Current issues in trace element nutrition of grazing livestock in Australia and New Zealand

Improving trace element nutrition of grazing animals, in a way that is cost effective and that meets consumer perceptions and preferences, is a continuing challenge. This review focuses on research over the past 10 years, addressing issues and perspectives on the roles, risks of inadequacy, and supplementary remedies of key trace elements, both essential and deleterious, which have an impact on the productivity and product quality of grazing livestock throughout Australia and New Zealand. The emphasis is on copper (Cu), cobalt (Co), iodine (I), selenium (Se), and, to a lesser extent, zinc (Zn) - the trace elements most frequently found to be deficient in parts of Australia and New Zealand. Research continues to elucidate new roles for trace elements in the animal, and as this leads to a better understanding of requirements, diagnostic criteria and supplementation strategies need re-evaluation. Newer perspectives on marginal deficiency for Cu, I, Se, and Zn are given and issues for risk management discussed. Advances in sustained delivery of trace elements such as Co (as vitamin B-12), I, and Se are reported. The diagnosis and management of marginal Cu deficiency continue to be difficult, especially in New Zealand where recent work has shown that dietary antagonists [iron (Fe), molybdenum (Mo), and sulfur (S)] may impair Cu absorption at lower intakes than previously thought. There is still a dearth of scientific data on the advantages of using so-called organic trace element supplements (metals complexed or chelated with amino acids or peptides). Nevertheless their usage continues to increase. There is increasing evidence that trace elements influence the pathophysiology arising from the ingestion of toxins. This review summarises recent work on the role of Co in annual ryegrass toxicity; Cu, Se, and Zn in lupinosis; Cu and alkaloid toxicity; Zn and facial eczema; and Co and white liver disease. Trace elements are required to support immune function (e.g. as imposed parasite infection) and marginal deficiencies may be exacerbated by an immunological challenge. The roles of Cu, Co, Mo, Se, and, to a lesser extent, chromium and Zn have attracted attention and under conditions of stress there may be an additional need for these elements. Diversification in farming has led to the introduction of species such as deer (Cervus), alpacas (Camilids), emus, and ostriches (Ratites) and the paucity of information on trace element requirements for these species, and also for horses, indicates the need for further work. The effect of supplementation on trace element composition of meat, milk, and wool is also reviewed, both in terms of product characteristics and human health. Of the deleterious elements, cadmium has attracted the most interest and concern because of its introduction into the pastoral system from phosphate-based fertilisers.