Guide to magnesium supplementation in cattle

New research has found magnesium from rock sources, such as magnesium oxide, has about half the absorption level as previously thought.

But with costs on the up and potentially a long winter to come, how can farmers get ahead for spring calving and set up their farms in light of this new research?

Farmers Weekly spoke to Karen Stewart, nutritionist at SAC Consulting – part of Scotland’s Rural College – to find out more.

See also: How to avoid mid-lactation milk fever in your herd

Why is magnesium important?

Magnesium is a key mineral for cows and plays a crucial role in mobilising calcium from bones and increasing gut absorption.

In pre-calving cattle, in particular, access to this calcium is pivotal in ensuring a smooth birthing process.

In the UK, magnesium oxide is one of the most commonly fed sources of magnesium.

However, a recent study carried out by Professor Bill Weiss of Ohio State University has found that absorption levels are much lower than previously thought.

This is important for farmers to take into consideration.

Magnesium requirements of suckler and dairy cows

As a guide, suckler cows require about 20-30g of magnesium a head a day. In the case of dairy cows, this will often be higher, though this is very much dependent on their diet.

However, if grass/silage is high in potassium, this can lock up magnesium, meaning it won’t be readily available to cattle and may lead to a need for higher supplementation.

In this scenario, farmers should look to target 30-40g intake from the total diet, although a forage mineral analysis is always advisable to know exactly what the potassium/magnesium levels are in your forage.

While the research from the US has shown that rock sources can cause reduced absorption levels, recent information suggests that feed availability (in sources such as silage) is actually much better than previously thought.

In theory, this could cancel out the effects of reduced absorption from rock sources and is worth considering when planning rations and calculating requirements.

Key supplementation considerations at calving

A magnesium deficiency prior to calving can cause metabolic issues at parturition such as subclinical milk fever or slow calving syndrome.

In a recent University of Edinburgh study of 988 spring-calving suckler cows from more than 80 farms across England and Scotland, 29% of cows had low magnesium blood levels in the last month of pregnancy.

Current supplementation guidelines suggest feeding at least 10% magnesium in a pre-calving mineral – though this will be dependent on the rest of the ration.

For example, straw-based rations will contain less magnesium and high-potassium silage rations may reduce magnesium availability.

As a guide:

• 100g of a 10% magnesium mineral supplies 10g of magnesium
• 150g of a 10% magnesium mineral supplies 15g of magnesium
• 100g of a 15% magnesium mineral supplies 15g of magnesium
• 100g of a 25% magnesium mineral supplies 25g of magnesium

However, given the most recent findings, it is worth considering upping this base level to 15% to cover any reduced absorption and prevent issues at calving.

This can also be achieved by adding additional calcined magnesite or a high-magnesium mineral. The rate should be discussed with your nutritionist.

Managing staggers and other potential issues at turnout

As it says in the name, staggers – or magnesium tetany – is caused by a lack of magnesium.

Grass can be variable, but in general has an average of 1.6g of magnesium/kg of dry matter (DM).

Therefore, if a cow eats 10kg DM – or 50kg of grass – this would equate to approximately 16g of magnesium.

While this may be adequate under normal conditions, if a cow eats less or there are factors reducing the absorption of magnesium, it will not be enough and staggers risk is high.

Cows in early lactation, older cows and cows under nutritional stress are most at risk and there are a number of factors that can cause levels to fall and subsequently cause staggers.

Lush, spring grass is often low in magnesium and passes through animals very quickly, meaning less is absorbed.

There are also some concerns over high nitrogen content reducing absorption, and high potash levels in grazing grass have been found to have a similar effect.

Low grass levels and poor weather conditions can trigger stress, which can in turn also result in reduced intake, and therefore lower magnesium levels.

Interestingly, vets in Scotland are rarely reporting cases of staggers in the spring now.

Instead, more occurrences are being seen towards the autumn, possibly because cattle are being kept out longer and grass levels aren’t as sufficient.

Therefore, staggers should be kept an eye on throughout the year.

There are a number of steps that can be taken to mitigate against staggers:

• Ensure cows are eating enough to prevent a negative energy balance/metabolic stress and subsequent low magnesium levels.
• Ensure cows have a good supply of magnesium prior to turnout.
• Pay close attention to cow behaviour, particularly changes that could result in reduced feed intake. For example, periods of bad weather can cause stock to shelter in corners of the field, meaning they’re less inclined to eat.
• In situations where grass levels are below 6cm, cows must be supplemented for energy as well as magnesium.
• Delay potassium fertiliser applications until later in the season to avoid antagonising magnesium absorption levels.
• Check the base ration and think about feed intake as a whole, rather than just focusing on magnesium supplementation.