Development of computer models of different selection strategies on poultry egg production.
Poultry have many behavioural, structural and biological features that are ideal for domestication and for meat and egg production (Appleby et al., 1992). Because of the importance of poultry meat and eggs to the human population, breeders and farmers are always looking for ways of improving these traits. Artificial selection is the primary method of trait improvement, and involves selecting individuals with the highest breeding values as parents in each generation. There are a number of different methods of artificial selection, including: individual selection, between family selection, within family selection, family-index selection and index selection. In order to maintain a good response to selection breeders are constantly striving to improve the effectiveness and accuracy of the different methods of artificial selection for traits of economic importance. One method of achieving this goal is the use of computer models. Computer models can be used to simulate selection strategies and to predict what strategy will be the most appropriate for the improvement of a particular trait. This is important as all traits are influenced by many different genetic and environmental factors (Falconer and Mackay, 1996). This investigation was designed to compare the effectiveness of five different artificial selection strategies, namely individual selection, between family selection, within family selection, family index selection and index selection. Five computer models were developed using Microsoft Excel 2000 and these models were then used to compare the efficiencies of the five selection strategies for four different traits. The selection techniques were applied to an artificially, randomly generated population of 500 chickens. The four traits were egg weight with a heritability of 0.51, egg production with a heritability of 0.22, age at first egg with a heritability of 0.41 and body weight with a heritability of 0.55. Firstly, each of these traits were selected for independently using the first four selection methods and secondly the traits were selected for two at a time using index selection. The most significant results obtained from the single trait simulations were that for all traits family-index selection produced the best response to selection in the initial generations and between family selection produced the best response in the later generations. The traits with a higher heritability (egg weight and body weight) responded better to individual selection than they did to within family selection and between family selection in the initial generations. However, within family selection and between family selection proved to be more effective for traits with a low heritability such as egg production. Individual selection and family-index selection resulted in a very rapid decline in the standard deviation of all the traits. Between family selection resulted in the slowest drop in the standard deviation of all the traits, which is why this technique produced the best responses to selection in the later generations. The impact of the correlations between the economically important traits were evident from the results of index selection. For example, egg production is negatively correlated with egg weight making it difficult to gain a correlated response in both these traits simultaneously. Furthermore, egg production is negatively correlated with age at first egg implying that early maturing birds will lay more eggs, however, these eggs will be lighter. The majority of the results obtained were to be expected. Family-index selection takes all the information about an individual's breeding value into account resulting in this method of selection consistently identifying the most desirable individuals being selected. It is therefore the preferred method of selection under all circumstances. It is, however, often not economically and practically efficient to incorporate this technique and the use of another method of selection usually proves to be more beneficial. Individual selection proved to be most effective when applied to traits with high heritabilities, due to the fact that this method selects individuals based on their own phenotypic values. For traits with a high heritability, an individual with a good phenotypic value will have a good breeding value. Between family selection and within family selection proved better for traits with lower heritabilities. For traits with a low heritability the phenotypic value of an individual is a poor indicator of its breeding value. Information from a number of relatives may thus improve the accuracy of prediction of the breeding value by accounting for the influence of environmental effects. The use of computer models to simulate the selection techniques proved very successful in illustrating the effectiveness of the different selection techniques under various genetic and environmental conditions. The models may also prove to be very effective from an educational perspective.