140 Effects of heat stress during late gestation on the dam and its calf.

Heat stress during late gestation in cattle negatively affects the performance of the dam and its calf. This brief exposure to an ad­ verse environment before parturition affects the physiological responses, tissue development, me­ tabolism, and immune function of the dam and her offspring, thereby limiting their producti­ vity. During the dry period of a dairy cow, heat stress blunts mammary involution by attenuating mammary apoptosis and autophagic activity and reduces subsequent mammary cell proliferation, leading to impaired milk production in the next lactation. Dairy cows in early lactation that ex­ perience prepartum heat stress display reduced adipose tissue mobilization and lower degree of insulin resistance in peripheral tissues. Similar to mammary gland development, placental func­ tion is impaired by heat stress as evidenced by reduced secretion of placental hormones (e.g., estrone sulfate) in late gestation cows, which partly explains the reduced fetal growth rate and lighter birth weight of the calves. Compared with dairy calves born to dams that are exposed to evaporative cooling during summer, calves born to noncooled dry cows maintain lower BW until 1 yr of age, but display a stronger ability to absorb glucose during metabolic challenges postnatally. Immunity of the calves, both pas­ sive and cell­mediated immune function, is also impaired by prenatal heat stress, resulting in in­ creased susceptibility of the calves to diseases in their postnatal life. In fact, dairy heifers born to heat­stressed dry cows without evaporative cooling have a greater chance leaving the herd be­ fore puberty compared with heifers born to dry cows provided with evaporative cooling (12.2% vs. 22.7%). Dairy heifers born to late­gestation heat­stressed dry cows have lower milk yield at maturity during their first and second lactations. Emerging evidence suggests that late­gestation heat stress alters the mammary gland microstruc­ ture of the heifers during the first lactation and exerts epigenetic alterations that might explain, in part, their impaired productivity.