PSV-16 - Different late gestation heat stress time-windows and their effects on newborn Holstein calves from northwest México

Abstract: Exposure to heat stress (HS) during the dry period not only exerts negative effects on the welfare of pregnant cows, but also impairs the development of the fetus in the utero. When dams are subjected to late-gestation HS, affectations to their placental function and blood placental flow occur, generating fetal hypoxia and growth retardation of their calves. Since on the last 60 d of gestation bovine fetuses undergo through a phase of accelerated growth where they accrue around 60% of their birth weight, the number of days that their dams become exposed to HS during this period could affect differently their weight and oxygenation status at birth. Therefore, the objective of this study was to determine the effects of different late-gestation HS time-windows on the birth weight and oxygen saturation of newborn Holstein calves. The study was conducted at the Yaqui valley in northwest México, a semiarid zone in where the temperature and humidity index (THI) exceeds the dairy cattle thermal threshold from mid-March to mid-October (~7 months). Under such environmental dynamic, all cows within this geographical zone experience HS at some point during their gestation; nonetheless, depending on the cows' conception date, different late-gestation HS time-windows arise. To assess the influence of the number of days of intrauterine HS exposure during late gestation (e.g., days with an average THI > 72 units), three intrauterine heat-stressed groups of calves were formed: G1 = calves born to dams exposed to HS less than 20 d within the last 60 d of gestation (n = 30), G2 = calves born to dams exposed to HS between 21 and 40 d within the last 60 d of gestation (n = 20), and G3 = calves born to dams exposed to HS for more than 40 d within the last 60 d of gestation (n = 16). Upon calving, variables such as sex, birth weight (BW, kg) and oxygen saturation (SpO₂, %) were recorded. A general linear model that included the effects of sex of the calf and intrauterine HS calves’ group was implemented to analyze BW and SpO₂. Harmonic means where compared via the Tukey-Kramer test. Harmonic means (±SE) for BW and SpO₂ among groups were 42.42±0.92, 40.84±1.11, and 34.73±1.24 kg, and 78.16±2.71, 82.52±3.28, and 82.29±3.68 %, for G1, G2 and G3, respectively. Significant differences (P < 0.001) were found only for BW, with calves of G3 exhibiting the lowest BW in comparison to calves within G1 and G2, which were not statistically different among themselves (P > 0.05). Results of this study indicated that intrauterine HS exposures of >40 d strive deleterious effects on fetal growth and reduce the BW of newborn Holstein calves from northwest México.