How you handle replacement gilts can impact your herd's health status for a long time. Nowhere has that been more evident than with the porcine reproductive and respiratory syndrome virus.

If your herd has encountered PRRS, any incoming breeding stock that has never been exposed to PRRS must be exposed to the strains found in your herd. This is critical regardless of whether your herd has recently had a clinical PRRS outbreak, says John Roberts, veterinarian at North Carolina State University.

Two key factors:

1. Incoming breeding stock must be exposed to adequate levels of PRRS virus, causing them to seroconvert the disease.

2. As many animals in the group as possible must be infected at the same time.

Of course, exposure should occur away from your main herd. Exposure can happen through natural infection or vaccination.

Natural exposure needed to immunize new animals is difficult to measure and can be variable. "It can be self-limiting because as soon as a herd starts to quiet down with PRRS signs, the virus may stop replicating and the value of using culls will be gone," says Roberts. That could lead to a break later on. Also, a 90- to 120-day off-site isolation and acclimatization is required before naturally infected animals can enter the breeding herd.

If that time frame ties your hands, and you're isolation/acclimatization period is limited to 60 days, then vaccination may be the route for you. The vaccination will need to occur early in the isolation stage. Work the details and timeline out with your veterinarian.