Maternal Heat Stress and Its Effects on Calves

Heat stress (HS) is a condition characterized by high ambient temperature and humidity. Twothirds of the calves' development takes place in the last two months of pregnancy; during thisrapid growth period of fetus, the stress of the mothers negatively affects the offspring. Feedconsumption, milk production, reproduction, udder development, morbidity and mortality rateand immunity are adversely affected in dairy cows exposed to heat stress and economic loses.Low birth weight (about 9%) in calves and low milk yield in the next lactations are obtainedin dairy cows exposed to stress in the last period of pregnancy. In heat stress condition, theblood flow to the placenta and nutrients transfers to embryo decrease, also, placentalhormones circulation decrease in the mothers exposed to heat stress in the late period ofpregnancy, and the cause give birth earlier. Calves with low birth weight can only compensatefor the weight loss in a long period of one year. Reduced birth weight of calves due to in uterostress can be attributed fetal hyperthermia and impaired placental function and so, a shortergestation period. In-utero heat stressed calves have lower plasma concentrations of insulin,insulinlike growth factor-I, prolactin and but no difference in glucose, or β-hydroxybutyrate(BHBA) and non-esterified fatty acid (NEFA) plasma concentrations.. Normally, postpartumcalves are not exposed to metabolic stress due to the lack of production, and therefore they areless affected by environmental heat stress than adult animals. However, studies have shownthat calves exposed to postnatal heat stress also have a decline in growth, an increase inmorbidity and death rates in calves. For a successful calf production, in the prenatal andpostnatal periods calves must be protected from heat stress.