My answer: True, rhubarb leaves contain oxalic acid, which is toxic. Never eat fresh or cooked rhubarb leaves and don’t feed them to animals/pets. It’s rare for rhubarb leaves to kill a human but it could cause of myriad of serious health problems involving the digestive tract, kidneys, etc.
There is the old story of the farmer who fed the rhubarb leaves to his pigs, and the pigs died. I’m not sure if the story is true but when we raised pigs on our farm, we often fed them fruits and vegetables from our garden. Giving the pigs rhubarb plants would have only seemed logical but fortunately we’d been warned about the toxic leaves so we were able to avoid mishaps. Thank you for you question, Alyce.