Managing Magnesium

Published 26 March 15

As we enter the grazing season it is crucial to ensure cow magnesium requirements are met. AHDB Dairy Research and Development Manager, Dr Debbie McConnell takes a look at the importance of magnesium supplementation at this time of year.

Magnesium is a key macro-nutrient in dairy cow diets. It is essential for bone growth and maintenance, a functioning nervous system and also aids fibre digestion in the rumen. It is also strongly linked to milk fever prevention enabling the mobilisation and absorption of calcium.

Magnesium is predominantly stored in the bones and consequently it is not readily available to the cow. Therefore it is important to ensure a sufficient supply of daily magnesium for absorption in the rumen.

 A number of factors affect the absorption of magnesium in the rumen. A large supply of rumen degradable protein, high levels of potassium and high rumen pH all decrease magnesium absorption. Stress or poor weather conditions may also indirectly reduce magnesium absorption in animals by reducing intake.

Although it increases throughout the year, in spring, magnesium uptake by grass is typically low (Figure 1). At the same time grass potassium levels are high, sometimes causing reduced magnesium uptake by grass, but also supplying a high level of potassium in the diet.

 Mag 1

Figure 1: Seasonal uptake of macro-nutrients in grass swards. Source: Potash Development Association.

In spring, high dietary intakes of potassium and rumen degradable protein from rapidly growing spring grass combine to pose a significant threat of magnesium deficiency and grass tetany in livestock. Pastures which have been heavily top-dressed with nitrogen and potash fertiliser are the most dangerous.

To combat this, supplementation of magnesium in spring grazing animals is strongly recommended.

Supplementing magnesium

The recommended daily amount of magnesium for a lactating cow ranges between 0.2 and 0.3% of the diet, with higher levels required for cows grazing pasture. For a cow consuming 15kg DM/day this equates to a minimum daily requirement of 30g of magnesium.

Magnesium can be supplemented via a variety of methods including: compound feeds, mineral supplements, magnesium bullets, pasture dressing, magnesium in water, licks and boluses.

The availability of magnesium in supplements can vary considerably, however smaller particles sizes will typically have higher levels of available magnesium. Magnesium oxide (magnesite) (120 g/day), magnesium phosphate (54 g/day) and epsom salts (MgSO4) are an effective way of ensuring a good daily intake of magnesium.

Offering supplementary hay may also help reduce the risk of tetany as it stimulates rumination and salivation, which prevents excessive build-up of ammonia in the rumen and aids magnesium absorption.

More information on supplementation techniques and animal requirements can be found here.

How do Forage for Knowledge Farmer contributors manage magnesium at this time of year?

Magnesium uptake at grass is all about managing the risk, says, AHDB Dairy Technical Extension Officer, Piers Badnell. Dr Debbie McConnell’s article above talks about the risk factors, such as high potassium levels or poor weather conditions, and it is crucial you know the risk factors on your farm and how they affect the cows. The farmers below describe how they manage the different levels of risk on their farms.

“We test the soil, therefore know we have high levels of magnesium in the soil here already,” says Tony Renwick, who has 300 autumn calving crossbred cows on a paddock grazing self-feed system. “We don’t add any mag to what the feed company often puts in at this time of year anyway. In fact it tends to be when we add mag that we get problems.”

Tom Mitchell, partner in Dynamic Dairying Ltd. which milks 320 cows on a contract-farming agreement in Hampshire, says “We just get as much extra mag put in the cake as possible from now until late May. I think it’s about 5% or 5ppm but I just specifically ask for them to put in as much as possible each time I order a load.

“Most cake companies put additional mag in during spring but I think it’s only 2.5%, and not sufficient for herds like ours that are on grass and small quantities of cake only. Touch wood, we've only had one mild case of staggers each spring which I just treat with the usual mag under the skin.”

“We’re not prone to staggers here,” says Phil Feeney who milks 470 spring calving and 230 summer calving cows, in partnership with Rob Bostock in Cheshire.

“We do put 100g of mag in the water, until about the end of May, but that’s really just a precaution.”