Relationships between models and data in growing poultry

1. Three plausible rules are used to define the problem that a growth function must solve. Rule 1, growth rate, dW/dt, is a function of weight, W; Rule 2; the standard deviation of weight, σ_w, is proportional to W so that the natural log of W, <i>ln</i>W, rather than W should be used; Rule 3, there is an upper limit to body weight, A which represents the animal's mature weight.2. Applying the rules identifies a problem: how is the rate of change in <i>ln</i>W over time, d<i>ln</i>W/dt, related to <i>ln</i>W? The problem is solved by using Rule 4, which is to make the relationship as simple as possible. The solution is a form of the Gompertz function: d<i>ln</i>W/dt=B.<i>ln</i>(A/W), where B is a parameter describing the rate of maturing.3. The following study proposed that this function sufficiently described the inherited post-hatching growth potential of domestic birds. It was used to explore whether any given data set can describe such potential at all times. It can be further used to explore errors in data collection and misreporting.4. Examples are given for tests of the function, its use in examining data for accuracy and extension to chemical and physical growth. For male broilers of the Cobb 700 strain, the mature weights (A) of protein, water, lipid and ash were 1194 g, 3984 g, 1541 g and 221 g, respectively. The common Gompertz rate parameter (B) for all four components was 0.0410/d.5. Models for the chemical body composition of broilers can provide the basis for calculating energy and amino acid requirements, responses and economically optimum feeding strategies.