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.
