In utero heat stress compromises whole-body growth and mammary development from postweaning through puberty

Late-gestation in utero heat stress derails organ development and growth, leading to reduced first-lactation milk yield. Yet, carry-over effects of in utero heat stress on peripubertal heifers remain unexplored. This study investigated the long-term effects of in utero heat stress on body growth, mammary gland (MG) development, and ovarian function in peripubertal heifers. Dry, pregnant cows were housed in freestall barns and provided only shade or shade plus evaporative cooling for the last 54 ± 5 d of gestation during a subtropical summer. The resulting in utero cooled (IUCL, n = 21) and in utero heat-stressed heifers (IUHT, n = 19) were raised as a single cohort until 12 mo of age. Body weight, hip height (HH), chest girth (CG), body length (BL), withers height (WH), head circumference (HC), and MG macrostructure was recorded monthly from 3 to 12 mo. Once puberty was confirmed with 2 consecutive weeks of plasma progesterone (P₄) concentrations >1 ng/mL, heifers were assigned to a protocol for the synchronization of estrus (D-7: GnRH + CIDR, D-2: PGF_2α, D-1: PGF_2α + CIDR removal). Blood and MG biopsies were collected on d 0 (high estradiol [E₂]/low P₄) and d 13 (low E₂/high P₄) to quantify mammogenic hormones and MG microstructure. Ovarian ultrasounds were conducted on d 0, 9, and 13 to analyze the cross-sectional area and volume of the dominant follicle and corpus luteum. Data were analyzed in SAS. A treatment by month interaction was observed for BW and CG, whereby IUHT heifers were lighter (3, 7, and 8 mo) and had smaller CG (3, 4, and 8 mo). Additionally, IUHT heifers had shorter HH (114.8 vs. 117.5 ± 0.6 cm; IUHT vs. IUCL), BL (100.6 vs. 102.2 ± 0.5 cm), and WH (108.3 vs. 110.4 ± 0.6 cm) and smaller HC (74.9 vs. 75.8 ± 0.3 cm). The distance between the teats and width of the MG did not differ, but teat lengths were shorter in IUHT heifers (front teats: 25.5 vs. 28.3 ± 0.6 mm; rear teats: 22.1 vs. 24.4 ± 0.6 mm). At 12 mo, MG length was shorter in IUHT than IUCL heifers (78.5 vs. 85.5 ± 1.4 cm). Fewer proliferating (0.38 vs. 0.85 ± 0.10%) and ERα-positive cells (10.1 vs. 12.8 ± 0.7%) were observed in IUHT heifers. In the MG fat pad, IUHT heifers had more adipocytes, particularly on d 13 (47.7 vs. 33.9 ± 3.3 cells). Although systemic P₄ was similar, E₂ concentrations were reduced in IUHT heifers on d 0 (3.8 vs. 5.0 ± 0.3 pg/mL). Additionally, IUHT heifers had a smaller follicle diameter and volume (diameter: 13.5 vs. 15.5 ± 0.6 mm; volume: 1,468.0 vs. 2,208.5 ± 262.0 mm³) and a smaller corpus luteum diameter and volume (diameter: 22.2 vs. 26.9 ± 0.7 mm; volume: 6,170.4 vs. 10,506.0 ± 617.7 mm³). Overall, exposure to in utero heat stress had lasting implications, leading to a disruption in the crosstalk between the MG and ovaries during the peripubertal phase. The observed effects during peripuberty may provide insight into the reduced lactation performance at maturity for in utero heat-stressed heifers.