Kids who had contact with cows and drank their milk also had a lower chance of getting hay fever.
Illi said it's possible the actual cows or straw might not be responsible for the reduced risk, but it could be that microbes in their vicinity, for example, have a protective effect.
This study and others suggest that "exposure to non-pathogenic microbes seems to be an important part of the education of the immune system," Gern told Reuters Health.
He said the link between straw and asthma could mean that exposure to certain plants is also beneficial for kids.
Although contact with cows and their milk and straw accounted for much of the lower asthma risk, about half of the farm effect remained unexplained, Illi told Reuters Health.
It would be valuable to find out what exactly is responsible for the lower asthma and allergy rates to ultimately develop some sort of intervention for kids who don't grow up on farms, according to Gern.
Still, Illi said there's currently not enough evidence to say bringing pregnant women or young children to a farm -- or exposing them to hay or raw milk -- could reduce later allergy or asthma risks.
SOURCE: bit.ly/JLQCG4 Journal of Allergy and Clinical Immunology, online April 23, 2012.