Published online by Cambridge University Press: 27 February 2018
By definition, the essential trace elements are required in very small amounts for optimum body function. The list of essential trace elements is steadily increasing, partly due to improved methods for the detection of very low concentrations and partly to the creation of experimental environments from which such elements can be excluded so that their essential properties can be demonstrated. However, this paper is considering field problems, and discussion will be limited to those elements which are known to be associated with disease in cattle and sheep. Pigs and poultry will be excluded because the major proportion are fed adequate amounts of the trace elements in compounded diets. In contrast, most of the sheep and a large proportion of the cattle population are heavily dependent on home-grown feeds, often only grass or grass products, and these animals are most at risk. Without supplementation, the intake of trace elements is dependent on the concentration in the herbage. It has been estimated that grassland provides almost three quarters of the total crude protein for ruminants in Britain and cereals provide another eighth. We must assume that home-grown feeds are a major contribution to animal diets.