Magnesium: The Most Critical Mineral

Accurate feed mixing is critical to controlling feed costs and optimizing cow performance. ( Mike Opperman )

Calcium is the most important mineral in milk, but magnesium may be the most critical mineral in the cow’s diet. Why? Because cows store very little magnesium, so they require adequate magnesium in their diet every day. There’s not much room for error.

I had a case last month where my client called the veterinarian because several mid-lactation cows had gone down with symptoms of milk fever. Their blood tests confirmed low calcium but also pinpointed low magnesium. I checked the new bag of grass haylage for protein and minerals, including potassium and magnesium and the test showed nothing out of the ordinary. The mineral premix also tested within specs. We discovered that due to faulty equipment in the mill, less than half of the targeted mineral premix was going into the grain mix. I’m not sure how long this had been going on, but it probably involved several loads of grain mix. The mineral premix provided all the supplemental macro and micro minerals. However, clinical magnesium deficiency symptoms appeared first.

Magnesium is needed in the blood to allow normal calcium metabolism. Secondary hypocalcemia in cows begins when blood magnesium levels fall below 1.7 mg/dL. and grass tetany or lactation tetany is exhibited when blood levels fall below about 1.2 mg/dL.

Magnesium oxide (MgO) is the most common magnesium supplement for lactating cows. It is fed as both a magnesium source and a rumen alkalizer. It’s typically 54-58% Mg. The bioavailability of magnesium in MgO depends on particle size and solubility and unfortunately this varies widely among commercial sources. Domestic (U.S.) produced MgO is always on top for solubility. Calcination is the process by which the raw magnesite ore is heated to drive off CO2 and form MgO. Pure MgO is about 60% magnesium. This make good steel, but poor animal feed. According to Dr. Jesse Goff at Iowa State University, the sweet spot is about 54-56% magnesium. This forms a loose crystal structure in which water and acid can penetrate and react.

Magnesium sulfate (Epsom salt) is another common magnesium source, primarily fed to pre-fresh animals to induce metabolic acidosis. It’s about 9% magnesium, so much lower than MgO and is not very palatable. 

Calcium magnesium carbonate supplements (22% calcium, 12% magnesium) are also widely used. They are palatable and can replace a portion of sodium-based buffers. Reactivity and hence bioavailability of both calcium and magnesium is improved with finer particle size.

Potassium is antagonistic to magnesium absorption. Magnesium is absorbed across the rumen wall. Potassium competes at the cellular level, reducing magnesium absorption. Magnesium absorption can be reduced by up to 50% over the range of potassium levels commonly fed in ruminant diets, according to Dr. Bill Wiess at The Ohio State University. It is important to increase supplemental magnesium as total dietary potassium increases. 

Monensin can help overcome the magnesium-potassium antagonism in lactating cows. It has been shown by Wiess and Tebbe, that 360 mg of monensin fed in diets containing 2.1% potassium increased the apparent absorption of magnesium from MgO by about 25%. However, magnesium absorption from magnesium sulfate was reduced by about 30% when fed with monensin. Since magnesium sulfate and monensin are commonly fed in pre-fresh diets, magnesium concentration should be increased by about 15% in these cases.

Several years ago, I wrote an article that included a simple and quick test developed by Dr. Jesse Goff that can be used to check the solubility of MgO sources. I had several requests for this test over the years from nutritionists and feed mills, so I elected to include it again.

Testing Magnesium Oxide Availability

  1. Weigh out 3 grams MgO into a large container.
  2. Add 40 ml of 5% acetic acid (white vinegar) slowly!
  3. Cap the container and shake well for 15 seconds, wait 15 minutes, shake again for 15 seconds, then check the pH at 30 minutes.

Vinegar is pH 2.6 – 2.8. The best MgO will bring pH up to 8.2. The worst up to just 3.8. 

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