Modelling the effects of environmental stressors on pig performance
Wellock, Ian J.
The performance of pigs reared commercially is often considerably below that of their potential as seen under good experimental conditions. At least some of this decrease in performance can be attributed to environmental stressors. The aims and corresponding chapters of this thesis were to; (1) Choose a suitable predictor of potential pig growth. (2) Develop a deterministic dynamic model to predict the effects of genotype and the nutritional and thermal environments on the voluntary feed intake, growth and body composition of growing pigs. (3) Test and evaluate the model developed in chapter 2 against experimental data from the literature. (4) Quantify the effects of social stressors on the performance of growing pigs and incorporate these into the previously developed model, including variation in ability to cope with encountered social stressors. (5) Extend the model to deal with individual pig variation. (6) Compare the variation predicted by the population model with that observed under experimental conditions.The Gompertz function was chosen as a predictor of potential pig growth and as the starting point for model simulation, i.e., to provide an upper limit to growth. It uses few parameters, holds over a wide degree of maturity and the values of its parameters can be estimated simply. Unconstrained voluntary feed intake, predicted from the current state of the pig and composition of the feed, is that required to achieve potential growth. Actual food intake and the consequent gain were predicted taking into account the capacity of the animal to consume bulk and its ability to maintain thermoneutrality. The physical environment, described by the ambient temperature, wind speed, floor type and humidity, sets the maximum and minimum heat the pig is able to lose and determines whether the environment is hot, cold or thermoneutral. Model predictions were generally in good quantitative agreement with the observed data over the wide range of treatments tested and give support to the models value and accuracy. The social environment was described by group size, space allowance, feeder space allowance and the occurrence or not of mixing. All of these factors may act as stressors and it is assumed in the model that they decrease performance by lowering the capacity of the animal to attain its potential. The parameter EX accounts or differences in ability to cope when exposed to social stressors. The introduction of individual variation in growth potential, initial state and EX allowed the mean population response to be compared with that of the average individual. Whether these responses differed depended in part upon the social stressors encountered. The addition of variation in initial state and EX allowed better estimates of the phenotypic variation observed in real experiments to be achieved.The developed simulation framework is able to explore, and at least in principle, predict the performance of both individuals and populations differing in growth potential, initial state and ability to cope when raised under given dietary, physical and social environmental conditions. One of the main advantages of simulation models is that they allow the effects of a multiple factors on animal performance to be considered simultaneously, including any interactions that may exist, in a way that cannot be done by direct experimentation. These interactions may be crucial in decision-making processes as different individuals and populations may react differently in response to the same environmental stressors.