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Are heavy metals an actual problem in animal feeds?

Are heavy metals an actual problem in animal feeds?

There is a perception that heavy metals are a concern in animal feeds, yet research to support this contention is lacking.


*Jon D. Nelson is with Southeastern Minerals Inc.

THE heavy metal content of animal feed is something that has been written about, talked about and totally misunderstood by the feed industry and its regulators.

Attempts have been made to categorize and summarize the subject in the publication The Mineral Tolerance of Domestic Animals, and the American Association of Feed Control Officials (AAFCO) has written guidelines for decision-making on the subject that appear in the AAFCO Official Publication, yet no one really understands whether or not this is a "real" problem.

The perception is that heavy metals are an item of concern in animal feeds, yet no one seems to understand why, nor can they support their contention with scientifically sound data. Therefore, trace mineral compounds and mineral compounds are the "usual suspects" for the major source of heavy metal contamination, primarily because of a lack of knowledge regarding these compounds and the difficulty most people have in making a mathematical conversion from ounces and pounds to milligrams, grams and kilograms and then on to parts per million, parts per billion, parts per trillion and so on.

Additionally, the fact that trace mineral ingredients and mineral ingredients are added at very low levels, thereby diluting any heavy metal content significantly, escapes the mathematical process.

Without getting into what is safe or not safe about the heavy metal content of trace mineral compounds, the Table provides a look at the cadmium content of a single compound, zinc sulfate, using 36% zinc content in the calculations. This compound was selected because the last heavy metal issue I am aware of occurring in the U.S. involved a source of zinc sulfate imported from China by a "broker" through the state of Washington that was actually a zinc sulfate-cadmium filter cake containing 20% zinc and 12% cadmium (120,000 ppm).

As far as I am aware, the material was primarily used in the fertilizer industry, but some feed companies jumped at the opportunity to purchase a lower-cost ingredient and incorporated it into animal feeds (remember melamine?). Hopefully, the new Foreign Supplier Verification Program will diminish the probability of this type of situation reoccurring.

So, the key question becomes, "Are trace mineral ingredients a primary contributor of heavy metals to complete animal feeds?"

In the Table, I have placed lines A, B and C. Let us consider line A first. If there is an elemental zinc content of 36% in zinc sulfate and it is added to a complete feed to reach an added zinc level of 175 ppm (essentially the highest supplementation level "normally" found in animal feeds), the "zinc sulfate" addition level becomes 486 ppm (175 divided by 0.36). Most "quality" zinc sulfates available on the market contain around 20 ppm of cadmium. Multiplying this number (20 ppm, or 0.00002) by 486 ppm of zinc sulfate provides a cadmium content of 0.0097 ppm in the finished/complete feed, or 9.722 ppb — a pretty simple calculation.

Is this a level that warrants concern? In my opinion, it is not, and I doubt that it would be considered significant by state and federal regulatory officials.

In line B, the same calculation has been performed using a metric measure (grams) in order to demonstrate that both methods can be used, and both will provide the same result. Note: There are 908,000 g in a "short" ton of animal feed.

In line C, I have demonstrated that, once there is a factorial relationship between the added level of zinc sulfate and the cadmium level of the same, the solution now becomes strictly a mathematical calculation. If the zinc sulfate being added contains 500 ppm of cadmium, (500/20 = 25), 25 becomes the multiplier; therefore, the zinc sulfate in this example would contain 9.722 x 25 = 243 ppb of cadmium.

Is it safe to have 243 ppb of cadmium in a complete feed? Again, that is up to regulators to determine; however, I believe a safety factor exists in the real world when considering that a "normal-quality" source of zinc sulfate will provide 9.722 ppb to a complete feed, whereas a source with an elevated level will only bring this level to 243 ppb. Consider that the maximum tolerable level of cadmium in a complete feed currently is listed as 5 ppm in all species in The Mineral Tolerance of Domestic Animals and the AAFCO publications. These two cadmium levels would provide a 514.3x safety factor at 9.722 ppb and a 20.6x safety factor at 243 ppb.

If trace mineral compounds are not a significant source of heavy metal contamination in animal feeds, what are the major environmental sources that contribute to elevated levels in the soil and the food chain? Normally, this list includes burning coal, gas and oil and incinerating waste material. The higher levels occur in the underdeveloped yet industrialized sections of the world, particularly in Asia and other non-European countries.

Cadmium is somewhat of a maverick in that it primarily comes from volcanic emissions and the weathering of rocks, but it also is emitted by anthropogenic sources such as smelting metals, burning fossil fuels, incinerating waste and using unrefined phosphates and sewage sludge fertilizers. Lead comes from a few additional sources, such as lead batteries, pluming solders, alloys, pigments and ceramics. Mercury is no different in its origin, but it accumulates and bio-magnifies along the food chain, particularly in the aquatic food chain. Additionally, it is volatile and undergoes a series of complex chemical and physical transformations as it cycles between the atmosphere, land and water.

Why are we, as an industry, so concerned about heavy metals in trace mineral ingredients? In reality, it is a good thing, because it's a strong indicator of the quality and biological value of a trace mineral ingredient. As an example, there is a direct correlation between the biological value of a zinc oxide and its lead content. We can probably assume that the same is true for other compounds containing heavy metals — not because the heavy metal content will cause a toxicity or a residue problem in the animals being fed but because, to a point, the cleaner, more refined products have an improved biological value in all animal species.

If this is true, should we be switching to USP ingredients in animal feed production? Absolutely not! The animal feed-grade materials are fine. USP, Tech and other compound grades have a much higher purity because it is necessary for laboratory use to ensure that low-level contaminants will not interfere with the desired chemical reaction. The added cost of using more highly refined compounds would be totally unjustifiable.


Heavy metals in animal feeds

A heavy metal, in this case the cadmium (Cd) content of a complete feed, can be calculated in a couple of different ways. The following is an illustration of how it may be calculated using ppm (A) or weight (B). There is an additional example of a zinc sulfate containing 500 ppm of Cd in order to demonstrate that, after determining a "base" number, extrapolating to higher levels/numbers is merely an arithmetic calculation.





% zinc




Zinc feed addition level

175 ppm

158.9 g

158.9 g





Zinc sulfate addition

486 ppm

441.4 g

441.4 g

Cd content

20 ppm/0.002%

20 ppm/0.002%

if 500 ppm


486 x 0.00002

0.008828 g


Cd content in complete feed, ppm

0.097 ppm

0.008828 g/908,000 g

500 ppm/20 ppm

Cd content in complete feed, ppb

9.722 ppb

0.00000000972 or 9.72 ppb

9.722 x 25 = 243 ppb


Volume:88 Issue:12

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