It is not advisable to feed wild deer because a rapid transition to new food sources negatively affects their digestion and health. Providing appropriate food for wild deer is complicated and expensive.
Feeding corn, bread, fruits or other unnatural food sources to wild deer causes digestive problems for the animals. Deer must adjust gradually to new food sources, especially those high in carbohydrates, and a quick transition causes acidosis and even death. Wildlife agencies have access to professionally formulated wild deer feed, but introducing these must be gradual, and the cost of providing such food is high.
In addition to causing health problems, artificial feeding of deer trains the animals to seek out human habitation for food and causes deer to become a nuisance. When deer concentrate around feeding sites, it increases the risk of disease transmission and makes them more vulnerable to predators. Also, competition at feeding sites is higher than it is for deer moving naturally through the ecosystem, forcing animals away from food that would otherwise suffer no such problems.
There is always some degree of winter deer mortality due to exposure or lack of resources. However, this is comparatively small and is actually beneficial for a population as it ensures the survival of the healthiest animals.