When we look at milk replacers, we have a tendency to look at the protein and fat content thereof and assign a value to that milk before feeding it to our calves. The real brain teaser is whether or not this value actually plays a role in our calves’ performance on farm. With the help of thorough research, we can dive deeper into the true value of protein and fat content in a milk replacer.
Before unravelling the importance of the nutrient content of milk replacers, one first has to look its composition. The ingredients used when making a milk replacer generally consist of butterfat, whey protein, palm oil, coconut oil, wheat germ, soya protein isolates and skimmed milk powder.
The way in which these are incorporated into a milk replacer is important, for instance we can use protein isolates and reach the desired protein level but still have these protein sources be insoluble and unavailable to the calves. A clear distinction also occurs between first and second generation milk replacers in terms of the caseination.
First generation milk replacers require caseination, whereas second generation does not. Therefore, the source of the critical nutrients is more important than the level in a milk replacer feeding program.
Firstly, fat levels in milk replacers
A study was performed by Suarez-Mena et al. on 100 calves, testing the fat content at 2 levels (17% and 24%). The study tracked calf performance based on many factors until day 112. This will be discussed in 2 separate periods starting with preweaning differences observed followed by the postweaning phase.
Calves fed on a 24% fat milk replacer (MR) performed worse than calves fed a 17% milk replacer. The differences in performance may be attributed to calf starter intake being significantly lower in the calves on a 24% fat level MR.
This is well researched as negatively influencing rumen development and maturation. This was the primary difference between the groups in the preweaning phase with lasting effects into the postweaning phase.
An important secondary factor to note is the width of the hips increased more in calves fed a 17% fat MR, this is important later in the heifer’s life for calving ease. Milk replacer fat content had no effect on body weight, hip height, ADG, fecal score or sick days. ADG in calves fed a 24% fat MR was 4.2% lower than the calves fed 17% fat MR.
The observations following weaning are due to calf starter feed intake differences with calves on the 17% MR eating 19% more starter than the calves fed the 24% MR. The improvements in calf starter intake led to improved development and maturation of the rumen. This translates into a lower body weight, hip width, ADG and feed efficiency.
“the source of the critical nutrient is more important than the level”
A milk replacer provides energy to a calf through one of 2 primary sources fats or lactose. As discussed above there are no benefits for feeding a higher fat milk replacer but we still need to provide energy to the calf. The gap in energy caused by lower fat levels in a MR is covered by an increased lactose level. This exchange of energy source has no effect on calf growth rates, starter intake and nutrient digestibility.
Therefore, a lower fat level and higher lactose level is ideal for calf rearing and performance.
Lastly, protein levels in a milk replacer
Morrison et al. conducted a study on 153 calves to determine if protein level of the milk replacer had any effect on calf performance and later in life lactation performance.
In the study they fed milk replacer at two protein levels; 21% and 27% and tracked the calves through calving and those who stayed in the herd up to and including 2nd lactation.
The findings showed no difference between the calves in performance post weaning by feeding a higher protein content in the milk replacer. Calves fed the 27% milk replacer were slightly lighter (2.1 kg on average) at weaning however this was not a significant difference in the study. Calves fed a 21% milk replacer weighed on average 68.2kg at weaning, The study went onto show that preweaning treatment had no effect on live weight gain from weaning until 720 days.
Protein and fertility
The heifers in the same study were artificial inseminated at a minimum age of 13.5 months and a minimum weight of 315kg. The protein content of the milk replacer had no effect on the performance of these heifers with no difference for age at first calving and no significant differences were found for heifer performance in the first or second lactation.
Furthermore, the milk yield was unaffected in the first or second lactation due to protein content of the milk replacer. While the milk yield was unchanged so was the milk solids levels (fat and protein value of the milk).
We are well aware of the fact that milk provides a calf with all of the necessary nutrients needed prior to the development of the digestive system. What is further demonstrated now is that the actual levels of protein and fat in a milk replacer have no effect on the performance of calves later in their life.
The digestive system requires a grain-based starter diet which releases specific volatile fatty acids to develop the rumen papillae in order to prepare the heifer to digest solid feed following weaning.
A calf requires 15 kg and more of easily fermentable carbohydrates in order for adequate development of the digestive system. This will be further discussed in a later article.
mado, L., Berends, H., Leal, L., Wilms, J., Van Laar, H., Gerrits, W. and Martín-Tereso, J., 2019. Effect of energy source in calf milk replacer on performance, digestibility, and gut permeability in rearing calves. Journal of Dairy Science, 102(5), pp.3994-4001.
Morisson, S., Wicks, H., Fallon, R., Twigge, J., Dawson, L., Wylie, A. and Carson, A., 2009. Effects of feeding level and protein content of milk replacer on the performance of dairy herd replacements. Journal of Dairy Science, pp.1570 – 1579.
Suarez-Mena, F., Dennis, T., Chapman, C., Aragona, K., Hill, T., Quigley, J. and Schlotterbeck, R., 2021. Effects of milk replacer feeding rate and fat content on Jersey calf nutrient digestion and performance to 4 months of age. Journal of Dairy Science, 104(6), pp.6768-6778.