Environmental temperature, particularly heat stress (hereafter referred to as HS ), negatively affects cow reproductive performance, gestation length, calf birth and weaning weights, and subsequent productivity of its offspring. We hypothesized that the effects of HS in the prenatal period manifest as lower calf birth weight, weakened immune function, and lower performance of heat-stressed offspring compared to offspring in a thermoneutral environment. Based on literature reviews, we concluded that heat stress has the greatest impact on the development of the embryo itself during gestation and on fetal growth during the dry period when the growth is most intense. The gestation period of dams exposed to HS during the dry period was on average 3 days shorter than that of chilled dams. Birth weight is the easiest method to assess growth performance during the prenatal period, so we compared results from eight studies conducted over the past decade. On average, the birth weight of calves exposed to HS during the prenatal period was 4 kg (9 %) lower than calves not exposed to this type of stress during the prenatal period. Passive immunity was weakened in calves exposed to HS due to poor absorption efficiency of immunoglobulins. Consequently, the impaired development of the mammary gland during fetal growth, is reflected in reduced milk production in the first lactation.
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