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* Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, Lennoxville, Quebec, Canada J1M 1Z3;
and
Animal Nutrition and Health Department, Scottish Agricultural College, West Mains Road, Edinburgh, EH9 3JG, Scotland, UK; and
and
PIC International Group, Schleswig, Germany
2 Correspondence: P.O. Box 90, 2000, Route 108 East (phone: 819-565-9171; fax: 819-564-5507; E-mail: pomarc{at}agr.gc.ca).
Pig production efficiency is the result of the responses of individual animals. However, experimental results are usually interpreted on the basis of mean animal responses with little emphasis given to variation around the means. Animals with different genetic potentials may respond differently to treatments, which makes it difficult to translate average population responses into either individual animal responses or across populations having different variation between animals. Nutritional theories that form the basis of current pig models are all at the level of individual animals. The problem of how to integrate across individuals to obtain population predictions is rarely addressed. To illustrate the effect of between-animal variation on population responses to dietary treatments, a pig growth model that predicts voluntary feed intake from parameters that predict the potential rates of protein and lipid retention was used. Feed intake is limited only by the ability of pigs to lose heat. Maximum heat production was set to always be that of a pig growing to its potential on a standard, balanced diet. An individual animal is defined by assigning values to three genetic parameters: protein weight at maturity, the ratio of body lipid-to-protein at maturity and a rate parameter. The model was made stochastic by assigning variation to these parameters. Population responses to increasing levels of available protein intake indicated that the linear-plateau model used to represent protein responses for an individual pig is compatible with the curvilinear response observed in experiments on populations. Increasing the time over which individual animal responses were measured also increases the curvilinearity of the response. Variation between animals has little effect on population feed intake, but it decreased population protein deposition rate, daily gain, and feed conversion ratio. It is concluded that mathematical models designed to simulate populations responses to treatments need to integrate the effect of population variation on growth and performance.
Key Words: Genetic Variation • Growth • Protein • Simulation Models
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