Dogs need copper but they cannot make it themselves so it must be taken in through the diet. Although all dog foods naturally contain a certain amount of copper from the raw ingredients, additional copper is routinely added to ensure there is enough for the dog's requirements.
While there are several forms of copper supplement available to pet food manufacturers, copper sulphate has been their number one source for near on 80 years. It is approved by both AAFCO in the US and FEDIAF in Europe and its use in pet food has never been directly associated with any pet illnesses. Despite this fact, though, copper sulphate remains a somewhat controversial ingredient with some parties claiming that copper sulphate is 'moderately poisonous' or even 'toxic' to pets.
Much of the criticism stems from the fact that pure copper sulphate is not a particularly pleasant substance. In the EU, for example, it is classified as 'harmful' and 'dangerous to the environment' as well as being an 'irritant'. Naturally, these are not the kind of terms you want to see linked with pet food ingredients but it is important to note that the amounts found in pet foods are absolutely tiny - far below the amounts that would be needed to cause harm.
Nevertheless, some manufacturers prefer to err on the side of caution and opt for less contentious forms of copper supplements like copper gluconate, copper carbonate or chelated copper (often listed as copper amino acid complex or copper proteinate and widely regarded as the safest and most absorbable form of copper supplement).
As a standard mineral supplement, copper sulphate is not required to be mentioned on the label so if you would prefer to avoid it, the only way to be sure is to contact your pet food manufacturer directly.
Find foods containing Copper sulphate See the full Ingredient Glossary