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Introduction: Feeding pregnant broodmares with cereal concentrates has been shown to increase maternal insulin resistance and affect foal metabolism in the short and long-term. These effects are likely to be mediated by the placenta. Here, we investigated feto-placental biometry and placental structure and function at term in mares fed with or without cereals concentrates.Material and Methods: From 7 months of gestation, 22 multiparous mares were fed forage only (group F (n = 12)) or received forage and cracked barley (group B (n = 10)) until foaling. Foals and placentas were weighed and placental samples were collected above the umbilical cord insertion at birth. Placental histological structure was studied by stereology. A RNAseq analysis was performed on 9 placentas of each group. Enrichment of gene sets was analysed using the Gene Set Enrichment Analysis (GSEA) software using the KEGG and GO databases.Results: No difference in feto-placental biometry was observed between groups. The volume of microcotyledonary vessels was decreased in B placentas and the vascular wall of allantoic arterioles was thickened. Gene sets involved in neutral amino acids, folate and anions transport and fatty acids, cholesterol and folate degradation were down-regulated while gene sets involved in RNA expression, inflammation and vascularisation were up-regulated in B placentas.Conclusion: Feeding pregnant mares with concentrates from mid-gestation alters the placental function and structure as observed in other species in cases of maternal insulin resistance. [ABSTRACT FROM AUTHOR]