Pigs must have free access to water even when normal levels of salt are fed, because excess salt is eliminated in the urine. Calcium and phosphorus must be in an acceptable balance with one another to function properly. The phosphorus in inorganic sources, such as dicalcium phosphate, and normal sources, such as meat and bone meal, are utilized more effectively by the pig than phosphorus in seeds. The trace minerals normally added to swine diets are iron, copper, iodide, manganese and selenium. Most serve a specific vital role in the body.
All trace elements can be toxic in high doses.
Selenium is unique in that the amount that can be added to swine diets is regulated by the Food and Drug Administration. The current legal maximum addition is 0.3 ppm. Selenium is the trace element with the greatest potential for toxicity.