As the European Union — and don’t tell me others will not follow suit soon — starts to look at zinc and copper as the next environmental hazards to be put under strict(er) legislative control, these nutrients require some rethinking from our side. Indeed, copper requirements are largely met by copper present in natural ingredients, but this might not be the case as we move to using more and more by-products. The same cannot be said about zinc, which becomes deficient unless an external source is provided. And, with existing zinc natural resources expected to be depleted within 20 years, prices will go only upwards. So, it's high time to rethink zinc and copper requirements, before others do it for us!
The first logical step is to do dose-response studies to establish actual requirements under commercial conditions. Surprisingly, not only are such studies scarce, but also they are so many decades old. And, they don’t cover all animal ages.
The second step would be to establish requirements not on a total basis, but on a bioavailable or digestible basis, like we do for amino acids, for example. It is of little relevance if requirement is set at 50 ppm total zinc, when zinc bioavailability is 33 percent from ZnO and 120 percent from organic zinc. We need to match actual needs with actual supply.
We all discount zinc and copper and overformulate diets and premixes, if only because they are relatively inexpensive. But, think again as they become more expensive and less available.