Genetically modified cows were born with ovaries that grew so large they caused ruptures and killed the animals.

The failed experiment happened during a study by AgResearch scientists at Ruakura, Hamilton, to find human fertility treatments through GM cows' milk.

AgResearch is studying tissue from one of the dead calves to find out what made the ovaries grow to the size of tennis balls rather than the usual thumbnail-size.

Details of the deaths - in veterinary reports released under the Official Information Act - have reignited debate over the ethics of GM trials on animals.

GE-Free NZ spokesman Jon Carapiet said details of the calf trial showed the animal welfare committee overseeing AgResearch's work was "miles away from the ethics and values of the community".

However,  AgResearch's applied technologies group manager, Dr. Jimmy Suttie, said the deaths were part of the learning process for scientists.

The calves died last year, aged six months. They were formed when human genetic code injected