Herd closure consists of stopping the entry of replacement females, heat check and teaser boars into the breeding herd for at least eight months. The breeding herd consists of the sows and gilts at the sow farm, as well as the growing gilts designated for the farm; these growing gilts could be housed on the sow farm or in an off-site GDU. The breeding herd may be loaded with gilts and boars as described previously, and then no additions, including saving internal replacement gilts post weaning, are made until the eradication program is completed.

For this method to be successful, it is critical that the breeding herd, including all gilts, is successfully exposed to M. hyopneumoniae and effectively colonized prior to the start of herd closure. Confirmation of this is considered Day 0 of closure. Common methods for exposure used in North America include natural contact with infected pigs and aerosolization or intratracheal administration of a positive inoculum source, such as lung tissue or pooled tracheal samples containing the farm-specific strain of M. hyopneumoniae.