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Are your Calves Getting too Much of a Good Thing?

12 August 2019

The importance of providing calves with a good quality milk replacer in early life is well documented.  However, not only is the nutritional value of the milk replacer important, the concentration of the liquid feed the calf receives needs just as much attention.


The definition of osmolality is the concentration of a solution expressed as the total number of solute particles per kilogram.

The osmolality of cow’s milk is around 300mOsm/kg, which is the same as a calf’s blood. This creates an ideal situation for the absorption and digestion of nutrients.

Osmolality will differ between milk replacers, even when mixed at the same concentration.  It is measured by adding the concentration of sugars, such as lactose, and minerals, such as sodium.

This can mean that high total solids does not necessarily indicate high osmolality.  For example, colostrum can have 26% total solids (compared to whole milk at 12.5% solids) but an osmolality of only 440mOsm/kg.

If your milk replacer uses fat as its main source of energy, it will have a lower osmolality than one that is relying more on lactose to provide energy. Whey based milk replacers can have varying osmolality depending on their lactose content.  Better quality whey based milk replacers will have lower percentage of lactose and thus a higher the fat percentage.  These will have a lower osmolality.

To keep osmolality at safe levels, it is recommended that liquid feed is made up with 150g of milk replacement powder per litre of liquid feed in 850ml of water to produce a litre of liquid feed.

What happens when osmolality is too high?

When the osmolality of a calf’s liquid feed is too high, fluid is effectively drawn from the calf’s blood into the intestine.  This can cause diarrhoea and lead to dehydration.

At this stage it would be normal practice to administer an electrolyte.  Simply adding this to the bucket of milk replacer may cause further problems. This is because the high sodium content of most electrolytes will increase the osmolality even more.  Ideally, the electrolyte should be given in a separate solution.

Delayed abomasal emptying can be caused by feeding milk replacer with an osmolality of over 600mOsm/kg.  This can put calves at risk of bloat.

Regular feeding of high osmolality milk replacer has also been shown to damage the lining of the gut.  This, not only, has an effect on the calf’s ability to digest nutrients properly, but an increase in the permeability of the gut lining will make it easier for pathogens to transfer into the bloodstream. This will have a detrimental effect on health in general.

How to increase intakes of milk replacer safely

There are occasions when increasing intakes of milk replacer are desirable.  These include achieving higher growth rates or using the increase in energy intake to combat cold weather. If the maximum concentration is already used, this should be done by increasing the volume of milk replacer fed at the same concentration rather than just adding a bit more powder when mixing up the feed.  As always fresh clean water should be available at all times.

Main points

  • Highly concentrated milk replacer can potentially damage your calves, depending on its osmolality
  • Use 150g milk replacer powder per litre of liquid feed
  • Do not add electrolyte to milk replacer
  • Increase volume of milk replacer rather than concentration, if you want to increase energy intake

Alasdair Scott for the Farm Advisory Service

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