OSLO (Reuters) - Comfortable Norwegian cows are producing more milk and have less udder infections since new regulations allowing them to relax for up to half a day on soft rubberized mattresses were introduced.

In 2004, Norway introduced rules to replace gradually all sheds where cows are kept in stalls with ones that allow them to move more freely and lie down on a softer surface.

"They lie about half of the day, which is natural for them," doctoral student Lars Erik Ruud told Reuters after researching the impact of the rules, the first of their kind in the world.

"Production increases by 5-6 percent," Ruud said, explaining that a more relaxed lifestyle boosted the volume of blood flowing through the cows' udders, which meant more liters of milk produced from each cow.

But the rules also have a drawback, he said, as the relaxed cows' hooves do not naturally wear down as they would if in contact with a hard surface.

(Reporting by Aasa Christine Stoltz)