Gestational heat stress alters maternal vaginal microbiome, plasma metabolic markers, and preweaning offspring performance in Bos indicus -influenced cow-calf pairs

This 3-year study evaluated the effects of gestational access to shade on maternal performance, vaginal microbiome composition, and offspring preweaning performance in beef cows. On day 0 of each year (92 ± 20 days prepartum), 176 multiparous, pregnant Brangus crossbred beef cows (25 to 37.5% Bos indicus) were stratified by bull, body weight (BW; 548 ± 51 kg) and body condition score (BCS; 5.52 ± 0.70; scale of 1 to 9), and randomly assigned to 1 of 16 bahiagrass (Paspalum notatum) pastures (11 cows and 8.1 ha per pasture). Treatments were randomly assigned to pastures on day 0 of each year (8 pastures per treatment) and consisted of cows provided (SH) or not (NSH) access to artificial shade (3.27 m2 per cow) from day 0 to 90. Cows were provided free-choice access to artificial shade from day 91 until calf weaning on day 330 (240 ± 20 days of age). Non-binary and binary data were analyzed as a complete randomized study using the MIXED and GLIMMIX procedure of SAS, respectively. Effects of treatment × day × year and treatment × year were not detected (P ≥ 0.11) for any variable, except for calf preweaning BW (P = 0.01). Shade access improved (P ≤ 0.05) cow BCS on days 30, 60, and 150 and modified plasma metabolites, including lower cortisol on day 30 and greater glucose and IGF-1 but lower urea nitrogen on day 60. Shade also reduced (P ≤ 0.05) body surface and rectal temperatures on day 60, respiration rates, and intravaginal temperatures from 1000 to 1400 h. Plasma non-esterified fatty acids, whole blood mRNA expression of heat shock proteins, and reproductive success did not differ (P ≥ 0.12) between treatments. Relative abundance of Blautia and Methanobrevibacter on day 30 were greater (P ≤ 0.05) in SH vs. NSH cows. At birth, gestational access to shade did not impact (P ≥ 0.41) calf plasma cortisol and serum immunoglobulin G but increased (P ≤ 0.05) calf plasma haptoglobin and BW. At weaning, calf BW was greater (P ≤ 0.04) in year 1 but was less (P = 0.05) in years 2 and 3 for NSH vs. SH calves. In conclusion, gestational shade access improved maternal thermoregulation, body condition, and metabolic profile, while modestly shifting the prepartum vaginal microbiome. These maternal responses were accompanied by greater calf birth weight, although weaning performance varied by year, underscoring the dynamic nature of gestational heat stress impacts in subtropical grazing systems.