Goats are clever animals. You watch them and they peacefully mind their own business which is eating browse (plants, shrubs, trees and reluctantly grass and weeds) Contrary to the stereotype, goats are fussy eaters and will turn up their noses at anything that doesn't look tasty. Only the most expensive hay will be eaten. Only the tastiest plants. The tastiest plants of all are the ones that they aren't supposed to be able to get to. Those plants are also the most expensive and require backbreaking labor to put in. Specifically, landscaping and the vegetable gardens. The goats will risk their lives to get a few mouthfuls of the garden.
Our craftiest goat is 12. She will eat grass until she sees you turn your back, then WHAM! She's through the fence and eating the tomato plants. She gives no thought that tomatoes are nightshades and therefore will make her sick, she just knows that they are forbidden and so are the tastiest plants of all. It is for goats like 12 that we are making our cactus fence.