelerate the oxidation of fat.
In addition, carcass quality should be considered based on human health, shelf life and cost of ingredients.
SOURCES OF FAT
Fats are used as high energy sources in broiler rations. The energy yielding potential of lipids is determined by degree of saturation and chain length (Wiseman, 2003).
The location of the fatty acid in the glycerol molecule and the proportion of free fatty acids affect its energy value. Saturated fatty acids are less absorbable and have a lower energy value than unsaturated fatty acids because unsaturated fatty acids are polar solutes and are therefore readily incorporated into micelles and absorbed (Wiseman, 2003).
Figure 2: Effect of age on the amount of energy provided by oil and tallow to chickens (Wiseman, 2003).
The composition of a source determines the amount of energy that source can provide to the animal. Most dietary fat sources contain more unsaturated fatty acids than saturated fatty acids. Based on the knowledge of the fatty acid composition, it is possible to manipulate the amount of energy to be provided to the animal.
Vegetable oils are highly digestible and are therefore the best source of energy for farm animals. Oils are expensive because of the competition with human food, which limits its use when formulating least cost rations (Wilson & Bayer, 2000).
According to Rose (2001) , animal fat or vegetable oils may be used as a source of energy up to a maximum of 6%. Above this level it becomes difficult to maintain pellet quality or to mechanically move the sticky feed when it is not pelleted. To improve palatability and reduce dustiness of the diet, 1% added fat is recommended regardless of other economic or nutritional considerations (Hamilton, 1999).
Figure 3a: Feeds for growing meat-line domestic fowl (Rose, 2001).
The diagram (Figure 1) shows the worldwide production of major oil crops used in the production of dietary fat.
The information in Figure 1 shows that soybean is the most widely used dietary fat source, contributing more than half the total dietary fat source usage, while palm kernels and copra are least used. This is probably because soybeans are also a major source of protein for humans and animas.
Other sources, such as sunseed, rapeseed and canola, cottonseed and groundnuts contribute significantly as dietary fat sources. The smal usage of some sources does not suggest that they are not important to the poultry industry.
Therefore, the usage of each source depends on various factors, such as, chemical composition, presence or level of antinutritional factors, its cost and the level of other nutrients that it can provide to the animal.
Figure 3b: Feeds for a domestic laying fowl (Rose, 2001).
Different sources have been used in poultry rations. Hamilton (1999) identified the following sources with respective energy values as the ones mostly used in poultry feeds (Table 1).
The data in Table 1 shows that most plant oils provide more energy to poultry than animal fat sources.
However, poultry fat and fish oil provide a high level of energy, which is even higher than some of the plant oils, because poultry fat and fish oil contain a high proportion of unsaturated fatty acids.
The data shows that poultry are able to utilize plant oils more efficiently than animal fat, which implies that plant oils will be used more readily than animal fat in poultry rations.
EFFECT OF AGE ON ENERGY AVAILABILITY
The requirements for poultry vary among species and with age. In young birds the limiting factors are the enzyme lipase and bile salt secretion, which causes reduced ability to digest lipids (Leeson & Zubair, undated) and less fat will be included in their rations. In addition, feeding a lot of energy to young broilers might lead to the accumulation of fat at a young age, which negatively affects growth. The effect of age on the amount of energy obtained from different sources of lipids is shown in Figure 2.
Figure 2 demonstrates that for all ages tallow, which contains saturated fatty acids, is less digestible and therefore, less available to the animal. It also shows that the ability to digest dietary fat increases with increasing age, because young birds have lower lipase levels than older birds.
Figures 3a and 3b are representations of the requirements of fowls of different strains within the same species. Broilers need more fat and oils and cereals and cereal byproducts than layers. The broilers need to grow fast and they need energy to utilize the protein for tissue development. On the contrary, layers do not need a lot of energy because they don't grow as fast and it might cause fat accumulation and impair the development of the reproductive system.
EFFECT OF DIETARY FAT ON CARCASS QUALITY
Unsaturated fats have a high-energy value and their merits can be observed when used in correct quantities. However, when used in excess in poultry feeds they result in a high risk of unsaturated fatty acids in carcass fat.
Unsaturated fatty acids should therefore be used early when only dietary energy is important, but not carcass quality. This is further motivated by the fact that young birds digest unsaturated fats better than saturated fats (Wiseman, 2003).
When the bird grows and carcass considerations become critical, more saturated fats should be fed. This is also because older birds are better able to digest saturated fats. Vitamin E, which is a biological antioxidant, is added into the diet to protect the carcass against oxidation. It does not have any antioxidant activity in the diet; so specific antioxidants are used to protect dietary fat against oxidation.
Dietary fatty acids are absorbed without significant modification. This makes it possible to manipulate fatty acid profiles in poultry tissue to suit human health. An increase in the human dietary n-3/n-6 ratio helps prevent coronary heart disease by reducing the plasma lipids. Increasing the level of n-3 fatty acids does not only reduce the level of saturated fatty acids in the carcass, but it also reduces these in the abdominal fat pads of broilers (Danicke et al., 2001). This results in healthier chickens.
The increase in n-3 fatty acids suppresses the formation of long-chain polyunsaturated fatty acids derived from linoleic acid. Danicke et al. (2001) warned that too much fat in broilers should be avoided. They emphasized that too much fat gets retained as abdominal and visceral fat, and decrease slaughter yield and increase slaughterhouse waste.
ENERGY TO PROTEIN RATIO OF THE DIET
Broilers eat to meet their energy needs and are conditioned by the desire to lay down fat (Saleh et al., 2004).
Protein requires energy to be deposited; therefore protein deposition will occur if energy is provided. This also occurs if other nutrients are present in adequate quantities so as to prevent deposition of abdominal fat (Wiseman, 2003). Diet quality is therefore examined in terms energy: protein ratio. Protein quality is assessed based on its constituent amino acids.
Variations in the ratio lead to changes in broiler performance. This ratio can be widened or narrowed as desired by manipulating the amount of protein or energy included in the ration. Saleh et al. (2004) mentioned that fat can be used to increase the energy content of the diet, while maintaining dietary protein concentration.
This will widen the energy: protein ratio.
The result of this will be increased energy consumption, which might lead to fat accumulation.
This brings about a conflict between two important aspects of production. The first one is to widen the energy: protein balance by providing as much energy as possible, to the bird so that it utilizes all the protein and grows fast.
The second one is to reduce the amount of energy so as to produce leaner carcasses. This suggests it is not the absolute amounts of energy and protein in the diet that matter, but their relative amounts. Therefore, if fat is to be included in the rations the protein content should also be increased to maintain the ratio.
DISCUSSION AND CONCLUSION
Broiler production has feed constituting a large proportion of production costs (Wilson & Bayer, 2000), of which energy is the major contributor (Saleh et al., 2004).
For the enterprise to be profitable feed costs should therefore be minimized. This is achieved by using nutrient dense rations. Fats are some of the ingredients used in formulating these rations. The chemical properties of the fat determine the amount of energy that will be provided to the bird (Wiseman, 2003).
Animal fats are less digestible than vegetable oils, as a result they provide less energy. Furthermore, the age of the bird has an effect on the nutrient availability (Wiseman, 2003). The differences in nutrient availability due to age are because of the variations in the ability to digest fats.
Young birds have lower lipase and bile salt levels than older birds, which are responsible for lipid digestion (Leeson et al., 2004). The type of fat fed, depending on the stage of production, affects carcass quality.
In order to formulate broiler rations all factors that directly or indirectly affect the endproduct must be considered together, not in isolation. The quality and cost of the endproduct will determine the profitability of the enterprise.