Should We Test Commercial Magnesium Sources Used for Dairy Cows?

Magnesium (Mg) is an essential mineral and plays a critical role in bone development, muscle contractions, and nerve function. Unlike calcium or phosphorus, Mg stored in bone cannot be easily mobilized during periods of deficiency, it needs to be consumed daily. This is especially important for high-producing cows, which have greater nutritional demands. Blood concentrations of Mg within 2.19 to 2.92 mg/dL are considered normal. When blood Mg levels fall below this range, cows are at risk of developing hypomagnesemia.

Clinical hypomagnesemia should be suspected when cows show signs resembling milk fever, such as muscle tremors, seizures, or sudden death, along with low blood Mg concentrations (<1.95 mg/dL suspicious of hypomagnesemia; < 1.7 mg/dL hypomagnesemia). In early lactation, hypomagnesemia is frequently associated with hypocalcemia. Cows in mid-lactation are also at risk of hypomagnesemia, particularly during heat stress or when diets are high in potassium. A case study from a California dairy reported some mid-lactation cows that experienced sudden seizures and death during the summer due to hypomagnesemia. After increasing the dietary Mg level, no further cases occurred that season (Urdaz et al., 2003). Magnesium deficiency can result from low dietary Mg, or reduced absorption of the Mg included in the diet.

What Affects Magnesium Absorption? 

Type and quality of Mg sources: Not all magnesium sources are created equal. Manufacturing processes, such as calcination (removes impurities) at ~800 °C, improve reactivity, and smaller particle size proportion (<200 mesh) significantly increases solubility and Mg uptake by the cow (Schonewille, 2013; Jesse et al., 1981). However, finely ground particles can be fluffy and difficult to handle, which may pose challenges during mixing and feed preparation.

Dietary antagonists: Potassium (K) is considered the main dietary antagonist of Mg; it interferes with Mg absorption in the rumen. For every 1% increase in dietary K, Mg absorption may decrease by up to 7.5% (Weiss, 2004). This is particularly relevant during summer, when dairies may supplement extra K in diets as a heat stress mitigation strategy.

Study. Our recent study at UC Davis evaluated the relative availability of five inorganic Mg sources commonly used in dairy diets (Silva-del-Río et al., 2024). The goal was to compare how different sources were absorbed and taken up by the cow.

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