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Feeding Pre-Calving Minerals

22 February 2022

An important factor in the pre-calving period is to ensure the cow is getting adequate minerals in her diet.  Mineral levels in UK silages are variable and are influenced by a number of different factors such as soil types, cutting date, fertiliser rate and sward mix.  It is advised to have your own silage analysed and work with your adviser/nutritionist to identify any shortfalls/ excess to requirements.

Minerals play a key role in the immunity, fertility and colostrum quality of the cow.  Any deficiencies can result in health issues for the cow such as slow/ difficulty calving, milk fever and retained foetal membranes.  As calves are born with a naïve immune system they are relying on the antibodies from their mother, therefore a maternal deficiency in late pregnancy can compromise the calf’s immune system leaving them more susceptible to pneumonia, scour, navel and joint ill.

Beef and sheep grass silage mineral averages 2021:

Macro MineralsAverage
(g/kg DM)
*Recommended levels in the total diet (g/kg DM)
Calcium6.22.1 - 3.6
Phosphorus3.30.8 - 2.7
Potassium226 - 7
Sulphur2.31.5
Sodium2.60.6 - 0.8
Magnesium2.12 - 3
Trace Elements(mg/kg DM)(mg/kg DM)
Copper6.710
Cobalt0.10.15
Iron22350
Manganese12850
Selenium0.10.1
Zinc3450

*These are the critical requirements based on the minimum level to overcome deficiency.  Antagonists are not accounted for, and a safety factor should be included appropriate to the ration being fed.

The table above shows the average figures for beef and sheep silages cut in 2021 analysed by the SRUC laboratory.  It’s important to bear in mind these are only average figures and there is a huge range as every farm will be different.

The data shows that trace elements tend to be undersupplied from grass silage alone.  While the macro minerals are generally well supplied it is important to look at the whole picture.  For example, magnesium appears to meet the minimum critical level, however the average level of potassium is very high and this is a direct antagonist to magnesium.  Recommended intakes for a pre-calving suckler cow are 20 – 30 g in the overall ration, however where potassium levels are high (2 0- 30 g/kg DM or 2 – 3% DM) then 30 – 40g of magnesium should be the target intake from the total diet.

As a guide, a cow eating 10kg of dry matter of the average SRUC grass silage would get around 21 g of magnesium which on a high potassium silage is not going to be sufficient.  A pre-calving mineral should contain at least 10% magnesium, fed at 120 g/head.  This would supply 12g bringing the total magnesium supply in the overall ration to 33 g.

Magnesium plays a vital role in the activation of calcium release from body stores leading to muscle contraction, therefore has a big effect on ease of calving and expulsion of the placenta.  It is a deficiency that is easy to correct by appropriate supplementation.  Blood sampling is another easy method of ensuring cows are receiving what they need and will accurately show if there are any deficiencies in the ration.

Top tips for supplying minerals: 

  • Analyse forage for minerals so you know what is being supplied through the ration to the cow.
  • Blood test a minimum of 6 cows from each calving group ideally one month prior to calving – this can be used to assess energy and protein as well as their mineral status.
  • Consider all the sources of minerals and trace elements being supplied (powdered minerals, bolus, blocks etc.). Over supply can also have a detrimental effect.
  • A pre-calving mineral should have a minimum 10% magnesium and 2000 IU/kg of vitamin E.
  • Mineral nutrition can be a complicated topic but taking some time to sit down with your adviser/nutritionist will help plan and improve overall herd health and production.

Mary Young, mary.young@sac.co.uk

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