The lower birth weights observed by the researchers suggest reduced placental function and earlier delivery, with less progressed fetal development. The lower weights of the heart, liver, and kidneys among calves born to heat-stressed dams also suggest compromised placental and fetal development.

Senior author Geoffrey E. Dahl, PhD, of the University of Florida, Gainesville, FL, USA, explains that “Calves, like all young animals, are prone to elevated rates of mortality and morbidity in the neonatal period, and preterm birth may further exacerbate that problem. Early-life losses may result from organ immaturity and dysfunction, notably of the gastrointestinal tract and the immune system.”

The team suggests that reduced weights of the thymus and spleen may be associated with slowed fetal growth and compromised immune function, as these organs play important roles in the development of a robust immune system. The higher rate of intestinal cell death among calves born to heat-stressed cows suggests that these calves may have a reduced ability to absorb immune factors from colostrum, essential to the transfer of maternal immunity to the immature and vulnerable calf, in the critical first hours after birth. “Passive uptake of immunoglobulins from colostrum is the only mechanism of immunoprotection in the bovine,” Dahl points out