187. Maternal Guanidinoacetic Acid Supplementation Modulates Cellular Heat Stress Responses in Late-gestating Hanwoo Cows and Their Calves.

Abstract Guanidinoacetic acid (GAA), a precursor of creatine, plays an important role in cellular energy metabolism and antioxidant defense. Therefore, dietary GAA supplementation may help mitigate heat stress in late-gestating beef cattle. This study aimed to investigate the effects of GAA supplementation on calf birth weight, serum metabolites and the gene expression of heat shock proteins, antioxidant enzymes, and pro-inflammatory cytokines in peripheral blood mononuclear cells (PBMCs) of late-gestating Hanwoo cows and their calves under summer heat stress conditions. Twenty-eight multiparous Hanwoo cows were randomly assigned to either a control (CON; n = 15) or GAA treatment (0.2% DM; n = 13) after stratification by parity, age at artificial insemination, and sire. GAA was supplemented via top-dressing from approximately 60 d prepartum until parturition. Blood samples were collected from cows after 1 mo of supplementation and at calving, and from calves at birth. Calf birth weight was also recorded. Data were analyzed using the TTEST procedure of SAS 9.4. No significant differences were observed in calf birth weight or serum metabolites between treatments in either cows or calves. However, total protein (TP) tended to be lower (P < 0.10) in GAA cows at calving, while albumin (ALB) remained unchanged, suggesting a potential increase in immunoglobulin transfer to colostrum. After 1 mo of supplementation, mRNA expression of heat shock protein 70 (HSP70), heat shock protein 90 (HSP90), glutathione peroxidase 1 (GPX1), and interleukin 8 (IL-8) in cow PBMCs was significantly decreased (P < 0.05) in the GAA group compared with CON. At calving, IL-8 expression remained lower (P < 0.05) in GAA cows, whereas the expression of other genes did not differ between treatments. In calf PBMCs at birth, the expression of heat shock protein 27 (HSP27), HSP70, and catalase (CAT) was significantly downregulated (P < 0.05) in the GAA group. These results suggest that dietary GAA supplementation during late gestation may attenuate heat stress-related cellular responses in both Hanwoo cows and their calves, as indicated by the downregulation of heat shock proteins, antioxidant enzymes, and pro-inflammatory cytokine expression in PBMCs. Collectively, maternal GAA supplementation may represent a promising nutritional strategy to mitigate summer heat stress in beef cow–calf systems.