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The Vital Role of Magnesium


The Vital Role of Magnesium

With cows soon to be dried off, or already dry on many farms around the country, the focus of farmers should now be on preparing the cow for a successful calving and strong start to lactation next spring. The key to this is avoiding diseases and metabolic disorders around calving time.

Milk fever is known as the ‘Gateway Disease’, as it leads to a number of other disorders including Retained Placenta, Displaced Abomasum and increased levels of Mastitis in early lactation. Clinical milk fever is easier to note, but sub-clinical milk fever can actually cost more due to more cows affected and associated metabolic disorders such as retained placenta, metritis, mastitis, ketosis and displaced abomasum. This cost can be three to four times more than a clinical case. Outlined in table 1 the result of a survey carried out by Nutribio across the country, which found an average cost of metabolic disorders per herd of nearly €5,000


Nutribio  survey – 16,141 cows Milk Fever Rt.Placenta Ketosis LDA
% Metabolic Disorders 3.8 4.7 1 1
Av.No.Of Cows /Herd – 106 4 5 1 1
Average Cost € (Vet Irl.) 423 448 320 515
€ per Herd 1,692 2,240.00 320 515


Milk fever can be prevented by feeding a good dry cow mineral, with high levels of Magnesium and Vitamin D3, along with other trace elements and vitamins that support a healthy immune system. Magnesium in particular, is a critical component of a good dry cow mineral.

The vast majority of Irish dry cows will be on a grass silage diet this winter, a diet that is typically high in Potassium. The high potassium levels are caused by the slurry applications to silage ground during the spring, which are required to grow the grass for good yields of silage. The excess Potassium is not necessarily a bad thing, but farmers must be aware of the need to balance the Potassium with supplementary Magnesium, in order to avoid costly Milk Fever issues at calving time. The Potassium to Magnesium ratio in the diet of the dry cow should be between 4.5:1 and 5:1 – any higher than this and Milk Fever is likely to be a significant problem in a few months’ time.

Using your silage mineral analysis, the below example demonstrates how to calculate the Magnesium requirements of your herd.


Grass silage Potassium = 2.0%

Grass silage Magnesium = 0.18%

Dry Matter Intake of Dry Cows = 11.75 kg

Potassium supplied by the forage = 235 grams

Target Magnesium intake = 52 grams (235/4.5)

Magnesium supplied by the forage = 21 grams

Supplementary magnesium required = 31 grams per cow per day

The magnesium supplied by the Dry Cow Mineral will depend on the % of Magnesium in the mineral, and the feeding rate of the mineral:

  • A Dry Cow Mineral with 26% Magnesium fed at 120g/cow/day will give 31 grams of Magnesium
  • A Dry Cow Mineral with 26% Magnesium fed at 80g/cow/day will give 21 grams of Magnesium
  • A Dry Cow Mineral with 20% Magnesium fed at 120g/cow/day will give 24 grams of Magnesium
  • A Dry Cow Mineral with 20% Magnesium fed at 80g/cow/day will give 16 grams of Magnesium


If additional magnesium is needed, it can be supplied by supplementing with Cal Mag or Sweetened Cal Mag supplementation. If cows require an additional 10g Magnesium per day on top of the dry cow mineral, feed 20g of Cal Mag (50% Magnesium) or 30g of Sweetened Cal Mag (33% Magnesium). It is important that total Magnesium levels in the diet do not exceed 0.6% of Dry matter intake (approx. 70g of Magnesium per cow per day), this is to avoid issues with Rumen function and scour.

Along with Magnesium, Vitamin D3 also plays a crucial role in the prevention of Milk Fever – requirements are approximately 22,000 iu’s of Vitamin D3 per cow per day during the dry period. In a mineral fed at 120g/cow/day, this would mean a minimum requirement of 180,000 IU’s per kg on the Dry Cow Mineral label.

Vitamins A and E, along with trace elements Copper, Selenium, Zinc, Iodine and Cobalt are all essential for the health and immune function of cows during the dry period, and also for colostrum quality and the health of the new-born calf in the spring time. Farmers should carry out mineral analysis on their silage at least once every 3 years to identify any trace element deficiencies, and plan what supplementation is required with a qualified Nutritionist.

Management of hypocalcaemia (milk fever) requires pre and post calving intervention, optimising calcium and magnesium status in the transition period. A lot of herds are under fed in calcium and magnesium in the first three weeks post calving as they rely only on the concentrate feed to supply these two macro elements. As one is introducing the concentrate feed to the freshly calved cow over two to three weeks one should top dress with a post calving mineral to meet the demands by the freshly calved cow for these two macros.

If farmers have any doubts about Magnesium supplementation, trace elements or any other queries regarding Dry Cow Minerals, contact the Technical team in Nutribio on 021- 4507303.