Broiler chickens have traditionally been perceived to produce lean meat. In recent years, however, chicken meat has increased in fat content, probably due to the selection of broilers on the basis of liveweight with little consideration given to other important criteria such as feed efficiency and carcass composition.
Adequate light is required to stimulate receptors responsible for the release of gonadotropin-releasing hormone (GnRH) in the hypothalamus. Photo: Hans Prinsen.
The selection of birds for liveweight gain as a major criterion has in many cases led to fat carcasses, since fat deposition and rate of gain are closely related. This increased fat content in the carcass is often linked with a number of problems. The 3 main issues are:
- The control of fat biosynthesis and the type of fat formed in the carcass are not guaranteed in all instances. Varying amounts of undesirable cholesterol, for example, may result from formation of a greater amount of saturated fatty acids relative to the unsaturated ones in each molecule of fat being formed in the chick's body. This inevitably leads to possible health hazards to the consumer, unless strict precautions are taken.
- With excessive fat in the body there may be a variety of health and production problems encountered by the bird itself on a longterm bases. The incidence of leg weakness has frequently been observed with fat broilers. Another problem such as reduced reproductive performance in the breeding stock is also possible with increased fat deposition in the body.
- The largest and most obvious fat depot is the abdominal fat pad which may comprise up to 4% of body weight. It is estimated that in just one country like the Netherlands, 10,000 tonnes of abdominal fat are produced by chickens and discarded at processing each year, the cost of which is directly or indirectly charged to the consumer, thereby presenting an additional economic burden.
This article is intended to review some nutritional and environmental aspects related to excessive fat formation in the chick’s body, and the strategies to be adopted when attempting to alleviate this problem and hence improve carcass quality.
Increased fat on broiler carcasses leads to numerous health and production problems as well as the loss cost issues when part of the body weight is discarded. Photo: Hans Prinsen.
Dietary energy and protein
If the ME level of the diet is increased, so too should the supply of the dietary protein in order to avoid excessive fat formation in the carcass (Figure 1). A proportion of the body fat formed with high energy-low protein diets is deposited around the viscera in the abdominal area, resulting in a reduction of at least 3-4% in the dressing-out percentage.
Unfortunately, the high-protein diets necessary to influence carcass composition do not seem to have any major beneficial effect on growth rate. The economics of such an approach, therefore, depends on being able to realise a premium price for these leaner carcasses, or there being sufficiently less fat trim during further processing.
Amino acid supplementation
There are numerous reports showing the importance of increasing dietary lysine and methionine concentration to reduce the carcass fat content of broilers. The response of chickens to such a strategy may vary, however, with the feeding program being adopted. As shown in Table 1 the effects of amino acid supplementation on liveweight, feed efficiency, and on fat pad percent at 49 days of age are more pronounced with feed being restricted for 4 days at 20% ad lib intake commencing at 7 days of age. The reason may be that the bird's ability to absorb and utilise the amino acids can be enhanced following feed restriction. Further aspects of feed restriction shall be examined later in detail.
Garlic powder and copper compounds
As indicated earlier, there is a problem of formation of various amounts of cholesterol in the carcass, which leads to health hazards to the consumer. This problem can be alleviated by incorporating garlic powder or copper compounds into the poultry diets. Garlic powder added to the diet at a level of 3-5% resulted in increased activity of the enzymes that convert cholesterol into bile acids, eventually being catabolised in the body so that a negligible amount is deposited in the carcass. The copper compounds were added at 250 mg/kg and resulted in better control of fat biosynthesis with the least amounts of saturated fatty acids being formed in the body. In both cases, the level of cholesterol in the carcass was reduced by 20-25%.
Pelleting of feed
One week old broiler chickens were fed up to 8 weeks of age on grain amaranth (A. hypochodriacus) either in mash or pellet form provided ad lib. As shown in Figure 2, the pelleted diets gave higher carcass fat and lower moisture and protein contents than the mash diet. This trend corresponded with the higher body and carcass weights obtained with the pelleted diets, probably due to higher dietary energy intake inducing higher fat deposition. The choice of either form of the diet depends again on the cost of processing and the premium price for the leaner carcasses.
Chemical manipulation of body fat
There have been several reports on the effects of beta-adrenergic agonists reducing fat deposition, either by reducing adipose tissue cell size, or by reducing tissue protein degradation. The beta-adrenergic agonists have been included in broiler diets at 0.2 and 0.4 ppm, and resulted in reduction in body fat from 10.8% to 8.7% and 8.5% respectively, without adversely affecting feed intake, growth rate, or feed efficiency. These compounds are usually included in the finisher diets, and are thought to depress growth rate if included earlier.
Lipolytic agents that regulate adipose metabolism, such as iodinated casein have also been tested at 50 and 100 ppm in commercial mash diets with broilers grown from 28 to 49 days of age. As shown in Table 2, best results were obtained with the 50 ppm level, leading to greater reductions in the abdominal fat pad (AFP) and the carcass fat content, with further improvement in all other production parameters.
A study was conducted to examine the production responses and fat deposition by broiler chickens under two different regimes of feed restriction (20% of ad lib). In the first regime, feed was restricted for 4 consecutive days commencing at 7 days of age. In the other regime, birds were subject to a broken feed restriction, whereby the restriction was imposed for 2 days, then lifted for 2 days, and re-imposed for a further 2 days commencing at 7 days of age as well. Results are given in Table 3. Obviously, the 2x2 regime resulted in better performance and less fat in the carcasses compared to both the control and the 4-days regimes. The improved performance here was probably due to the fact that broken feed restriction has allowed the chicks to have frequent chances for compensatory growth, leading to heavier weights at 49 days of age with a greater feed efficiency. The difference in fat deposition between the ad lib and the restricted feeding regimes could simply be attributed to the use of body fat during the periods of feed restriction, or to the better absorption of amino acids following restriction as indicated earlier. The reason why the form of restriction affected fat content differently may not, however, be justified.
Role of the environment
Temperature and light are the key environmental factors affecting fat deposition in the chick's body. The following is a brief description of the mechanisms through which these factors affect carcass quality, with particular reference to fat deposition.
Temperature appears to influence carcass composition partially by its effect on feed intake and partially by altering the response of the bird. In hot environments, feed intake is less than normal, creating a nutrient deficit that may be server enough to alter the metabolism of the liver, causing excess fat to accumulate in the carcass; whereas in cold temperatures, the demand for energy may force depletion of adipose reserves for energy. Furthermore, increasing the energy density of the diet in cold weather will alleviate, to some extent, the demand to match intake with expenditure of energy and thus maintain carcass composition at a status quo. During heat stress, when feed intake is reduced, the need for nutrients is alleviated to some extent by increasing nutrient density thus assuring less of a chance for a nutrient deficit, which can cause fat to accumulate in liver and carcass.
Dim light results in an increased fat level of the carcass. The light-fat relationship has been attributed to the decreased activity of birds kept in dim light, and has been demonstrated by the increased percentage of thighs, drums, breast skin and wings which act as primary reserves of fat.
In addition to the activity aspect, a physiological response may also be involved. Adequate light is required to stimulate receptors responsible for release of gonadotropin-releasing hormone (GnRH) in the hypothalamus because these receptors are sensitive to light directly passing through the skull instead of perception of light by eyes.
The release of GnRH helps secretion of sex steroids and growth hormones (GH) which act as lipolytic agents in the body. Under dim light conditions, however, the concentration of these hormones in decreased, resulting in a higher level of fat deposition.