Researcher John Harper says sheep were brought in never having tried grape leaves and then allowed to stuff themselves on vines. Then they fed them leaves doused with lithium chloride… 

"…and that proceeded to give them a mild stomach upset that they associated with eating grape leaves and manipulating their behavior in that way is one way to create this taste aversion." 

Sheep that were given the chemical left the grapes alone when they were set loose at an experimental vineyard.

Harper says farmers may soon be able to let sheep graze the floor of a vineyard, instead of using herbicides.