We get some calls on sudden deaths in pasture and often it is Nightshade. This is a weed that commonly grows in fence rows, waste areas, grain and hay fields. It can easily be killed with 2-4-D and new areas should be inspected before sheep or goats are allowed to graze. Probably every pasture has some, but if it is prevalent it can cause a problem. Treatment is not rewarding, prevention of consumption is the key to prevention.
Drying the weed as hay or waiting until after a freeze won’t reduce the toxicity; neither will fermenting it as silage.