Dog owners are being asked to pick up after their pets amid fears cows are losing unborn calves due to infections from dog mess.
Neospora caninum is a parasite which can be carried by dogs.
The National Farmers Union (NFU) said picking up dog mess was vital to stop the spread of disease.
It comes after farmers in Denbighshire had to cull some of their herd thought to have been infected from their feed.
Farmers John and Stephen Tudor, of Gwerclas Farm near Cynwyd, have taken to social media to ask dog walkers to pick up after their pets.
The pair said tests showed the cows, which lost their young in late pregnancy, identified 'neospora caninum' as the cause.
They tweeted: "The vet stated that the likely source is silage contaminated by dog faeces. There is no vaccine to prevent or cure this condition.
"Unfortunately, infected cows had to be culled as they will continue to abort every year.
"We have no issues with people walking their dogs along the footpaths, and know that some do clean up after them, therefore, this is an appeal to those who don't."
Denbighshire vet Dyfrig Williams said the infection was one of the most "frequently diagnosed causes of abortion in cows".
"I'm not sure that it's something you'd think of as a dog owner," he said.
"It's probably something we could do with further information for dog owners that walk on farmland, just to make sure that they do clean up after their dogs and have respect for livestock in the fields."
The NFU called on dog owners to pick up after their pets and to keep them away from animal food and water troughs to stop the spread of disease.