The most conclusive way of determining whether animals are deficient in a trace element is to measure production responses to supplementation in a field trial. However the opportunity, expertise and resources necessary to run such trials are not always readily available and there is often a considerable delay in reaching a diagnosis. Provided the degree of a production response can be closely related to a tissue level of the element or its metabolite then analysis of tissue samples can replace the need for field trials. The paper uses data from a series of cobalt liveweight response trials with lambs to outline a proposed methodology for constructing response curves which, for any specified level of Vitamin B12 in serum, can be used to determine (a) the expected liveweight response to supplementation, and (b) the probability of getting a response at least as great as some given level eg. that considered sufficient to just cover the costs of rectifying the deficiency. A protocol for future production response trials is described. It is planned that all appropriate production response trial data will be used to derive ;response; and ;probability of response; curves for use in diagnosing cobalt, selenium and copper deficiencies in sheep and cattle. It is suggested that the methodology could be applied in many biological systems involving deficiencies.

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Changes in blood levels of zinc, copper, selenium, glutathione peroxidase, vitamin B12 and total and free thyroxine in sheep removed from pasture and held without food for 50 hours.

Blood was collected from a group of ten sheep immediately after removal from pasture and thereafter at regular intervals for 50 hours whilst maintaining them without food but with adequate water. The samples were analysed for parameters normally examined as an aid to the diagnosis of zinc, copper, selenium, cobalt and iodine status. Serum vitamin B12 levels increased with time. Mean plasma zinc concentrations and mean serum concentrations of both T4 and fT4 followed significant parabolic trends. Maximum mean zinc levels (50 +/- 10% higher than initial values) were recorded after 30 hours. T4 and fT4 levels peaked at approximately 26 hours. Zinc levels at two hours were 20 +/- 5% lower than the initial values. Whole blood copper, selenium and glutathione peroxidase levels did not change significantly with time the largest differences were 11 +/- 6%, 5 +/- 4% and 10 +/- 8% respectively.

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