Magnesium. The Most Critical Mineral in a Cow’s Diet

Calcium is the most important mineral for milk production but for bodily processes and the cow’s poor holding capacity, Magnesium is vital! As we approach the shoulder of the grazing season magnesium reduces in fresh grass. Spring calved herds on low feed rates and/or mineralised compounds are particularly low in supply of magnesium.

Milk Fever specifically in mid lactation, where bloods show low magnesium (<1.7ml/dL) and calcium, may be that the low magnesium is reducing calcium metabolism.

Magnesium is vital for the cow providing:

• Neuromuscular activity
• Hormonal activity
• Enzyme activation
• Fat, Protein & Carbohydrate metabolism
• An insulin balance
• Bone & teeth maintenance
• A key element of cell respiration

Cows store very little, so consistent daily absorption is key.

Absorption occurs via the rumen, reticulum, and kidney reabsorption. As cows become older the ability to mobilise magnesium from the bone reduces. Absorption can be hindered by high levels of Calcium, Phosphorous, Potassium, Iron, oil/fats and oxalic acid.

High fertiliser levels of Potassium & Nitrogen can reduce magnesium absorption. Magnesium is concentrated in the stem rather than leaves, meaning poorer grazing practices where more stem is left could be leading to insufficient magnesium intake.
10-25% of forage magnesium is available compared to,
30-40% in grains and concentrate

Absorption can be increased with the aid of protein, lactose, vitamin D, ionophores (transport ions across cell membranes) and readily degradable carbohydrates. pH is also important as this affects magnesium solubility.

A first look should be made via nutrition plans and anticipated fresh grass mineral samples. Water, forage and fresh grass mineral analysis are important to check on availability whilst veterinary blood samples from a good average of 9-12 cow (3 – 40 DIM, 3 – 100 DIM and 3 – 180 DIM) is the best cow signal.

For further advice on Ration Formulation, please contact Wesley at or your local FCG Office.

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