Whole-blood selenium concentration and glutathione peroxidase activity across the grazing season in three beef herds

This exploratory field study characterized whole-blood selenium (Se) concentration and glutathione peroxidase (GPx) activity in three commercial beef herds in the Czech Republic. Blood was collected before turnout to pasture (D0; 20 April 2024), 90 days after baseline (D90), and 150 days after baseline (D150). Ten cows were sampled per herd and occasion; because different cows could be selected, the design was repeated cross-sectional. Each biomarker value was the mean of three technical determinations. Sampling-occasion differences were tested separately within herd by Welch’s ANOVA with Games–Howell comparisons. Se–GPx associations were estimated by occasion using herd-adjusted linear models with HC3 robust standard errors. Both biomarkers differed among occasions within every herd (all P < 0.001). Mean ± SD Se concentrations from D0 to D150 were 60.05 ± 7.62 to 148.40 ± 17.77 µg/L in herd A, 64.32 ± 6.12 to 159.51 ± 25.01 µg/L in herd B, and 24.65 ± 4.15 to 150.15 ± 20.82 µg/L in herd C; D90 and D150 did not differ in herd C. GPx activity was highest at D150 in herds A (1020.47 ± 126.27 µkat/L) and B (937.58 ± 116.93 µkat/L), but in herd C it was higher at D90 (943.65 ± 75.62 µkat/L) than D150 (703.87 ± 167.38 µkat/L). The herd-adjusted Se–GPx slope was not significant at D0 (β = 4.67, 95% CI −0.65 to 9.99; P = 0.082) but was positive at D90 (β = 7.79, 95% CI 5.55 to 10.03; P < 0.001) and D150 (β = 4.83, 95% CI 3.13 to 6.53; P < 0.001). The findings describe the monitored herds but do not establish breed effects, individual responses, or mineral-lick efficacy.