Selection for litter size and its consequences for the allocation of feed resources: a concept and its implications illustrated by mice selection experiments

Reproductive failure contributes to more than 50% of total cullings in sows after first weaning. Prolonged rebreeding intervals are often associated w ith metabolic imbalance, disease or stress. Furthermore, a pre-weaning mort ality of more than 20% is not unusual and is mostly due to problems of deve lopment and adaptation. The present study describes the consequences of sel ection for litter size for the allocation of feed resources, in relation to reproductive performance and pup development. Implications are illustrated by mice selection experiments. Average litter size of females from a line selected for high litter size at birth (S-line) is about twice (20) the lit ter size of females from a non-selected control line (C-line; 10). Lactatin g S-line females reallocate more buffer resources towards lactation than la ctating C-line females. Furthermore, they mobilize body stores for a longer period of time. This means that S-line females produce more offspring but at a greater cost to their own metabolism. This process was insufficient to supply offspring with adequate resources, resulting in reduced pup develop ment and increased pre-weaning mortality rates. Results are discussed in re lation to pig production. Increased genetic gains will be compromised in th e long-term if the short-term focus is on a single production trait only.