Making future udders: Mammary development and perinatal programming of dairy cattle.

The perinatal period, described herein as the time spanning the final 2 mo of gestation through 2 mo postnatal, is a critical window of developmental plasticity for many organs in placental mammals, including the mammary gland (MG). In dairy cattle, early-life MG development involves foundational morphogenic events that are highly sensitive to environmental and nutritional factors. Emerging evidence challenges the long-standing belief that substantial MG development begins after weaning and lasts until puberty, showing instead that preweaning mammary parenchymal (mPAR) growth is allometric and its degree of development can influence future lactational capacity. Overall BW typically doubles from birth to 60 d, whereas both whole udder weight and the mammary fat pad increase by ∼2.2 to 3 times over the same period. In contrast, the mPAR exhibits pronounced allometric growth, expanding 15 to 35 times during this time. Early-life management strategies, such as heat abatement to maintain thermal homeostasis and enhanced nutrition through higher milk intake supporting greater average daily gain, further promote mPAR development and can positively influence future lactation performance of dairy cows. These findings underscore the importance of integrating developmental biology into heifer-rearing strategies and emphasize the need for precise environmental and nutritional management during this critical window to support lifelong mammary function and optimize herd performance.