Not entirely correct . Ungulates digest via fermentation , so they can extract nutrients from very low energy food sources such as grass efficiently , and hay is mostly grass . Ever watch a deer browse a round bale ? Suddenly dumping an entirely different browse into the mix like he does can harm or even kill them , but not for the reason the other guy stated . A deer's digestive system takes time to adjust from one browse type to another , once most of the wild mast and ag crops are gone they're pretty much living on bark , evergreen needles , and other low energy browse , along with their fat reserves . Their system is aclimated to the currently available browse , switching suddenly doesn't allow their system time to accomodate the new matter so they can't extract the nutrients efficiently enough to sustain themselves . Deer have been found starved to death with full bellies , which is why feeding them is illegal in some states .