Although early feed restriction in broilers reduces growth performance, compensatory growth in the refeeding period will be attained to accelerate growth so as to reach the target weight of birds at the time of marketing.
Feed restriction has been adopted in broiler production to avoid rapid growth rate, which is associated with ascites, lameness, mortality, and poor reproductive results. In addition, feed restriction in the early stage is beneficial for improving the feed efficiency and decreasing the rearing cost. This article will provide some insight into feed restriction methods, duration, and advantages, along with the strategies to be considered for maximum benefits of that system.
The following are the procedures that can be applied to manipulate the feeding strategies of poultry in order to decrease growth and metabolic rate to some extent and so alleviate the incidence of some metabolic diseases as well as improving feed conversion in broiler chickens.
Physical feed restriction supplies a calculated amount of feed per bird, which is often just enough to meet maintenance requirements. Practical application of physical feed restriction, however, is not simple due to the problems of regularly weighing birds, and calculating feed consumption on a daily basis. Moreover, it is necessary to provide sufficient feeder space in order to prevent competition among restricted birds and to prevent the unequal growth.
The feeding of high fibre diets has been used as an alternative to the physical feed restriction method. In some cases, however, birds were able to adapt to high fibre diet containing up to 55% rice hulls. Such adaptation enables them to digest and get more energy (and possibly other nutrients) than is required to achieve the desired growth retardation during the under nutrition period. This situation will obviously interfere with the total energy and another nutrient intake of the birds during under nutrition, thereby changing their growth pattern.
Removing feed for 8-24 hour periods during the starter period has been reported to decrease early growth and reduce the incidence of ascites without affecting the final body. It has also been reported that the skip-a day feeding for 3 weeks starting at day-old would improve carcass quality and reduce sudden death syndrome which is often associated with birds that are on ad libitum feed intake.
Lower cumulative feed intake and significantly improved feed conversion and compensatory growth were observed in chickens under an intermittent program (1 light:3 dark from 8 to 49 days) compared with those under a continuous lighting schedule (23 light: 1 dark). The use of intermittent lighting programs also has the advantage of reducing electricity costs, the incidence of leg abnormalities and sudden death syndrome with no reduction of weight at market age. Genotype, sex, feeder space, diet composition and stocking density are the main aspects that can interact with the lighting program and affect the broiler’s final performance.
Early growth can be retarded by the restriction of intake of specific nutrients, such as protein. Broilers require 220, 200 and 180 g/kg dietary crude protein during the starting, growing and finishing periods, respectively for optimal growth. They tend to increase their feed intake to make up for deficiencies when fed with diets that are marginally deficient in crude protein. However, feed intake is depressed by feeding diets that are severely deficient in crude protein. Studies have shown that ad libitum feeding of a diet containing only 94 g/kg crude protein from 8 to 14 days decreased the feed intake of broilers by some 57%. This decrease in feed intake resulted in 41% growth retardation, which was not recovered after 6 weeks of re-alimentation.
Low energy diets can also be used to reduce the body weight of broilers by 4 weeks of age. In one study, 2 levels of diet ME (13.0 and 14.2MJ/kg) were used during an under-nutrition period of 6 to 12 days of age and were fed to birds at 167 KJ ME/bird/day. All growth retarded birds observed complete growth compensation by 56 days of age. Female birds, however, required less energy to recover from the lost body weight occurring during the preliminary feeding of low energy diets than did males.
The consumption of mash feed at different phases of the broiler’s growth may be employed as a method of limiting feed intake. Birds offered mash spend more time consuming their feed compared to birds fed pellets and, therefore, expend more energy in this process.
In one study, the use of 50g/kg of calcium propionate as an appetite suppressor resulted in gains of birds close to those obtained under a recommended program of quantitative feed restriction between 2 to 6 weeks of age. Glycolic acid has also been used as a chemical means of restricting feed intake of broilers. The feed intake of birds given diets supplemented with 1.5% and 3% glycolic acid was depressed by 17% and 45%, respectively. These reductions in feed intake due to glycolic acid supplementation resulted in growth retardation during the under nutrition period to 71% and 41% respectively, relative to the growth of control birds. Male broilers exhibited complete body weight recovery at 49 days of age, with no difference between the birds restricted by the dietary glycolic acid addition or those subjected to physical feed restriction. Due to its natural occurrence, glycolic acid may serve as a safe and useful anorectic compound for restricting feed intake in poultry.
The longer the period of underfeeding, the more difficult it is for broilers to compensate for the reduction in weight gain. Most workers recommended feed restriction of not more than 7 and 5 days for male and female broilers, respectively, to allow for full body weight recovery. The restriction of the feed intake of birds to 90% of that of control birds from 7 to 49 days of age has resulted in a significantly lower final body weight at 56 days of age, probably due to the insufficient time allowed for the birds to exhibit complete growth compensation.
Initiation of 6 days feed restriction at any age between 3 and 11 days of age seems to permit complete body weight recovery by 8 weeks of age in broilers. On the other hand, when initiating a feed restriction program at 3 weeks of age, very little evidence of compensatory growth has been noticed, probably because of the little time allowed for recovery.
