I would think they would love the berries.Not sure if this is a wife's tale but I was told rhubar has toxic leaves.
Rhubarb is a high oxalate food. Which means that it contains high levels of oxalic acid. It wont kill your animals to eat small amounts of it but large amounts causes calcium to be robbed from the bones in order to digest it and will result in weak bones that will break more readily. Hi oxalate feeds are mainly a problem for growing animals and include kikuyu grass, silverbeat and beets (as well as the rhubarb).