The body condition of breeding heifers should be closely monitored, and their total ration should be adjusted to prevent both excessive fattening and growth retardation. Live weight gains must be kept under control. Heifers should not be fed with excessive amounts of energy-rich corn silage and grains. In breeding heifers, the period of 3–8 months of age is when the mammary glands and ducts develop rapidly. If young heifers become fat during this stage, instead of glandular tissue, fat tissue (fatty udder) develops in the mammary gland. This condition results in reduced milk yield after calving. Therefore, during this period, young heifers should not be fed with energy-dense feeds such as corn silage and grains that could cause fattening. Overfeeding replacement and pregnant heifers, leading to fattening, inevitably causes dystocia (difficult calving), low milk yield, retained placenta, and post-calving metabolic disorders.