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Milk replacer percent solids, osmolarity … What in the concentration are we talking about?

Drew A. Vermeire for Progressive Dairy Published on 30 October 2020

I read mixing directions on a milk replacer tag recently, and it said “mix at 15% solids” which seems simple enough. But what does “15% solids” mean? Is it 15% by weight or 15% by volume?

We see terms thrown about like “percent solids” or “grams per litre,” but I’m never sure if the person using those terms knows what they mean. We also hear terms like “osmolality” (with an “L”) and “osmolarity” (with an “R”), which only adds to our confusion. Have no fear. This article will help you cut through the confusion.

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Concentration as weight per weight and weight per volume

The common feeding practice with veal calves in the U.S. is to speak of concentration in terms of ounces of milk replacer powder in a specified number of pounds of total solution. This is a measure of weight (ounces) per weight (pounds).

Both veal and replacement calves outside the U.S. are fed a certain number of litres of total solution with a specified number of grams of milk replacer powder per litre of total solution, which is an example of weight per volume.

The term which is least understood is “percent solids,” since we don’t know if it refers to weight per volume or weight per weight. Expressing concentration as grams of powder per litre of total solution is the most accurate descriptor we could use and, as an industry, it would minimize confusion if feeding directions were expressed in this way.

Weight and volume

While it’s easy to add a known weight of milk replacer into a known volume of total solution, it is not easy to calculate weight/weight when we typically feed by volume. It is very difficult to make calculations between the weight of a solution and the volume of the solution because the composition of milk replacer has some impact on weight and volume. Four litres of water weighs 4 kilograms. Four litres of cows’ milk weighs 4.12 kilograms but varies with fat content. In the same way, 4 litres of milk replacer solution weighs more than 4 litres of water.

If we add 2 cups (a volume of 473 millilitres) of dry milk replacer powder to 4 litres (4,000 millilitres) of water, the total volume of milk replacer solution is more than 4,000 but substantially less than 4,473 millilitres because after the milk replacer powder is dissolved into solution, the total volume is less than the volume of water plus the volume of dry powder. When we add 149.2 grams of milk replacer powder per litre, the total volume is 1,030 millilitres, which illustrates this point.

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Weight and volume of 10 onces milk replacer powder

To illustrate some relationships between weight/weight and weight/volume, we did some testing in the laboratory in which 283 grams of milk replacer powder was mixed into 1,893 millilitres of total solution.

Our mixing in the lab is similar to how we recommend that milk replacers be mixed for replacement calves. Fifty percent of the total water is added to a mixer, milk replacer powder is added and mixed, and additional water is added to bring the total volume to a prescribed level. In our testing, 1 litre of the milk replacer solution weighed 1,033.3 grams, but a different milk replacer would likely have a different weight per litre due to different composition.

Osmolality and osmolarity

The milk replacer solution from our test was tested by an outside laboratory to determine the osmolality, which is the number of dissolved particles per kilogram of solution, and we weighed the solution to determine how much a litre of solution weighed so we could calculate the osmolarity, which is the number of dissolved particles per litre of solution. The differences between these two terms are again a question of volume or weight; however, what is being measured are the number of dissolved solids.

Osmolarity refers to milliosmoles (mOsm) of dissolved solids per litre, while osmolality refers to mOsm per kilogram. Guidelines and published data are often shown as mOsm/L (osmolarity), but the testing instruments often test osmolality, which is mOsm/kg. For some solutions, these values are very similar – but that’s not true for all solutions, so be careful which values you are comparing.

There are some recommendations about the osmolality, based on limited number of research studies, but the composition of dissolved particles has an impact that has not been widely researched. High-ash milk replacers are high in sodium, potassium and chloride in addition to protein, fat and lactose present in low-ash milk replacers. High osmolarity associated with high ash has a much greater impact, leading to bloat and other digestive problems.

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Go metric

As if we didn’t have enough confusion, the system of weights and measures in the U.S. includes a unit of weight called “ounces” and also a unit of volume called “fluid ounces.” An ounce by weight is 1/16 of a pound (28.35 grams), while a fluid ounce is 1/128 gallon. There is not a fixed relationship between the weight (in ounces) and the volume (in fluid ounces) using this system. In the metric system, 1 millilitre of water weighs one gram, so there is an established relationship between weight and volume using water.

We can eliminate some confusion about ounces and fluid ounces by using metric measurements, and it’s easy to make calculations. Units of weight are grams (g) or kilograms (kg). One kilogram weighs 1,000 grams, and 1 pound (lb) weighs 453.6 grams. Units of volume are litres (L) and millilitres (mL) with 1 litre = 1,000 millilitres and 1 gallon = 3,800 millilitres. If we’re feeding in quarts, it is easy to know how much milk replacer we are feeding a calf if we know the concentration (180 grams per litre for example) x 1.9 litres (2 quarts) = 342 grams per bottle. To determine ounces, 342 grams ÷ 453.6 grams per pound x 16 ounces per pound = 12 ounces milk replacer powder per bottle.

Automated milk replacer feeding systems

With automated milk replacer feeding systems, the common term is “grams per litre,” but for some machines, the actual term means grams of powder mixed in addition to 1 litre of water while for other machines it means grams of powder mixed into 1 litre of total solution. The simple way to interpret these two concepts is to consider the grams of milk replacer powder added “on the litre” or grams of milk replacer powder added “in the litre.” Check your machine to see what the terminology actually means. What difference does it make? If your machine feeds 6 litres total solution, and it’s mixed at 160 grams per litre, the calf is either fed 960 grams per day if the machine mixes 160 grams in the litre or less than 960 if it feeds 960 grams on the litre.

What’s the best concentration?

While cows’ milk has approximately 125 grams per litre, research shows that calf growth is most efficient when milk replacer is fed in the range of 150 to 180 grams per litre, with two exceptions. The first is with automated feeding systems, where experience shows that concentrations higher than 160 grams per litre can result in more cross-sucking among calves, and feeding a more dilute concentration can reduce this negative behaviour.

The second exception is with high-ash milk replacers. Veal milk replacers typically contain less than 7% ash, while many herd milk replacers contain in excess of 11.5% ash. I am a calf nutritionist, but if we asked a dairy nutritionist what would happen if we added 4.5% salt into a TMR for cows, he or she would excitedly describe all the negative impacts of adding so much salt to the total diet. The difference between high and low ash is the sodium, potassium and chloride content because these minerals replace lactose, which is a source of energy for calves.

A producer can spot high-ash milk replacers by a listing of “dried whey product” on the label because delactosed whey and whey permeate are usually listed with this term. High-ash milk replacers often carry a warning to not feed with “high-sodium water” or not to mix with a concentration higher than 150 grams per litre because of the risk of sodium poisoning due to the high sodium content of the milk replacer associated with high ash.  end mark

Drew A. Vermeire, Ph.D.

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