Problems in the determination of the trace element requirements of animals

The impact of feeding supplemental minerals to sheep on the return of micronutrients to pasture via urine and faeces

The form (organic versus inorganic) of minerals (Se, Zn, Cu and Mn), supplemented to sheep (Charolais × Suffolk-Mule (mean weight = 57 ± 2.9 kg) at two European industrial doses, on the return of micronutrients to pasture via nutrient partitioning and composition in sheep urine and faeces was investigated. This gave four treatments in total with 6 animals per treatment (n = 24). The form of the supplemented minerals did not influence the excretory partitioning of micronutrients (Se, Zn, Cu and Mn) between urine and faeces, nor on their concentrations in the excreta. The two doses trialed however, may influence the Se flux in the environment through altering the ratios of Se:P and Se:S ratios in the faeces and Se:S ratio in the urine. Administration of the mineral supplements also improved the retention of P in sheep reducing its excretion via urine. Although the concentrations of readily bioavailable micronutrients in the faeces were not affected by the mineral forms, there were differences in the more recalcitrant fractions of Se, Zn and Cu (as inferred via a sequential extraction) in faeces when different forms of supplemental minerals were offered. The potential impact of these differences on micronutrient flux in pasture requires further investigation.

A technique for measuring the biological availability of copper to sheep, using hypocupraemic ewes

1. Ewes were made hypocupraemic by feeding a copper-deficient diet and the subsequent responses in plasma Cu obtained when Cu was added to the diet were used to assess the biological availability of the added Cu.

2. The uniformity of responses was investigated by repleting thirty-six ewes for 33 d on two occasions with a standard diet containing 5.0 mg Cu/kg dry matter (DM). The mean responses were 0.36 and 0.33 mg/l and the coefficient of variation was approximately 53% on each occasion. The marked individual differences were largely repeatable, the correlation coefficient (r) within individuals being 0.68 (P< 0.001).

3. A dose: response relationship was investigated by giving five groups of seven ewes diets containing 2.7, 4.2, 5.7, 7.2 or 8.7 mg Cu/kg DM for 33 d. Each increment in dietary Cu above 4.2 mg/kg significantly increased the response in plasma Cu. The relationship between plasma Cu response (y, mg/l) and Cu intake (x, mg/d) after 21 d wasy= 0.0871x−0.250 (r= 0.99; 3 df).

4. The source of individual variation was investigated by comparing the responses of three ‘slow’ and three ‘fast’ responding ewes from Expt 1 to Cu given as a continuous intravenous infusion at rates of 0.05–0.3 mg/d. The relationship between plasma Cu response (y) and infusion rate (x, mg/d) after 17 d, wasy= 2.135x–0.156 (r= 0.86;P< 0 .001). The regression coefficients for ‘slow’ and ‘fast’ responding ewes were similar, as was their metabolism of intravenous64Cu, suggesting that the individual differences were due to differences in absorption rather than in the metabolism of absorbed Cu. Faecal endogenous Cu excretion was estimated to be 0.127 ± 0.019 (mean ± SE) mg/d.

5. The relative responses to oral and intravenous Cu were used to estimate the true availability of dietary Cu; in one experiment it was 4.1% and for individual ewes in another experiment, availabilities ranged from 4.5 to 11.4%. The figures are compared with assessments by conventional techniques.

6. It is concluded that the repletion technique provides a sensitive means of assessing the availability of Cu in ruminant diets.