Osmolality is the concentration of soluble particles (sugar, salt, ...) in a liquid. Different osmolalities of different liquids always strive to balance each other out if, for example, a permeable membrane allows an exchange. This means that liquid (water) migrates through the membrane to the higher concentration (osmosis) until the concentrations on both sides are equal (isotonic).

Similar problems can occur when a calf consumes milk with too high an osmolality (hypertonic). If the milk replacer mixture is too concentrated or minerals are added, the calf feed may become hypertonic, possibly leading to problems with diarrhoea.

This can happen firstly because fluid in the intestine is no longer properly absorbed by the body.

Secondly, it may lead to cell damage in the intestinal wall, which can then cause what is known as "leaky gut syndrome". Damaged cells become permeable to pathogens, which are then able to enter the metabolic cycle unhindered and cause a variety of problems.

Moreover, multiple studies have noted inflammation and ulceration of the abomasal wall, in addition to diarrhoea, when hypertonic calf feed is given over the long term (e.g. "Intestinal adaptations to energy source of milk replacer in neonatal dairy calves" A.C.Welboren, B. Hatew, J.B.Renaud, L.N.Leal, J.Martín-Tereso, M.A.Steele 2021).

Very high osmolality (over 600 mOsm/kg) can also be the cause of abomasal bloating. The milk or also the rennet cake can remain too long in the rennet stomach and is not sufficiently transferred into the small intestine.

But what does this have to do with calf rearing? It's simple: natural whole milk is ideally suited to calves. Whole milk also has an osmolality of around 300 mOsm/kg. Osmolality in milk is primarily determined by minerals and lactose. Because milk and blood have approximately the same osmolality, there is no problem with harmful osmotic processes in the intestine.

Milk replacers (CMRs) are made from by-products of food production.

When butter is made, fat is removed from the milk, leaving skimmed milk, which is then dried and used as skimmed milk powder in the CMR. Skimmed milk still contains all the protein components, including high-quality casein. It still contains casein and pepsin in a ratio of 80:20.

However, when cheese is made, pretty much all the casein is converted into cheese. The fat content is also completely incorporated into the cheese. What remains is what is known as sweet whey. It only contains whey proteins and all the minerals (ash).

Since nearly all of the milk fat is extracted in both butter and cheese production, extra fat needs to be added. Vegetable fats are unproblematic in milk replacers and are easily digested. They have no influence on osmolality, either. However, fats imported from overseas, such as palm oil and coconut oil, are increasingly coming under criticism from society because they generally come from regions with former tropical forests. But that is surely a different issue.

It is therefore evident that milk substitutes, regardless of quality, are fundamentally different from whole milk. Levels of lactose and ash in particular are significantly higher. This means that the osmolality of the milk replacer is also higher (hypertonic). It ranges between 400 and 600 mOsm/kg, depending on the product. This means that we continuously expose the calves to stress which, as described above, can lead to problems with diarrhoea and ulcer formation in the abomasum. Osmolality and the risk of diarrhoea increase particularly with the increased CMR concentrations that are often recommended.

This means that care should be taken to ensure that lactose and ash levels in the milk replacer are not too high. This is difficult because it is not compulsory in Europe to show lactose content on the product declaration. The ash content (minerals) in the milk replacer should not exceed 8 %, with 7 % better.

It is not easy to give a clear-cut answer.

Whole milk

Whole milk is generally best for the calves to digest, as its osmolality and protein composition meet calves' needs. However, aspects of hygiene need to be borne in mind when feeding whole milk. You can read about what is at stake in this blog post: "The H&L whole milk story"

Skimmed milk replacer

Pure skimmed milk powder contains high-quality casein, which calves can easily digest. However, whey products are usually also added to skimmed milk powders in order to achieve the necessary protein content or even to reduce costs. As the whey content increases, the casein content decreases, meaning increased osmolality.

A common belief expressed when calves are sick or also suffering from stress is, "Electrolytes always help". However, there are a few things to bear in mind:

1. Electrolytes mixed in milk will massively increase the osmolality of the feed, especially in powdered milk feeds (see above). This is therefore something that should be avoided. When using whole milk conditioners with a high mineral content, the relevant product information must be observed.
2. An "isotonic" solution should always be prepared when dosing the electrolytes. This is because if calves develop diarrhoea due to hypertonic calf feed, you do not want to stress them with a hypertonic electrolyte as well. Please follow the manufacturer's instructions carefully in this case too. As a rough guide, an osmolality of 290-300 mOsm/kg in blood corresponds to a 0.9 per cent NaCl water solution.

Besides choosing the right type of milk feed, how it is prepared is also crucial. There are three key things to consider here:

1. Continuity.
2. Continuity.
3. Continuity.

Calves like things to be boring. Always prepare the same milk feed, at the same temperature, in the same concentration and in the same quantity. The best way to ensure this is by using modern feeding technology. Automatic feeders need to be calibrated and adjusted correctly and care must be taken to use the correct amount of milk replacer when hand feeding. The MilkTaxi offers the ideal aid for mixing the exact milk replacer concentration in the form of Smart-Mix.