In preparation for herd closure and exposure, a sufficient number of replacement gilts will need to be secured, based on the production goals of the farm and anticipated selection and mortality rates for the complete closure period. The structure of lot groups is defined by the age of gilts to be used throughout the closure process. Gilt source considerations include ability to supply enough gilts at the appropriate age distributions, health status, desired genetics, etc. Incoming gilts need to be housed in a dedicated on-site or off-site gilt development unit that will facilitate the exposure process to be explained at the next station. Gilts should arrive at the gilt developer unit (GDU) prior to planned exposure and targeted Day 0 of herd closure. The GDU(s) housing exposed gilts are considered part of the breeding “herd” for the duration of herd closure.