Chapter Four - Factors influencing elemental micronutrient supply from pasture systems for grazing ruminants

Sustainable ruminant livestock systems are based on forage from pasture comprising the major source of feed, due to its low unit cost, animal health benefits, and lack of land competition with human food production. Macronutrients (energy and protein as well as elemental) in pasture systems have been well-researched but there has been much less research into elemental micronutrients despite their essentiality for livestock health and productivity. Inadequate micronutrients in pasture, as well as elemental antagonisms in the gastrointestinal tract, can affect micronutrient absorption, and hence animal health and production. Direct supplementation of livestock is often adopted to prevent deficiency of micronutrients, which is used prophylactically and routinely as part of standard practice. Subsequently animal excreta can be the major input of micronutrients to pasture. More studies are required to understand how animal feed and micronutrient supplements affect the quantities and chemical forms of micronutrients in excreta, and their ultimate fate in pasture systems. Uptake of micronutrients by a forage is related to the available content in soil, which is influenced by the sources and forms of micronutrients, soil properties, the forage type and agricultural management. Forage species differ in their micronutrient concentrations, but it is unclear how this is affected by plant species' interactions with soils of different types and different environmental conditions, or if this may be significantly affected through plant breeding. The design of multispecies swards for optimal micronutrient content for ruminant health may be feasible, but requires an understanding of the biological, temporal, environmental and management controls on plant uptake.