There are different types of closed herds, says Rick Tubbs, DVM, Bowling Green, Ky. He offers a synopsis of what they involve.
1. No introduction of outside genetic material, period. Some breeding stock companies, particularly at the nucleus level take this approach. In this system, genetic improvement relies on internal testing, indexing and selection.
2. Closed to animal introductions except for cesarean-derived piglets. Herds at the genetic nucleus level use this approach to introduce animals with desirable genetic traits from both sire and dam lines.
3. Closed to animal introductions, which involves introduction of genetic material via frozen semen or embryos only. This allows the semen or embryos to be stored for a defined period of time while testing and observation is done in the source herd. When it’s confirmed that no disease outbreaks occurred during the semen or embryo-collection period, the material can be brought into the farm.
4. Closed to animal introductions. In this case, genetic material is introduced via fresh semen. This approach, combined with long-term semen extenders, allows for some testing and observation at the source herd before the genetic material is introduced. It does not allow the same security (because of time) that using frozen semen allows.
5. Closed to gilt introductions. Here, replacement gilts are raised internally in a roto-terminal or maternal indexing system. Boars are purchased externally and are the only live-animal introductions. This approach limits but does not eliminate the risk of animal introductions. Sire-line needs could be met in this system by purchasing all semen, thus further limiting animal introductions.
6. Closed to parent gilt introductions. Grandparent gilts and parent boars introduced periodically to meet the needs of the farm. This approach also limits the number of animals introduced and the number of times animals need to be introduced, but does not eliminate the need to introduce some animals and the associated risk. Sire-line needs could be met in this system by purchasing all semen, thus further limiting introduction risks.
7. Closed to gilt introductions from external sources, but with gilts raised within the system on another site introduced periodically. This approach keeps the health status and related decisions in the hands of the herd owners, but does not eliminate risks to “unknown” diseases or those associated with transport.