PS3-5. Effects of Dietary Substrate During Late Gestation on Transcriptional Changes in the Placenta of Beef Cows.

Abstract Maternal nutrition during late gestation plays a critical role in shaping placental function and fetal development in beef cattle. This study evaluated how differences in dietary substrate, independent of energy intake, influence placental gene expression and potential adaptations in nutrient utilization. Briefly, forty-nine pregnant (236 d ± 10 of gestation) Angus and Simmental-Angus cows (BW = 606 ± 68kg) were blocked by breed, age, and BW and assigned to one to two treatments: 1) forage-based diet with intake limited to 1.5% BW (HFOR; n = 25); and 2) high concentrate corn-based diet, with intake limited to 1.2% BW (HCON; n = 24). Diets were limit-fed to maintain an equal intake of metabolizable energy (21.56 and 22.08±0.43 Mcal/d for HFOR and HCON respectively). Cows were housed in a group-pen equipped with electronic feeders and waterers. Treatments started at enrollment (∼47d ± 10 pre-calving) and continued until d 70 ± 10 d post-calving. Calving was observed for each cow, and placentas were collected immediately following natural expulsion. The largest cotyledon closest to the umbilicus was dissected and rinsed with phosphate-buffered saline solution. A 2.5 ´ 2.5 cm2 of cotyledonary tissue (n = 8 per treatment) was snap-frozen in liquid nitrogen and stored at -80 °C. Total RNA was isolated from the samples and subjected to RNA sequencing (HCON = 8 and HFOR = 8). After quality control, the reads were mapped to the Bos taurus reference genome using STAR aligner, and differentially expressed genes (DEGs) were identified using DESeq2. Transcriptomics analysis revealed 1,686 differentially expressed genes (P ≤ 0.05 and |Log2FC| > 1). Among them, 566 genes were upregulated, and 1,120 were downregulated in the placenta of HFOR cows. Functional over-representation analysis of DEGs through WebGestalt retrieved biological processes (BP) and KEGG pathways. While SLC2A5 coding transporters were not identified, members of the SLC25 family (SLC25A1 and SLC25A10), and mitochondrial coding genes were upregulated in HCON. Thus, if glucose is being taken up, these genes modulate how that glucose is used for energy (ATP) vs. biosynthesis in the placenta. Additionally, overrepresented pathways included genes involved with chromosome separation and sister chromatid segregation indicating an increased hyperplasia. Therefore, HCON likely improved placental growth and result in a greater functional surface area for nutrient exchange later in gestation. This data suggests that dietary substrate rather than energy intake drive transcriptomic changes, suggesting adaptations in placental function and, ultimately, fetal development and offspring performance.