As a parent, there will be many times when your child will not enjoy what you are doing.  For example, taking your little one to the doctor for regular check-ups is important for monitoring growth and development, despite their discomfort of receiving vaccines. Formula feeding is as important as those check-ups.  It is more than just a cow's milk-based product meant for babies.  It is specifically formulated to support an infant's nutritional needs.  Whole cow's milk is inappropriate because it lacks sufficient vitamin E and essential fatty acids that promote weight gain and cognitive development (source).  Infant kidneys are not mature enough to handle the high amount of sodium and potassium.  Cow's milk also lacks iron, and giving it to babies too soon can cause anemia (source).  For these reasons, even if it becomes difficult, you should try to continue formula until the first birthday.  If the amount your infant takes significantly decreases, be sure to offer a variety of supplemental foods that provide proteins, fats, iron, and vitamins. Sometimes, switching formula brands may appeal better to your baby's taste preferences.  You can compare the differences between Enfamil and Similac formulas here.