Doctor Alan Adler, of the Medical College of Wisconsin, says researchers don't yet know why.
More than 4,000 rural Wisconsin families filled out questionnaires. Researchers wanted to compare the prevalence of asthma in farm and non-farm children.
They also found the amount of time a child lives on a farm is important.
They conclude that asthma rates are lower for kids who lived on a farm before age five, but climb for those who lived there only after age five.
The research will be published in the Journal of Allergy & Clinical Immunology.