Many people welcome sightings of deer in their backyard. It's hard not to admire the proud stance, the tawny coat, and the soulful brown eyes of deer. You might even want to put food out for them, especially in winter when their food sources gets scarce. But that act of kindness could cause the deer more harm than good.
Deer have very sensitive stomachs, and feeding them "people food" such as lettuce, table scraps or corn can make them sick. In addition, these kinds of foods don't give deer enough nutrition to keep them going. If you do feed deer, give them specially formulated deer or horse pellets. These foods carry a risk too. Left outdoors, even in feeders, this food is likely to get wet, which will cause it to mold. In cold weather, the deer may not smell the mold. Eating it will cause them to get sick or die.
Deer tend to congregate around artificial feeding stations. Predators, such as bobcats and coyotes, will notice and start seeking deer in those locations. Even domesticated dogs that are allowed outside can attack the visiting deer. Another threat to consider is traffic. Deer that come to expect food in your backyard will not always travel a safe course to get there. Some will cross busy roads in front of the house, which will increase the chance of a collision with a car. Deer rarely come away from these accidents alive.
When winter comes, deer move to find better food sources and a habitat that better protects them from severe weather. If they come to expect food in your backyard, they'll stick around for that and put off their migration instincts. In addition to the lack of suitable winter food they would normally get themselves, they are more at risk of freezing to death.
In the wild, deer range over large areas. Being spread out like this means each deer has its own food sources. An artificial feeding station will attract all the deer into one small area. Setting out food for one cute doe and her fawn will eventually bring other deer that want this easy meal. The bigger and stronger deer will push out the smaller and weaker ones to get to the food.
Attitude Toward Humans
Deer in the wild are naturally afraid of people; it's part of their survival instinct. If you feed them, they will start to associate people with food and lose their natural fear. The deer won't recognize that some people have other intents and that other person could harm them.