Abstract
Trade‐offs between life history traits are expected to occur due to the limited amount of resources that organisms can obtain and share among biological functions, but are of least concern for selection responses in nutrient‐rich or benign environments. In domestic animals, selection limits have not yet been reached despite strong selection for higher meat, milk or egg yields. Yet, negative genetic correlations between productivity traits and health or fertility traits have often been reported, supporting the view that trade‐offs do occur in the context of nonlimiting resources. The importance of allocation mechanisms in limiting genetic changes can thus be questioned when animals are mostly constrained by their time to acquire and process energy rather than by feed availability. Selection for high productivity traits early in life should promote a fast metabolism with less energy allocated to self‐maintenance (contributing to soma preservation and repair). Consequently, the capacity to breed shortly after an intensive period of production or to remain healthy should be compromised. We assessed those predictions in mammalian and avian livestock and related laboratory model species. First, we surveyed studies that compared energy allocation to maintenance between breeds or lines of contrasting productivity but found little support for the occurrence of an energy allocation trade‐off. Second, selection experiments for lower feed intake per unit of product (i.e. higher feed efficiency) generally resulted in reduced allocation to maintenance, but this did not entail fitness costs in terms of survival or future reproduction. These findings indicate that the consequences of a particular selection in domestic animals are much more difficult to predict than one could anticipate from the energy allocation framework alone. Future developments to predict the contribution of time constraints and trade‐offs to selection limits will be insightful to breed livestock in increasingly challenging environments.
Highlights
Most livestock breeding programmes currently generate hyperproductive animals: in less than half a century, the chicken body growth rate has increased almost fivefold (Collins et al, 2014; Havenstein et al, 2003; Zuidhof et al, 2014), annual egg production of layer hens has doubled (Preisinger & Flock, 2000), pig litter size at birth has increased since the nineties by one piglet every 5 years (Merks et al, 2012), and annual milk yield of dairy cows has increased by 140 kg every year from the seventies
Observations in livestock do not conflict with the compelling evidence that supports resource allocation in the broad sense—that is the fact that changes in energy and material apportioning are part of selection responses
As observed in our study, genetic changes in energy allocation to maintenance are clearer when feed intake is part of the selection objective than when selection is based on a productivity trait
Summary
Most livestock breeding programmes currently generate hyperproductive animals: in less than half a century, the chicken body growth rate has increased almost fivefold (Collins et al, 2014; Havenstein et al, 2003; Zuidhof et al, 2014), annual egg production of layer hens has doubled (Preisinger & Flock, 2000), pig litter size at birth has increased since the nineties by one piglet every 5 years (Merks et al, 2012), and annual milk yield of dairy cows has increased by 140 kg every year from the seventies. In limiting environments (path 3 in Figure 1), trade-offs show up because any increase in productivity should result in reduced energy allocation to maintenance during the productive phases of animal life (cf prediction 1). If this reduction is associated with insufficient maintenance and repairing processes, the capacity to start over a new productive cycle or to remain healthy should be impaired (cf prediction 2). Energy allocation trade-offs are unlikely to occur as far as the feed supply keeps up with genetic gains in productivity (paths 1 and 2 in Figure 1), unless the levels of productivity approach physiological limits stemming from time constraints. Compared with Unselected line Unselected and opposite lines Unselected and opposite lines Unselected line Unselected line Unselected and opposite lines Opposite line Unselected line Taiwanese native strain Slow-growing egg-type strain
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