Determining the genetic growth parameters
The method used in the simulation model of growth and food intake described here requires a description of the genotype of the bird, from which its potential growth rate and carcass composition under non-limiting conditions are predicted. Potential growth defines the upper limit to nutritional response and is thus of paramount importance in ensuring the accuracy of the model.
The parameters of the Gompertz growth curve that must be defined are the mature protein weight (Pm), the rate of maturing (B), and the time taken to reach 0.368 of mature size (t*). The-lipid-to protein ratio at maturity is required to describe the chemical composition of the bird at different stages of maturity. When simulation models become more popular, and more accessible to the broiler industry these values might well be provided by breeding companies in the future, as a means of describing the diverse genotypes available from their company, but at present they must be estimated from other sources.
A number of methods for estimating the values of these parameters exist. Where growth curves are available, and if the birds providing that information were grown under optimal conditions, optimisation routines would provide estimates of the parameters. Recently (Hancock et al., 1995; Gous et al., 1996, 1999) the growth parameters of different strains of broiler have been measured in this way to determine the variability between breeds available to the industry at present.
In the absence of such information, relative growth rates may be used to estimate the parameters. The Gompertz growth function predicts that the relative growth rate will decline linearly to zero as the logarithm of (protein) weight increases to maturity. Measurements of the relative growth rate of broilers at two or more intervals during the growing period can thus be used to determine two components of the growth curve, namely Pm and B.
Examples of three different genotypes are illustrated in Figure 2. This is a particularly useful means of estimating these two parameters as optimal conditions do not have to be provided for the duration of the growing period. It is likely that maximal growth will not be sustained by many birds over the entire growing period, and that physiological problems such as weak legs and ascites would be prevalent in birds that were encouraged to grow at their maximum rate to maturity. Estimates of the genetic parameters of male and female broilers of a slow growing strain, a present-day strain and a strain predicted to be available in the future are presented in Table 1.
Emmans (1989) has used published data on the growth of turkeys to show that the mature protein weight has been increased substantially by genetic selection for high weight at an age. This has had the effect of increasing the scaled rate parameter B*, calculated as B.Pm0.27, and which, according to Taylor’s (1980) time scaling rule, is uncorrelated with Pm. The value of B* has been shown not to vary across genotypes, except where a high degree of genetic selection has been applied to growth rate. This has had the effect of increasing B* in broilers, in turkeys and in pigs.
Genetic selection has recently been directed towards reducing the lipid content of the gain, and this has presumably had the effect of reducing the lipid to protein ratio at maturity. This ratio can be expected to decline still further in the future. This effect is difficult to measure as birds will vary the amount of lipid deposited as a means of overcoming deficiencies in the food offered, so the ratio measured is valid only where birds have been reared under non-limiting conditions.
| Strain | Sex | Pm (kg) | B (per d) | t* (d) | LPRm |
|---|---|---|---|---|---|
| Past | M | 1.05 | 0.028 | 58 | 0.6 |
| F | 0.73 | 0.031 | 51 | 1.2 | |
| Present | M | 1.20 | 0.037 | 43 | 0.5 |
| F | 0.84 | 0.041 | 39 | 0.4 | |
| Future | M | 1.26 | 0.042 | 39 | 0.4 |
| F | 0.88 | 0.046 | 35 | 0.9 |
TABLE 1
Estimates of mature protein weight (Pm), the rate of maturing (B), t* and the lipid to protein ratio at maturity (LPRm) of three strains of broiler.
FIGURE 2
Relative growth rates of four strains of broiler, representing past, present and future males and future females, plotted against the natural logarithm of body weight.