A resource allocation model describing consequences of artificial selection under metabolic stress.

Long-term selection on production results in increased environmental sensitivity. This often is expressed through decreased fertility and increased health problems. The phenomenon has been described in all common farm animal species. One theory is that potential resource intake is insufficient to express production potential. Additional resources are drawn away from fitness-related traits, such as fertility and health, to further increase observed production. In addition, resources for maintaining fitness depend on the demands by the environment. In a harsh environment, more resources are required for fitness-related traits than in an optimal environment. Literature results show that selection in an optimal environment will increase sensitivity to less optimal environments. The objectives of this paper were to increase understanding of the underlying mechanism behind the development of environmental sensitivity and to gain insight into correlated response(s) when selection is on observed production. A resource allocation model was defined where observed production depended on production potential, resource intake potential, and the allocation of resources to production or fitness, including maintenance, health, and reproduction. Penalties for reproductive performance and probability of survival were included when the proportion of resources assigned to fitness dropped below a certain, environment-related, threshold. Mass selection was practiced on observed production during 40 generations using stochastic simulation. Depending on the heritabilities of the underlying components and on the environment, selection on observed production resulted in a decrease in reproductive rate and in the development of environmental sensitivity when resource intake becomes limiting. Correlations of observed production with underlying components changed across generations, following a nonlinear pattern. The proposed model is simple, but increases the understanding of underlying mechanisms and consequences of selection for production when resources are limiting.