Late-Gestation in utero Heat Stress Limits Dairy Heifer Early-Life Growth and Organ Development

Abstract

Dairy calves exposed to late-gestation heat stress weigh less, have impaired immunity, produce less milk across multiple lactations, and have reduced productive life. However, less is known about the relationship between in utero heat stress and organ morphology and development. Herein, we characterized the consequences of late-gestation in utero heat stress on body and organ growth trajectories during early-life development. Holstein heifers were either in utero heat-stressed (IU-HT, n = 36, dams exposed to THI > 68) or cooled (IU-CL, n = 37, dams exposed to THI > 68 with access to active cooling) during late gestation (54 ± 5 d prepartum). All heifers were reared identically from birth to weaning. Upon birth, calves were weighed and fed 3.78 L of colostrum followed by 0.87 kg DM/d milk replacer (MR) over two feedings and ad libitum starter concentrate daily. Weaning began at 49 d and ended at 56 d of age. Feed intake was recorded daily, and body weight (BW) and other growth measures were recorded at 0, 28, 56, and 63 d. Blood was collected at d 1 then weekly. Subsets of heifers were selected for euthanasia at birth and 7 d after complete weaning (n = 8 per group each) to harvest and weigh major organs. Reduced BW and stature measures persisted in IU-HT heifers from 0 to 63 d of age with a 7% lower average daily gain and reduced starter consumption relative to IU-CL heifers. IU-HT heifers had lower hematocrit percentages and reduced apparent efficiency of absorption of IgG relative to IU-CL heifers. Additionally, IU-HT heifers had reduced gross thymus, spleen, thyroid gland, and heart weight at birth and larger adrenal glands and kidneys but smaller ovaries relative to BW at 63 d. The mammary gland of IU-HT heifers was smaller relative to IU-CL heifers at birth and 63 d adjusted for BW, suggesting mechanisms leading to impaired milk yield in mature IU-HT cows are initiated early in development. In summary, in utero heat stress reduces whole-body size and limits development of key organs with potential repercussions on dairy calf metabolic adaptation, immune function, and future productivity.

In utero hyperthermia in late gestation derails dairy calf early-life mammary development

“…Reduced overall birth and weaning BWs are a hallmark of late-gestation in utero heat stress ( Tao et al, 2019 ). This was also reported in the present study ( Dado-Senn et al, 2021 ), though the subsets of heifers euthanized did not have a difference in BW at the time of euthanasia. Regardless of the lack of BW difference, the difference in mammary size remains when accounting for BW.…”

“…A detailed description of dam and calf treatment and management can be found in Dado-Senn et al (2021) . Briefly, multiparous pregnant Holstein dams were dried off at 54 ± 5 d before parturition and simultaneously enrolled into heat-stress ( HT ; n = 41) or cooling ( CL ; n = 41) treatments.…”

“…Continuous recording of pen temperature–humidity index ( THI ) averaged 79 across the treatment period, indicating cows were above the THI thresholds for dry period heat stress ( Ouellet et al, 2021 ). In addition, dam respiration rate (53.5 vs. 77.3 ± 0.6 breaths per min for CL vs. HT dams, respectively) and skin temperature (34.1 vs. 36.0 ± 0.05 °C) were different between maternal treatments ( P < 0.001; Dado-Senn et al, 2021 ), confirming the cooling treatment efficacy in restoring thermoneutrality. Heifers born to these dams were either in utero heat-stressed ( IU-HT ; n = 36) or in utero cooled ( IU-CL ; n = 37) during late gestation.…”

“…Starter grain concentrate ( Supplementary Table S1 ) and water were offered ad libitum from birth. Dry matter intake was recorded daily and averaged weekly, and body weight ( BW ) was recorded at 0, 28, 56, and 63 d of age ( Dado-Senn et al, 2021 ). Briefly, the entire population of IU-HT heifers weighed 4 to 6 kg less than IU-CL heifers from birth (34.4 vs. 39.2 ± 0.9 kg for IU-HT vs. IU-CL, respectively; P < 0.001) to weaning (78.9 vs. 84.9 ± 1.0 kg; P < 0.001).…”

“…Our lab recently demonstrated that late-gestation maternal heat stress, and consequent in utero hyperthermia, reduced offspring organ mass in early life, including the MG ( Dado-Senn et al, 2021 ). However, additional investigation into mammary morphology and tissue architecture was warranted.…”