When choosing a milk replacer, calf raisers often look at two key ingredients: fat and protein. However, according to Skip Olson, technical services veterinarian for Milk Products LLC in Chilton, Wis., “Milk replacers are made with four major components: protein, fat, lactose and ash. All four work in collaboration with one another.”

Protein and fat are traditionally used to describe a specific milk replacer’s formulation, with the first number representing protein and the second representing fat. For example, a 24:20 milk replacer would contain 24% protein and 20% fat.

The ash (mineral) percentage is determined by the ash content of the various ingredients. “Ash is a naturally occurring substance in milk- and plant-based feed products,” Olson said. “It is made up of minerals, including sodium, potassium, chloride, calcium and trace minerals. Many of these essential elements contribute to the cellular function of calves that allow for both growth and strong immunity.”

“Whole, raw milk contains about 5.5-6.5% ash,” Olson said. “Milk replacers usually contain more ash because they often are manufactured using primarily milk-based ingredients that concentrate natural minerals.” An example is reduced-lactose whey, or “delac,” which is a co-product of cheese production.

The fourth ingredient, lactose, varies in percentage because it completes the formulation based on the protein, fat and ash content. In a 20:20 milk replacer with 10% ash, lactose would make up the remaining 50%. If the protein level is 24%, lactose would drop to 46%.

Milk replacers also often contain other additives — like trace minerals, vitamins and medications — that increase the ash content. The result is a 7-12% range in commercial milk replacers, with most containing about 9% ash. Olson said a higher number is not necessarily detrimental to the calf for a number of reasons:

1. Protein, fat and lactose are variable influencers. Protein is required by the calf for lean tissue and structural growth. Fat and lactose combine to provide energy to fill in the skeletal frame. Ash levels only become “too high” and limit growth when they don’t allow for enough lactose to provide a total energy package that maximizes growth based on the amount of protein in the ration. Even in those cases, the setback in weight gain is minor. For example, when a 25:20 milk replacer containing 9% ash is fed at 1.5 lb. of dry matter per day, the National Research Council (NRC) predicts that gain at 60°F is reduced by only 0.03 lb. per day compared to a formulation containing 7% ash. Over a five-week period, that amounts to just a 1 lb. difference in total gain.

2. Feeding rate matters. Using the same formulation of 25:20 milk replacer but increasing the feeding rate to 2 lb. of dry matter per day means the 9% ash level won’t have any influence on predicted gain. That’s because feeding at this higher rate provides more than enough available energy (fat plus lactose) to accommodate the protein in the ration.

3. Cost of adjustment usually is not recouped in gain. The primary challenge in manufacturing milk replacers is providing high-quality nutrition at the least cost. “Delac” often is a key ingredient to accomplish both. It is higher in ash than other common ingredients. However, given the examples above, it can be used to develop milk replacer formulations that still result in acceptable ash levels. For instance, choosing a 12% ash product versus a 9% product saves $1.50-2.00 per bag while sacrificing only 2 lb. of total predicted gain for the first five weeks of feeding.

4. Moisture also is a factor. Probably a more important variable than ash is moisture level in milk replacer. Moisture can vary from 2.5% to 7.0%. As an example, a bag of milk replacer with 7% moisture has 4% less dry matter (and 4% more water) than a bag with 3% moisture. That represents a 50 lb. bag of milk replacer with 2 lb. less dry matter and nutritional value.

A high moisture content has a much more detrimental effect on calf growth and cost efficiency than a high ash content. A 20:20 milk replacer with 7% moisture, fed at the same volume as a 20:20 formulation with 3% moisture, will result in 0.07 lb. lower average daily gain, or a total of 2.45 lb. of gain in the first five weeks of feeding.

Olson said the last example is especially telling, because the two bags of milk replacer likely would sell at a very similar price. “Unlike ash, where a small sacrifice in gain would be offset by a more affordable product, moisture levels at the high end of the acceptable range usually do not come at a lower price, but they have similar, or more profound, impact on gain,” he said.

“Ash is not added to milk replacer as a ‘filler,’” Olson added. “It is a natural and varying component based on the ingredients selected for the formulation.”