Acta Veterinaria Scandinavica 2007, 49:1 http://www.actavetscand.com/content/49/1/1 ence material. During the project period, the measured concentration of the reference material was 0.19 ± 0.01 µg/g (N = 17). For practical reasons, the National Veteri- nary Institute calculates plasma and blood Se concentra- tions in µg/g. 1 µg/g blood corresponds to 1.052 µg/ml, and 1 µg/g plasma to 1.026 µg/ml [15]. The detection limit was 0.01 µg/g. Statistical calculations All correlations were evaluated with Pearson correlation analysis. To evaluate the temporal changes in plasma vit E and Se concentrations in the weanling pigs statistically, a regression model analysis was used, with pig number as a cluster variable to adjust for repeated measurements (Stata for Windows, SE 8.2, Stata Corp. College Station, TX; regress procedure). The correlation structure between measurements and the degree of variance at individual and litter level was tested using the xtgee procedure in Stata, which allows for incorporating the correlation struc- ture in a repeated measurement model. Model assessment was done by comparing estimates from the regression model and the xtgee procedure, and by examining predic- tions and residuals by x-y plots and normal quantile plots. In addition, the statistical differences between the average Vit E and Se concentrations at day -1 and day 4 were ana- lysed for each litter, with student's t-test (paired analysis). The relative contributions of farm and litter to the total variance of plasma vit E, plasma Se concentration and body weight at day -1 were evaluated with variance com- ponent analysis, using SAS statistical software (ver. 8, SAS institute inc.). The same method was used to evaluate the relative contributions of farm and litter to the variance in the size of plasma vit E and Se changes from day -1 to day 4. Because herd 1 differed significantly from all the other herds in age and bodyweight at weaning, both the analysis of correlations with age at weaning and the variance com- ponent analysis were done twice; with and without herd 1 included. Results Mean values for plasma vit E, plasma Se and body weight in all sows and weanling pigs for each sampling day are shown in Table 1, with standard deviations and ranges. In Fig. 1, the relation between plasma vit E and Se concen- trations in each sow and the average level in her litter at day -1 is illustrated. No significant correlation was found between plasma concentrations in the sows and the aver- age plasma concentrations in their litters at day -1, neither for vit E nor for Se. In the entire data set, plasma vit E in the piglets at day -1 was negatively correlated to age at weaning (r = -0.40), while both plasma Se (r = 0.38) and bodyweight (r = 0.79) were positively correlated to wean- ing age. When herd 1 was excluded from the analysis, only the correlation between bodyweight at day -1 and wean- ing age remained statistically significant (r = 0.35, p < 0.01). Mean plasma vit E concentrations in the weanling pigs fell substantially from day -1 to day 4, and stayed low for the rest of the period. Plasma Se concentrations increased from day -1 to day 4, and continued rising slightly till the end of the study (Table 1). In the regression model analy- sis for the temporal changes in plasma vit E and Se, no specific correlation pattern between repeated measure- ments was found, and thus a simple cluster adjustment for the repeated measurement was used, by implementing the robust estimator in the regression model procedure. The final model for both vit E and Se included day post wean- ing and herd as explanatory variables, while litter, individ- ual age and weight were dropped. Regression diagnostics did not reveal any serious deviations, model fit was acceptable, and estimates from the regression model and the xtgee analysis showed only marginal differences. Clear and significant differences between day -1 and each of the days after weaning were shown for both plasma Se and plasma vit E. However, the 95% confidence intervals indi- cated that the model predictions for plasma vit E at day 4, day 8 and day 18 were not significantly different, while the differences between these three days for plasma Se were clearly significant. Table 1: Plasma vit E and Se in sows and weanling pigs – summary table Sampling day: Vit E (µg/ml) Se (µg/g) Body weight (kg) Mean; std dev Range Mean; std dev Range Mean; std dev Range Sows (n = 18) Day -1 4.0 (1.0) 1.8–5.6 0.22 (0.02) 0.18–0.26 Weanlings (n = 72) Day -1 Day 4 2.6 (0.8) 0.7–4.7 0.08 (0.01) 0.05–0.11 11.9 (4.0) 4.9–22.3 1.6 (0.6) 0.4–4.1 0.10 (0.01) 0.08–0.13 12.8 (4.3) 3.7–24.0 Day 8 1.5 (0.5) 0.4–2.6 0.11 (0.01) 0.07–0.13 14.2 (4.7) 5.5–27.1 Day 18 1.7 (0.7) 0.7–3.6 0.12 (0.01) 0.09–0.15 19.2 (6.1) 8.6–35.9 Plasma vit E (α-tocopherol) and plasma Se in sows and weanling pigs, and bodyweight in weanling pigs, in six Norwegian herds in relation to weaning. Means, standard deviations (in brackets), and ranges. Day 0 is the day of weaning. Page 3 of 9 (page number not for citation purposes)