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The Importance of Copper, Phosphorus and Magnesium in Diets...

There is an optimal requirement for all minerals and vitamins in animals. Feeding more is not always better, as an excess can be costly and may cause issues to animal health and the environment. In this blog we will focus on Copper, Phosphorus and Magnesium as they are very topical for supplementation discussion.



Trace Elements

Trace Elements are needed in small quantities per animal per day, mg/kg (quarter of a teaspoon). Supplementation is required through mineral packages. Trace elements are needed for enzyme and hormone functions, e.g. Selenium for the immune system, Iodine for metabolic rate, Zinc for feet and skin health.


Macro Minerals


Macro Minerals are needed in grams per day (teaspoon). They are required for bone structure, Calcium, Magnesium and Phosphorus. Other Macro Minerals such as Sodium and Potassium are needed for water movement and are called electrolytes. Sodium is needed to bring water into the body, whilst Potassium pushes water out into the milk. A balance is required and often supplied in mineral packs.  

Mineral Efficiency


Good vs Bad minerals: It’s easy to fall into the trap and assume that higher levels of minerals are always better, but this is not always the case.


You run the risk of moving from deficiency, through optimal and on to toxic levels. Too much can not only be costly, but it can also cause health and environmental issues.



For a typical dairy cow at 30 litres milk with 22kg DMI, an ideal amount within the diet would be 18-22ppm.


Copper comes from the background in forages and feeds, and from supplementation. It is absorbed into the blood stream, a bit too much is banked in the liver, and some will go back out via the bile into the manure.


In sheep there is the same system, but they excrete less bile and therefore more goes into the liver, which is why we need to be more careful with sheep levels; target 11ppm in total diet.


If feeding forages with 3kg of compound and minerals, you are likely to meet adequate levels.


If feeding forages with distillery by-products, mineral, cake, and pot ale syrup, you may end up over-feeding Copper as pot ale and distiller’s by-products can contain particularly high levels.


Mineral supplementation needs careful consideration to ensure it is not too high in copper. Extra supplementation, such as boluses and buckets, can also add to this and cause long-term issues.


The ideal level of Copper in the liver should be 400ppm. An excess each month can add 100ppm and side effects are not seen until the danger level is reached – 1000ppm. The animal is then on a knife edge. Copper can release from the liver, bursting red blood cells if the animal comes under any stress such as calving.


Research has shown 80% of farms in the UK over feed Copper, with 20% feeding above the maximum legal limit.



Phosphorus is needed for bone structure and energy transitions in animals. It is the 2nd largest mineral in the body and if deficient, it would affect rumen microbe function. There are issues with the environmental output, which could have a negative effect on the diary industry.


Bones are a bank for Phosphorus and can help with supply in early lactation, which is replaced in mid lactation. Milk Phosphorus is at a constant level. If the cow is fed excess Phosphorous, she absorbs less from her diet and the excess then ends up in the manure.


A traditional Phosphorous diet at 0.45% gives 99g P and at 30 litres milk 33g P goes in the milk with 60g into the manure. Feeding a lower Phosphorus level gives the same amount into the milk but less is excreted in the manure, therefore reducing input and saving money.


Most dairy diets have enough Phosphorus in their background. Care should be taken if the feeding regimen is maize silage based, as this will be a lower Phosphorus level.


There is a common myth that Phosphorus is required for fertility, but there is no proven direct response.



Magnesium is more topical in the Spring and is required for structure – bones and teeth, nerve and muscle function and metabolic pathways.


We need to adjust levels precisely to find the correct balance; with bones holding 200g, 5g is available circulating in the blood and 4g is needed each day for milk. Very little Magnesium is available per day, so adequate amounts are required each day in the diet as there is no safety net. Remember, every day is a new day with Magnesium. 


Spring Magnesium Risks


Grass Mineral Content – Low in Magnesium, low in Sodium and high potash. Magnesium and Potassium complete for uptake and Sodium is needed to help Magnesium absorption.


Rumen Conditions – Fast passage rates so the Magnesium uptake is not as good.


DMI – Variable and unknown due to grazing conditions and changes in the diet.


Action Points


1.      Address Grass Risk – It’s better to add potassium mid-season, rather than early. Add Sodium and Magnesium in fertiliser

2.      Stabilise the Rumen – Introduce grass slowly, feed a buffer diet to complement grass and steady the rumen

3.      Get the Magnesium provision right – 30g added Mg

4.      Correct Compound at the Correct Level is Essential

5.      Check Buffer Feed – Reformulate Mineral, effective fibre, energy and protein

6.      Question / Calculate other Magnesium Sources per day – For example Boluses or Water supplementation


For more information and advise, contact your DN Sales Specialist...






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