Benefits of magnesium for suckler cows ahead of calving
If beef farmers are assisting a high proportion of cows at calving, or if stillbirth rate is higher than wanted, then cow magnesium status could be to blame.
Magnesium is commonly supplemented at turnout as part of grass staggers prevention, but vet Lucy Hepworth of Friars Moor Livestock Health believes most beef farmers could benefit from introducing it four to six weeks before calving.
See also: Advice on optimising suckler cow fertility with better nutrition
“Magnesium is really important to supplement in the run-up to calving – it is needed for calcium hormone mechanisms to be effective at mobilising calcium from bones and kidneys, and absorbing it from the gut,” she explains.
Calcium supports strong muscle contractions to help the cow deliver her calf more quickly, reducing the chance of oxygen deficiency, and making the calf more likely to get up and suckle quickly.
It also increases the chance a cow will cleanse, increasing her ability to get back in-calf, Lucy says.
An inability to mobilise enough calcium around calving often manifests as clinical milk fever in dairy cows, which have a huge demand for calcium for milk production.
In beef cows, it is more likely to be an underlying subclinical issue and would show up as a higher stillbirth rate or assistance rate because of slower calvings, she explains.
A University of Edinburgh study on 988 spring suckler cows found that nearly 30% had low magnesium blood levels in their last month of pregnancy. This could potentially predispose them to calving issues.
Case study: Rupert Coffin, Seven Ash Farm, Sherborne, Dorset
Farm facts: Seven Ash Farm, Dorset
- 130ha, all grass
- 125 suckler cows, mainly Hereford cross Friesian and British Blue cross Friesian
- Calving March to May
- Limousin and Aberdeen Angus stock bulls
- Some sold finished to ABP, rest sold as stores
Ellie and Rupert Coffin with vet Lucy Hepworth (right) © Aly Balsom
Since increasing magnesium supplementation in the run-up to calving, the number of assisted calvings has dropped dramatically at Seven Ash Farm, Sherborne.
Rupert Coffin, who farms in partnership with his parents, Allan and Nikki, was unhappy with the level of assistance needed at calving. He had been supplementing cows with magnesium for some time, but it was the Friars Moor beef suckler discussion group that highlighted he might be underdelivering.
Last year, he increased the level of magnesium chloride supplementation from 7.2g a head a day to 12g a head a day. Along with about 12-15g magnesium delivered from hay, this provided the minimum daily requirement of 20g a head pre-calving.
Correct supplementation
Assisted cow calvings dropped from 24% to 6%, while all other management factors, such as trace element boluses, bulls, and hay, stayed the same. Having previously blamed breeding for calving problems, Rupert says he now believes it might have been mineral status.
“It’s only been one year of magnesium supplementation at the right level but looking back at cows I’d got rid of due to calving difficulties, if I’d got the magnesium right, I might not have needed to get rid of them.”
Having experienced the occasional uterine prolapse and milk fever in the previous two years, there were also no calving issues in 2024. “Calves were a lot livelier this year and we had to help a lot less to suckle,” says Robin’s wife, Ellie, who works part time on the farm.
The herd at Seven Ash Farm © Aly Balsom
Magnesium status
Lucy says if stillbirth rates are above 2%, or assisted cow calvings are higher than 5%, it is worth considering magnesium status alongside other contributing factors such as genetics, body condition and management at calving.
A vet can blood test at least six cows in each group that are within one month of calving for magnesium status. Where deficiency is a problem, cows can be supplemented accordingly.
What are the requirements?
- Dry beef cows require 20g magnesium/day, and lactating cows need 30g/day.
- An extra 10g should be added if forage potassium levels are high (above 2%) as this can impact magnesium absorption.
- A full forage mineral analysis will determine the level of supplementation needed to deliver the daily target.
- Work out the amount of magnesium delivered by specific minerals – for example, 100g of a 10% magnesium mineral delivers 10g of magnesium.
- Magnesium chloride flakes are 12% magnesium, so 150g a head a day will provide 18g magnesium.
How should it be fed?
- Dissolving magnesium chloride flakes in water troughs is a relatively cheap method.
- Magnesium can be included in a pre-calving mineral. Feeding this in the total mixed ration is the most reliable way of feeding. It can also be sprinkled on top of forages, although this reduces accuracy.
- Magnesium can be incorporated into blocks and licks, although it is difficult to know which cows have eaten what.
- A mineralised dry cow roll is a further option.
Case study: Adam Frampton, Manor Farm, Hammoon, Dorset
Farm facts: Manor Farm
- 190ha organic farm, including 61ha arable grown for beef
- 105 suckler cows including Aberdeen Angus-crosses
- Nine-week spring calving from March
- Aberdeen Angus and Fleckvieh bulls
- Finished calves go to ABP
Adam Frampton © Aly Balsom
When two calves were born dead in the same week last season at Manor Farm, Hammoon, Adam Frampton, who farms with his mother, Sue, was keen to get to the root of the problem.
“I couldn’t get my head around it. It shouldn’t be like that. They were born dead, but without assistance,” he says.
Blood tests on one of the dams within 24 hours of calving showed she was low in calcium, which could have negatively impacted her ability to push the calf out, increasing the risk of a stillbirth.
Adam had already started introducing magnesium chloride at calving, rather than waiting until cows were at grass. That meant that in 2024, assisted calving rate in cows was 1%, versus 9% in 2023. Stillbirth rate had also started to improve, dropping from 5% to 4%.
The right timing
However, the blood results highlighted he needed to ensure the correct amount was being given – and earlier. “I wasn’t on it enough. You’ve got to put more effort on the amounts, rather than just chucking it in,” he says.
This year, magnesium chloride will be increased and provided four weeks before calving. Forage mineral analysis will be carried out on haylage and silage, and magnesium chloride levels balanced accordingly to deliver the target 20g magnesium a head a day.
Adam adds half the daily requirement to the water troughs each morning and the rest at night. This reduces the risk of poor palatability as it can make the water bitter.
Having cows and heifers in different groups also reduces competition at the troughs.
With older cows also at increased risk of poor calcium status, “an extra eye” will be placed on these animals near calving so any issues with a slow calving are picked up promptly.