Why is it important to cool dry cows and youngstock cool during hot summers: From physiology and epigenetics to economics

Exposure to heat stress during late gestation significantly im­pacts the development and health of calves, both in the short term and long term. In utero heat stress can immediately alter calf growth and immune function post-birth. Long-term ef­fects include reduced survival rates, shortened lifespan and decreased milk production. Furthermore, intrauterine hyper­thermia affects the daughters (F1 generation) and extends to their offspring (F2 generation), highlighting multigenerational consequences. Economically, the financial losses due to in utero heat stress in the U.S. are estimated to exceed $500 mil­lion annually. Heat stress during gestation disrupts mammary gland development in the offspring and induces organ-specific epigenetic changes and common methylation modifications in the mammary gland and liver. These epigenetic alterations may contribute to decreased survivability and production outcomes in affected animals. Postnatal heat stress significantly affects newborn calves dur­ing their critical developmental window before weaning. It elevates thermal stress indices, reduces feed and grain intake, and disrupts the normal development of the immune system. Environmental heat stress is detrimental to calves both before and after birth. Therefore, effective heat abatement strategies should be implemented to protect young replacement heifers and ensure their healthy development and survival in the herd.