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It takes deer take 2-4 weeks to adjust to a new diet, so start feeding them gradually. Slowly introducing new types of food into a deer's diet will help their digestive system learn to process it. Deer typically eat woody vegetation that is found in forests, so feeding them foods that are rich in fats and carbohydrates can be extremely harmful. Begin introducing new foods by combining supplemental foods with deer's natural diet. Start by including a small amount of the new food, and gradually incorporate more and more, ultimately replacing their natural diet. In winter, their natural food supply will decrease significantly (if not completely), so it is recommended to begin this transition before winter hits. These mixes can typically be found at feed mills or pet supply stores. Deer feed mixes are usually a mix of alfalfa, oats, soybeans, molasses and several vitamins and minerals. This type of food is easiest for deer to digest, which is what makes it ideal for supplemental feeding. You must be careful when feeding deer supplemental foods, as they have a sensitive digestive system. If formulated deer food mixtures are unavailable, oats are the next best supplemental food for deer. Oats provide deer with a healthy mixture of fiber and carbohydrates without disrupting their digestive system.  A wide variety of fruits and vegetables – including apples, grapes, cherries, pears, carrots, and snap peas – are eaten in nature by deer. Therefore, it is safe to feed deer these fruits. Acorns are another safe food source. A deer's digestive system is complex, and corn happens to be one of the worst types of food for them. Unfortunately, many deer end up extremely sick or dead because animal lovers believe that corn is good for deer. When deer are offered a sudden supply of corn, they aren't able to adjust to the high carbohydrate diet, and they end up dying. In nature, deer eat twigs and other natural forest vegetation. To provide them with more of their natural food source, cut down branches that are out of their reach. This is one of the safest and most natural ways to keep deer fed throughout the year.

Summary:
Introduce deer to a new diet slowly. Purchase formulated deer food mixes. Feed deer the right foods if you cannot find formulated deer mixture. Do not feed deer a corn diet. Cut down tree branches to feed deer.