In a context of increasing societal concerns around animal production, consumers asks for products coming from grazing dairy systems and. The segmentation of collection rounds to offer milk specifically from these systems results in higher seasonal variability in milk composition that needs to be better understood. Our objective was to characterize the monthly variation of milk and plasma mineral composition and heat stability, in a factorial arrangement of two breeds of cow and three grazing systems. The experiment involved 48 cows, half Holstein-Friesian (HF) and half F1 Jersey Holstein-Friesian crossbred (JFX) at Teagasc Moorepark. The grazing systems were based on either a monoculture of perennial ryegrass, or a perennial ryegrass and white clover sward, or a multispecies pasture. Milk, blood and pasture were sampled monthly between April and June 2021. Milk heat stability increased constantly between April and June and was unaffected by either the breed or the grazing system. Milk Ca, P, Zn and citrate contents were higher with JFX compared with HF, whereas that of Cl was lower. Milk soluble Ca and P contents were unaffected by the breed. Milk contents of a large part of elements (Ca, P, Mg, K, Fe, Cu, Zn, Mn), except Na and Cl, varied strongly according to the month and differently according to the considered element in interaction with the grazing system. No dominant parameters, among those prone to affect milk heat stability, could be clearly related to milk stability results. Plasma contents of all mineral elements, except Fe, strongly increased between April and June, in interaction with the grazing system for inorganic P, Mg and Zn. An important part of this variability could be related to herbage mineral content although plasma mineral contents were not clearly related to milk mineral contents. Funding by SmartCow project (H2020 N°730924) is acknowledged.