Diagnosis of the Cu and Se status of dairy cattle in New Zealand: How many samples are needed?

AIM: To determine the minimum number of samples required to obtain a robust estimate of the Cu and Se status of dairy herds, as assessed by determining liver Cu and serum Se concentrations. METHODS: Results were collected from analyses of samples of liver from 18 dairy herds and serum from 19 herds, for concentrations of Cu and Se, respectively. All herds were in either the Manawatu or Rangitikei regions of the North Island of New Zealand. Data were used to determine the required sample size for each herd; firstly to estimate the population mean with 90% confidence with a precision of 27.5 nmol/L for Se in serum, and 100 μmol/kg fresh weight (FW) for Cu in liver; and secondly to ensure that the 90% CI of the sample mean did not include specified thresholds for concentrations of Se or Cu. RESULTS: For Se concentration in serum, the SD of each batch varied from 0.5-147 nmol/L, and for Cu concentration in liver, the SD varied from 173-829 μmol/kg FW. For Se, the minimum sample size required to estimate the population mean to within 27.5 nmol/L with 90% confidence was >10 for 13/19 batches. For Cu, the minimum sample size required to estimate the population mean to within 100 μmol/kg FW was >10 for 17/18 batches. When estimating required sample size based on 90% CI and a threshold value, the minimum sample size to confirm the population mean of Se was >140 nmol/L was four in 17/18 batches where the sample mean was>140 nmol/L. For concentrations of Cu in liver, ≤8 samples would have been sufficient for a threshold of 45 μmol/kg FW in 16/18 batches. For the 95 μmol/kg threshold, the minimum required was 12. For the threshold of 300 μmol/kg FW, 6/17 batches with a mean >300 μmol/kg FW required≤20 samples. CONCLUSIONS: From this dataset of 21 herds, the sample size recommendation for ensuring that the population mean of Se concentration was not below the marginal threshold was similar to previous recommendations. For Cu concentrations in liver, the estimated sample size recommendations for ensuring that the population mean was not below the marginal threshold was much larger than currently recommended. CLINICAL RELEVANCE: In dairy cattle, five to six blood samples per group should be taken to determine Se status, and to effectively monitor Cu status a minimum of 12 liver samples should be taken, preferably in the autumn. © 2013 New Zealand Veterinary Association.