At Bonanza Calf Nutrition’s #CalfChat event last week, protein content in milk replacer was the subject under discussion.

Deciphering the ingredients labels on different milk replacers can be very confusing as it is not always evident which contain the best formula for calves.

“The protein element alone is enough to boggle the mind,’’ says Dr Christine Cummins, of Bonanza Calf Nutrition.

In the first weeks of life, calves get all of their nutrients through their milk feed – a time when the gut is still developing. Their ability to digest feed can therefore vary greatly.

This is why it is essential to consider the type of protein included in a milk replacer and to understand the need and ability of the calf ingesting the product.

A calf needs protein to maintain biological processes, tissue growth and repair and for many more functions therefore it is essential that it can process the protein it takes in.

Dr Cummins says young calves are not able to sufficiently process all types of protein therefore care is needed when selecting the milk feed.

“Ingested protein is broken down in the gut and subsequently absorbed, the young calf only has the ability to utilise milk proteins,’’ she says.

In a poll held during #CalfChat, participants expressed mixed opinion on what they believed to be the most digestible form of protein for their calves: 39% believed it to be whey, 47% skim and 4% plant, while 10% thought any form of milk protein was suitable.

Milk proteins – proteins that are derived from cow’s milk – include skim and whey. Milk replacer formulas can contain both or just one.

For calves in the first weeks’ of life, milk proteins are key for calf health.

Dr Cummins advises that the most digestible formula for a young calf includes both skim and whey combined as this can be digested in a similar way to cow’s milk.

The calf’s gut is still developing and learning to become efficient in its ability to process the feed ingested therefore a milk replacer with protein which is digested most similarly to cow’s milk is the safest option, she says.

“Skim is digested in the same way as casein in whole milk, that is slowly through clotting action, and whey is digested by passing through the calf’s system more rapidly, without a clot, as described in Figure 1.’’

Figure 1 Digestion of whole milk and milk replacers in calves; source: Bonanza Calf Nutrition

A skim-based milk replacer is therefore the most digestible to the young calf.

Furthermore, a more digestible protein will help protect the calf if a disease outbreak occurs as a greater chance to digest skim and protein means there is a lower demand on the gut to absorb the necessary nutrients.

This was demonstrated during an outbreak of salmonella during a trial in New Zealand (see Table 1).

Table 1: Effect of dietary & feeding regime treatments on calf health during a salmonella challenge. Source: Thomson et al, 2018. NZ Journal of Animal Science and Production 2018. Vol 78: 157-160


When considering which milk replacer to feed, Dr Cummins recommends being mindful that protein percentage is determined by a chemical test measuring crude protein, not the digestibility of the protein.

Therefore, it is more important to examine a list of ingredients to determine if milk proteins are mostly or wholly included.

“If skim is listed first or second, then the powder contains mostly milk proteins,’’ Dr Cummins explains.

In another poll held on #CalfChat, opinion was split on how much protein a calf should consume.

Forty six per cent of participants believed more protein resulted in more growth.

However, while the digestibility of the protein is crucial, the calf has a certain threshold, says Dr Cummins.

“It can’t process any amount of protein just because it is digestible, it has a limit regardless of the digestibility of the protein.’’

In nature, a cow doesn’t produce endless amounts of protein, she adds.

“When considering the level of protein required by the calf, we often think that more protein equals more growth however this is not the case.’’

Using the cow as an example, the milk she produces post-calving demonstrates that more protein is not necessary.

In the first 20 days after calving the protein content of her milk declines by 30% – it only starts to increase after day 40 (Figure 2).

Figure 2 Change in milk protein concentration from calving to day 160 of lactation; source: Gellrich et al., 2014. Czech J. Anim. Sci., 59, 2014 (3): 97–106


After day 40, a calf is not only relying on milk as feed but is digesting concentrates too which adds to the protein content in the diet.

If the protein fed to the calf is too high or alternatively too high and not very digestible, it means more energy is needed to process this protein and this can result in negative effects on both the growth and health of the calf.

“It is about optimal protein, that is the right level of the right type,’’ says Dr Cummins.

Consider this; do we need an additive for whole milk to bring amino acids to an adequate amount?

“Feeding a 20% crude protein milk replacer with the right type of protein is therefore better than feeding a higher percentage of the wrong type.’’

The amino acids in milk replacer must also be considered. These are the building blocks of protein.

“Consider this; do we need an additive for whole milk to bring amino acids to an adequate amount?’’ asks Dr Cummins.

“The answer is no. Then why is it done in milk replacer? There are more than 20 essential amino acids required by livestock. These amino acids are added to milk replacer when necessary but a good quality skim-based milk replacer does not require this addition as there is a sufficient amount present naturally.

“Like with a lot of things in calf rearing, keeping it simple is key when it comes to milk replacer.’’