16 Overview of trace minerals and considerations in defining their requirements

At least eight trace minerals are classified as essential for beef cattle, based on identification of one or more metabolic functions. Essential trace minerals are under homeostatic control mechanisms that alter their absorption and/or excretion, resulting in tissue concentrations of most minerals being maintained within a fairly narrow range. Without whole body homeostatic mechanisms to control mineral concentrations in the body, it would be extremely difficult to formulate bioavailable trace mineral levels in diets within a range that prevents both deficiencies and toxicities. Cellular homeostasis is also important in preventing adverse effects of free metal ions, especially iron and copper, in the cell. Trace mineral requirements are estimated by the National Academies of Sciences Committee on Nutrient Requirements of Beef Cattle. Requirements are based on the concentration of each trace mineral necessary to maximize overall performance (growth, reproduction, and health), and are based on published research. A number of factors can affect requirements of certain trace minerals such as bioavailability, which is affected by antagonist and trace mineral source. There is little recent research on trace mineral requirements. Most recent trace mineral studies have not included a negative control treatment and have evaluated adding additional concentrations of trace minerals to a diet already formulated to meet or exceed NASEM (2016) recommendations. Much of the older trace mineral research was conducted with British cattle breeds. Research has indicated that Continental and Bos indicus breeds differ in regard to copper and possibly selenium metabolism. Trace mineral requirements of breeds that have been developed by crossing Bos indicus with Bos taurus are largely unknown. There have been genetic improvements over time that have resulted in heavier calf weaning weights, faster and more efficient feedlot gains, and heavier slaughter BW. These genetic improvements have resulted in improved energy efficiency. It is unclear if the improved energy efficiency has also been associated with enhanced trace mineral utilization. Use of more aggressive implant programs and beta-agonists in recent years has also improved growth rate and efficiency of feedlot cattle. Well-designed studies are needed to more accurately define trace mineral requirements for beef cattle.