Do you have Periwinkle (
Vinca minor) growing there?
According to "The Complete Herbal Handbook For Farm And Stable", periwinkle is an old peasant's way to dry off an animal in the last months of pregnancy.
This dosage is for goats (so I would double it for cows):
One and a half handfuls of leaves & flowers brewed in 2 pints of water.
Give one cup morning, and evening.
Good luck.