Mycoplasma hyopneumoniae (Mhyo) is often neglected unless pigs are contagious at merchandise. Antibodies play an important role in the defence towards disease (1), and the ability to produce such antibodies is poor in young piglets (4). Vaccinations initiated as pigs are aged 10 weeks have prevented spread of disease by 7 month old replacers sold from nucleus herds infected with Mhyo (5). The present study aimed to evaluate whether successful vaccinations also could be initiated earlier, i.e. when the piglets in breeding herds are selected as potential replacers at the age of three weeks. Herd, management routines and initial health status: The study was made in a nucleus herd with 170 Yorkshire sows. Groups of 28 sows farrowed batchwise every 4th week in previously emptied and cleaned units. As the piglets were aged 3 weeks approximately 110 gilts and 25 boars were selected as potential breeders and given an identity. All piglets were weaned at the age of 5 weeks and remained in the pen of birth until the age of 11 weeks. At that age the gilts were transferred to age segregated facilities, while the boars were transferred to a unit also housing other growing boars. The different rearing systems were mirrored by the time for seroconversion to Mhyo, taking place earlier among boars (age 15 to 20 weeks) than among gilts (age 19 to 27 weeks). Animals not selected for breeding were sold out at the age of 12 weeks.
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