The choice between feeding whole milk (cow’s milk) or milk replacer is not just economics and logistics. It can also significantly affect the calf’s health. A study from Aarhus University and SEGES Innovation has shown how three different feeding strategies influence blood iron levels, liver enzymes, and copper status in milk-fed calves.

Low iron in whole milk and high copper in milk replacer

Researchers divided 24 newborn Holstein calves into three groups and followed them for 8 weeks. During this period, the calves received either whole milk, whole milk with iron supplements, or milk replacer with high levels of iron and copper. While feeding with either whole milk or milk replacer is common practice, adding iron to whole milk is an experimental treatment not typically used. The calves’ blood values were measured weekly, and after 8 weeks, liver function and mineral content in the organs were assessed.

The results showed that calves fed whole milk without iron supplements experienced a significant drop in hemoglobin and hematocrit (the proportion of red blood cells) – two parameters that may indicate iron deficiency. In contrast, calves fed milk replacer had high concentrations of hemoglobin and hematocrit. At the same time, they developed copper accumulation in the liver, which can lead to chronic copper toxicity.

Blood values reveal hidden problems

As early as week 2, researchers began to observe clear differences in hematocrit and hemoglobin levels in the blood: calves fed whole milk showed declining hematocrit and hemoglobin values until week 5. These calves fell below the EU’s minimum threshold for hemoglobin (4.5 mM) for two weeks and below the recommended level (5.3 mM) for five weeks.

Conversely, hemoglobin levels increased in calves receiving either whole milk with iron supplements or milk replacer from week 2 onward and continued to rise. Hematocrit values followed the same trend and had already increased by week 2–3 of the trial.

“The problem is that we actually don’t know what the optimal values for hemoglobin and hematocrit should be,” explains Associate Professor Niels Bastian Kristensen, one of the lead researchers behind the study. “Initially, we thought it was only positive that we found higher values in calves receiving iron supplements either in whole milk or milk replacer. But excessively high hematocrit values can also be a health concern and may be caused by factors other than iron content in the feed. That’s why we need to better understand the calf’s actual needs,” he continues.

From necessary to toxic

Calves fed milk replacer showed another problem: extremely high copper concentrations in the liver. These calves had 636 mg of copper per kg of dry matter in the liver – well above the threshold for chronic copper toxicity. In comparison, calves fed whole milk or whole milk with iron supplements had only about 170 mg/kg. Although liver enzymes in the blood did not indicate acute damage, copper accumulation is a sign of slow poisoning, which can lead to liver damage later in life.

Too Much of a Good Thing

Milk replacers sold on the Danish market can contain up to 14–16 mg of copper per kg of dry matter – significantly more than the calf’s needs. “Calves are born with the copper they need during the milk-feeding period and absorb copper from milk very efficiently,” says Niels Bastian Kristensen. The researchers conclude that adding copper to milk replacer has no nutritional value for calves and may be harmful. “In general, we believe that cattle are fed far too much copper. There has been far too little focus on the problems this can cause,” he adds.

Liver mineral balance: complex interactions

When iron levels in the liver increased, manganese levels decreased. Research in humans has shown that iron status affects manganese absorption – and now it appears the same applies to calves. It is therefore important to consider the complex interactions between different microminerals.

What Does It Mean for the Farmer?

Calf growth was not affected by their iron or copper status, so it may be difficult for farmers to detect. Nevertheless, it is important to pay attention to the calves’ mineral supply. The researchers’ recommendations are clear when it comes to copper: minimize copper content in milk replacers – it is unnecessary and potentially harmful. “It’s a clear message, and it actually applies far beyond just milk-fed calves,” says Niels Bastian, and continues: “cows have lived off grass for thousands of years and are simply not built to handle large amounts of copper.” “We are therefore pleased that our results have already led to some milk replacers having their copper content halved. That shows us our findings are being used,” adds Senior Researcher Mogens Vestergaard, who also participated in the study.

We need to know more about the animals’ requirements

“We are continuing our collaboration between SEGES Innovation and the Department of Animal and Veterinary Sciences to follow up on these results, where we have conducted a new calf trial with different iron doses. Currently, our hypothesis is that a healthy calf will not have problems with high hematocrit due to high iron intake. If the calf develops diarrhea and becomes dehydrated, the situation is entirely different. In that case, elevated hematocrit levels could have serious consequences,” explains Niels Bastian Kristensen.

“We now know that for many years we have been feeding calves incorrectly with iron and copper. We have already presented our results to the industry, but the optimal mineral recommendations for calves are still not fully understood,” concludes Niels Bastian Kristensen. “To get closer to real-life conditions in cattle herds, we are currently conducting practical trials with iron levels for calves,” adds Mogens Vestergaard.