What feed additives and supplements are permitted? Natural (nonsynthetic) feed additives and supplements are permitted, as are synthetic substances that are on the National List. These include synthetic vitamins and minerals, which are limited to the amount necessary for adequate nutrition. Slaughter by-products are prohibited for feeding to mammals and poultry, and urea and manure re-feeding is prohibited for all livestock. Synthetic amino acids are not included on the National List, with the exception of a temporary allowance granted for methionine for use in poultry until October 2005.