The effects of feed restriction on the performance of broiler chickens are shown in Table1. Feed intake, body weight gain, feed conversion ratio (FCR), and final weight at 1-42 days were not significantly affected by feed restriction. The lack of significant effects of feed restriction systems may be due to the gradual physiological adaptation of the birds to the different feeding regimes which probably improved the efficiency of conversion of the feed available.
The largest and most obvious fat depot is the abdominal fat pad which may comprise up to 4% of body weight. The abdominal fat is usually discarded at processing each year, the cost of which is directly or indirectly charged to the consumer, thereby presenting an additional economic burden.
With feed restriction, fat accumulation is reduced (Table 2). This reduction may be explained by two basic mechanisms: 1- Feed restriction results in lower hepatic acetyl-CoA carboxylase activity, a rate-limiting enzyme for fatty acid synthesis. This may limit hepatic triglyceride synthesis causing lower serum triglyceride concentration and hence reduces fat accumulation in the body. 2- Feed restriction influences the recovery of the fat cell number during re-feeding and hence decreases total body fat and abdominal fat contents.
In addition to the economic problems associated with the excessive fat formation, the type of fat formed in the carcass is not guaranteed in all instances. Varying amounts of undesirable cholesterol, for example, may result from the 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 for the consumer, unless strict precautions are taken.
The cholesterol problem may be alleviated by feed restriction. In one study, the cholesterol level was reduced from 132 to 109 mg/ml when chickens were feed-restricted. This was probably due to the fact that feed restriction decreases the hepatic 3-hydroxy-3-methyl glutaryl Co-A reductase activity in chickens, a rate-limiting enzyme in chlostrogenesis and that the activity of this enzyme is not recovered by re-feeding.
Early fast growth in modern broilers is associated with increased stress on the birds and can result in metabolic and skeletal disorders that lead to economic losses due to reduced animal performance, high mortality rates, and carcass condemnation at slaughter houses.
In one study, ascites mortality was significantly reduced when a feed restriction regimen was used from either 4-11 or 7-14 days of age, consisting of limiting daily intake of the birds to 75% of the ME required for normal growth.
Sudden death syndrome (SDS) is also a metabolic problem that occurs in all countries where broilers are grown rapidly under intensive conditions. Mortality may start as early as 3 to 4 days, but most often peaks at around 3 to 4 weeks of age, with the affected birds being found dead on their back. Mortality may range from 1.5 to 2.0% in mixed-sex flocks and as high as 4% in male flocks. Lowering energy intake by changing feed texture or density (mash), or management methods such as feed restriction or long dark periods will reduce mortality from SDS. A mortality level of 0% has been reported for feed-restricted birds compared to 3.33% under ad libitum feeding.
It has been generally assumed that rapid weight gain has been a major cause of skeletal defects occurring in leg bones and joints. Despite the evidence that there is no genetic correlation between skeletal disorders and body weight, nutritional evidence suggests that dietary strategies that depress growth rate by altering dietary energy and protein levels and offering various feed forms decrease the incidence of skeletal disorders. In one study, the incidence of skeletal disease was 3-fold lower in feed restricted birds compared to the full-fed birds. A similar reduction in the incidence of skeletal was also observed in broiler chickens exposed to intermittent light or a step-up lighting regimen.
Feed restriction has a beneficial effect on the immune response of chickens through the expression of some cytokines including IL-4 and iNOS. Cytokines are small glycoproteins produced by a number of cell types, predominantly leukocytes that regulate immunity, inflammation, and hematopoiesis. They regulate a number of physiological and pathological functions including innate immunity, acquired immunity, and a plethora of inflammatory responses.
Feed restriction may also be used as an effective means of combating the cold stress effects on poultry. Under cold temperature, there is an increase in the metabolic rate of the broiler, with a resulting high oxygen requirement and thus an imbalance between the respiration system and the high oxygen requirement, leading to a state of systemic hypoxia. The resulting systemic hypoxia triggers increased generation of reactive oxygen species (ROS) in mitochondria. When the rate of generation of ROS exceeds the protective capacity of antioxidant, the excess ROS oxidises proteins and DNA within the cell, damages the polyunsaturated fatty acids on the bio-membrane, and initiates lipid peroxidation chain reaction. Lipid peroxidation severely damages the cell membrane and compromises cell function, mainly by damaging the vascular endothelium cells, causing vasoconstriction, thrombosis and vascular remodeling, which then lead to an increase in pulmonary artery resistance and pulmonary hypertension. These changes had been attenuated by early time feed restriction. Feed restricted broilers had a lower body weight and consequently reduced metabolic rate and oxygen requirement, which leads to a reduced generation of ROS and lipid peroxidation and hence improves resistance to the invasion of diseases and reduce mortality.
From the economic point of view, feed costs per kilogramme live weight and return over feed costs per bird were in many cases affected by restricted feeding as a result of improvements in health, viability, and feed conversion rates. Also, when feed is restricted through manipulation of the lighting system, the electricity costs will be reduced and the feed efficiency will be improved, as indicated earlier, both factors will contribute to higher profits.
References available upon request.