An experiment was carried out to collect data suitable for testing methods used to describe the potential growth and body composition curves of broilers. Males and females of two commercial broiler strain-crosses were grown to 16 wk of age with birds taken at 0, 2, 4, 6, 8, 12, and 16 wk of age for chemical analysis and for the measurement of feather weight and breast meat (Pectoralis major and Pectoralis minor) weight at these ages. The data were used to test the Gompertz growth equation and the assumption of chemical allometry, as well as to estimate the values of the growth parameters for the different genotypes. Feeding and environmental conditions were intended to be such that potential growth and body composition could be attained. The weights of the chemical components for each of the four genotypes were described in terms of the mature weight of these components, their rates of maturing, and the time taken to reach the maximum rate of growth of each component. Allometric relationships between the weights of the chemical components and that of body protein were estimated. The ratio of ash to protein was essentially constant. Water matured more slowly, and lipid faster, than protein. For males, and for females up to 8 wk, the models were satisfactory. For females after this age, lipid growth was faster than expected from the earlier period, probably in preparation for egg production. There were small, but important, differences in the values of some parameters between the strain-crosses. For each of the four genotypes the changes in weight of feathers and breast meat with time were described in terms of the Gompertz growth function, which described the data very well. The parameters of the function for each component and genotype-mature weight, rate of maturing, and the time taken to reach the maximum rate of growth B were evaluated. For the feathers, the value of the rate parameter was higher than that estimated for the body as a whole. For the two breast muscles, and for their total weight, the value of the rate parameter was similar to that for the body as a whole. There was a simple allometric relationship between the weights of the breast muscles and that of the whole body. As a consequence, the development of the yield of breast meat for a given genotype could be described by the values of the two parameters: mature yield and the allometric exponent. A description of each genotype of interest is seen as an essential first step in using a simulation model either to predict requirements, or to predict the effects of different feeding programs, and environmental conditions, on the performance of broilers.

The World's Poultry Science Association (WPSA) is a long-established and unique organization that strives to advance knowledge and understanding of all aspects of poultry science and the poultry industry. Its 3 main aims are education, organization, and research. The WPSA Keynote Lecture, titled "Modeling as a research tool in poultry science," addresses 2 of these aims, namely, the value of modeling in research and education. The role of scientists is to put forward and then to test theories. These theories, or models, may be simple or highly complex, but they are aimed at improving our understanding of a system or the interaction between systems. In developing a model, the scientist must take into account existing knowledge, and in this process gaps in our knowledge of a system are identified. Useful ideas for research are generated in this way, and experiments may be designed specifically to address these issues. The resultant models become more accurate and more useful, and can be used in education and extension as a means of explaining many of the complex issues that arise in poultry science.

Three experiments were conducted on male broiler chickens between one and three weeks of age to determine their response to dietary lysine concentrations. Serial dilutions of a summit diet shown to be first-limiting in lysine were fed in all experiments. The balance between amino acids in these diets was maintained within narrow limits. Intake of the most-limiting amino acid was the most important factor determining growth rate; protein intake as such was of little or no importance. The efficiency of utilisation of dietary lysine for protein growth was calculated to be 65.05 mg/g protein gain, representing a net efficiency of 0.85. The diet dilution technique overcomes the major disadvantage of the graded supplementation method for determining the requirements of amino acids, namely that of the amino acid balance changing systematically in successive dietary treatments.

Commercial broilers are increasingly being subjected to environmental temperatures that are above their comfort zone. This is mainly because birds are growing faster than before and are therefore larger at any given age, but also because broiler production is being introduced as a farming system in environments that are unsuitable for such production. It is not economic for most producers to modify the environment within the broiler house to account for these problems and so it would be useful to know of nutritional strategies that could be used to reduce the effects of heat on broilers in the finishing stage. Nutritional strategies discussed in this paper include the use of feeds with a high ratio of net energy to metabolisable energy, feeds whose amino acid composition is closer to that required by the birds, feeds with additional salts or vitamins and the use of pelleted feed and timed feeding. However, dietary modification will increase the cost of feed and the producer will usually not reap a net benefit. Some advantage may be gained by adding vitamins C or E to the feed, because of their action in reducing lipid peroxidation resulting from the increased body temperature of the bird: but it is impractical to reduce the heat increment of a broiler feed unless poor quality ingredients are currently being used. Heat production by the broiler may be lowered by reducing activity, by feeding pellets instead of mash, or by withholding access to feed before the temperature increases to stressful levels. Some improvement in performance can be obtained by increasing water intake. This can be achieved by cooling the drinking water and by adding salts, though these are only effective if the water is kept cool. Most nutritional strategies that have been proposed as a means of reducing the heat of digestion in the broiler result in a maximum theoretical saving in metabolic heat production equal to the effect of lowering dry bulb temperature in the broiler house by about 1ºC. None of these strategies is as effective in terms of growth rate, feed conversion, liveability or carcass quality as reducing the radiant heat load on the birds by making appropriate modifications to the structure of the broiler house and to the husbandry practices employed. ReferencesBACON, W.L., CANTOR, A.H. and COLEMAN, M.A. (1981) Effect of dietary energy environment and sex of market broilers on lipoprotein composition. Poultry Science 60: 1282-1286. BASILIO, V. DE, VILARIÑO, M., YAHAV, S. and PICARD, M. (2001) Early age thermal conditioning and a dual feeding program for male broilers challenged by heat stress. Poultry Science 80: 29-36. BOULAHSEN, A.A., GARLICH, J.D. and EDENS, F.W. (1993) Calcium deficiency and food deprivation improve the response of chickens to acute heat stress. Journal of Nutrition 123: 98-105. BRAY, D.J. and GESELL, J.A. (1961) Studies with corn-soya laying diets. 4. Environmental temperature-a factor affecting performance of pullets fed diets suboptimal in protein.

1. An experiment was conducted to measure the potential growth of males and females of 6 commercial broiler stocks, from which information the growth rates of these genotypes could be characterised by the Gompertz growth equation. 2. Feeding and environmental conditions were designed to ensure that the birds remained comfortable throughout their growing period, which was to 26 weeks of age. A choice of diets differing in protein content was offered from 3 weeks of age. Because of leg weaknesses among the male broilers after 11 weeks of age, and because many females reached sexual maturity at about this age, the growth analyses were conducted on weights collected up to 11 weeks of age only. At this weight, broilers had achieved approximately 0.76 of their mature weight. 3. Birds representative of each genotype were killed for carcase analysis at weekly intervals to 9 weeks of age, and every two weeks thereafter. The contents of gut fill, feathers, water, protein, ash and lipid were measured on each of these birds; from these, equations were derived for each genotype that allowed the estimation of the weights of these components in the birds remaining on the experiment. 4. The body weight, body protein, body water and feather weight of the 12 genotypes were described in terms of the mature weight of these components, their rates of maturing and the time taken to reach the maximum rate of growth of each component. These descriptors of the growth of each component were then compared between genotypes. 5. No statistically significant differences existed in the rates of maturing of the different genotypes, either between strains or between sexes. Highly significant differences were evident between strains and between sexes in their mature weights, indicating that their rates of growth differed. 6. Estimates of mature feather weights indicated that this component of the body comprised 0.062 and 0.050 of the mature body weight of female and male broilers respectively. The protein content of feathers increased steadily, and the water content decreased steadily, throughout the growing period. 7. Differences between the genotypes evaluated in this experiment indicate that the nutrient and environmental requirements of these genotypes would differ. A description of each genotype, therefore, is an essential component of any simulation model that attempts to determine the optimum economic feeding programme and environmental conditions for broilers.