Magnesium Supplementation of Suckler Cows – Are You Covered?

As turnout approaches, now is a good time to reassess your mineral supplementation strategy for grass staggers prevention in the suckler herd (also known as hypomagnesaemia or grass tetany).  Whilst access to spring grass is one of the key risk factors for lactating cows, with the high passage rate of lush, wet grass through the digestive tract reducing magnesium absorption, there are several other factors to consider.  These include method of supplementation and its magnesium content, potassium level in grass, grass availability and when to start supplementation. 

The prevalence of poor magnesium status in suckler cows was highlighted last year through a Scottish Government funded project involving 12 spring calving herds.  Results from metabolic profile testing showed that over one third of cows had low blood magnesium levels pre-calving, increasing their risk of slow calvings and milk fever related disorders.  Post-calving, a quarter of cows also had inadequate magnesium status, which, if left uncorrected, could lead to grass staggers.  Cows under nutritional stress, as well as older cows are also more at risk from this condition.   

 Target Intake

A minimum target intake for lactating suckler cows is 30g of magnesium (and 20g for dry cows), which includes background levels from forage.  In areas where potassium levels are high in grass (i.e., over 2%), then 40g should be the target from the total diet (and 30g for dry cows).  A high nitrogen level in grass is also thought to interfere with magnesium absorption, although the exact mechanism is not fully understood.  Sufficient magnesium must be provided on a daily basis as it is not stored within the body. 

It is worth discussing appropriate supplementation with your nutritionist, particularly in light of recent research by Professor Bill Weiss of Ohio State University.  He found that magnesium from magnesium oxide (also known as calcined magnesite), the most commonly used source of magnesium in the UK, has about half the absorption level as previously thought.  

While the magnesium content of grass can be variable, average levels are about 1.6g/kg of dry matter.  Therefore, if a cow eats 10kg of dry matter from grass she takes in 16g of magnesium.  Supplementation with 100g of a mineral containing 15% magnesium would then meet her 30g requirement.  

If using free access minerals, ensure sufficient is provided fresh every day and that they are being consumed at the correct rate.  If using molassed mineral buckets, ensure there is adequate access for the number of cows in the field and check intakes meet manufacturers recommendations.  A good alternative for providing magnesium are compound feeds (i.e., magnesium rolls) or some barley with minerals mixed in (although this is not so practical outside for feeding on the ground, with wastage and loss of mineral in wet weather).  Concentrate feeding also allows extra energy to be fed for first calvers or cows struggling with their condition and is essential if there is insufficient grass available (<6cm).  

Note that magnesium is not very palatable and getting cows used to magnesium supplements two to three weeks prior to turnout will help ensure good intakes when they go to grass. 

Anything that affects dry matter intake will affect the cows’ magnesium status and increase the risk of grass staggers.  A good example where this can happen is with rotational grazing when aiming for a low target residual before a shift to a new paddock.  Bad weather, where stock tend to shelter in field corners or next to a fence means they are less inclined to eat grass.  Also, wet weather can greatly reduce the dry matter of the grass and hence dry matter and magnesium intake, increasing susceptibility to grass staggers.  

Lorna MacPherson, lorna.macpherson@sac.co.uk