Cows and buttercups

Every year at around this time we get cows in the buttercup field below our garden. This year it’s a group of young heifers of different kinds. Actually they are probably technically still calves looking how fluffy they still are.
  
And every year I ask the same question – Do cows eat buttercups? And I still don’t know the answer. I thought they were poisonous. So do they eat them, or do they avoid them and trample them down?