353. Programming Effects of Removing Maternal Heat Abatement on Beef Offspring Growth, Thermotolerance, and Liver Transcriptomics During Backgrounding.

Abstract The objectives of this study were to evaluate the fetal programming effects of removing maternal heat abatement during late gestation on offspring growth, thermotolerance, physiological responses, and liver gene expression during the backgrounding phase. Fifty-six Angus-cross calves [225 ± 5.3 kg body weight (BW); 9 ± 1.5 months of age] were enrolled at weaning for a 112-day summer backgrounding period. Calves were born from cows given (SS, n = 28; steers = 16 and heifers = 12) or not given access to shade (NS, n = 28; steers = 16 and heifers = 12) during gestation. Pre-weaning, all cow–calf pairs were managed as a single group and fed bermudagrass hay ad libitum during winter. At weaning (d 0), calves were moved to a partially covered drylot for 7 days, then allocated to one of four 10-ha bermudagrass pastures (14 calves/pasture) and rotated biweekly. Calves received group-fed supplement (18% CP; 1 g/kg BW) from d 7 to 112. Body weight and blood was collected on d 0, 3, 7, 14, 28, 48, 56, 84, and 112. Intravaginal temperatures (IT) were recorded every 30 min in heifers (n = 12/treatment) from d 48 to 56 and 77 to 84. Liver biopsies were collected on d 56 from 10 calves per treatment for transcriptomic analysis. SS calves were heavier at weaning and on d 14, 48, and 112 (P ≤ 0.03). Data was analyzed using the MIXED procedure of SAS considering maternal pasture as experimental unit for all dependent variables. NS calves tended to lose more BW from d 0 to 3 (P = 0.09), although overall average daily gain did not differ (P = 0.89). From d 77 to 84, SS heifers had lower IT than NS heifers (38.6 vs. 38.9 ± 0.04 °C; P = 0.03), with no treatment differences from d 48 to 56 (P = 0.18). Concentrations of glucose, total protein, globulin, creatinine, SUN, and activities of AST, ALT, and GGT were not affected by treatment (P ≥ 0.19), although SS calves had higher SUN on d 0, 3, and 7 (P = 0.04). Nine genes were differentially expressed between treatments: eight were downregulated (AOX4, USP25, CFHR5, FGG, MBL2, FGB, GBE1, FGA) and one upregulated (FBLN2) in NS calves. KEGG analysis showed NS calves had depleted metabolic pathways, including oxidative phosphorylation and mitochondrial/protein degradation pathways. GO Biological Process analysis revealed reduced metabolic and immune-related processes in NS calves, including small-molecule catabolism, humoral immune response, and adrenergic receptor signaling. In summary, removing maternal heat abatement during late gestation impaired offspring growth and heifer thermoregulation during postweaning, and altered liver gene expression.