Final answer:
Breeding sows in Seed Stock production systems are kept until they farrow their first litter. The focus is on the success of their gestation and farrowing to evaluate their genetic contribution.
Step-by-step explanation:
In the context of Seed Stock production systems for swine, breeding sows are generally kept in the system until they farrow their first litter. The mentioned gestation period for sheep of approximately 150 days provides insight into the management of breeding animals in livestock production, but does not directly apply to swine.
In this Seed Stock system, sows stay in production until they complete a successful gestation and farrow, after which their value as contributors to the genetic stock is assessed. During this time, the sows are responsible for nourishing and raising their piglets. Once the piglets are weaned, the sows may or may not be bred again depending on the specific breeding goals of the operation.