Sessions

In pasture-based beef cattle production, a fundamental question in assessing selenium status is how much selenium available in the soil reaches the animals through vegetation. Inadequate intake of this essential trace element, which plays a crucial role in antioxidant defence, immune function and reproduction, may adversely affect herd health and productivity even in the absence of clinical signs.

The aim of our study was to characterize seasonal changes in selenium status and identify soil and vegetation factors associated with plant selenium concentration in an organically managed beef cattle herd. In autumn, winter and spring, blood and hair samples were collected from 50 cows per season, while soil, plant and total mixed ration (TMR) samples were also analysed. Selenium concentrations were determined using GFAAS and ICP-MS methods, and the data were analysed using linear mixed models and Spearman’s rank correlation.

All three animal biomarkers showed significant seasonal variation (p<0.001). Median plasma selenium concentrations were 31.9, 81.6 and 46.4 µg/L in autumn, winter and spring, respectively. Whole-blood selenium concentrations were 66.3 µg/L in autumn, 97.3 µg/L in winter and remained at a similar level in spring (99.7 µg/L). Hair selenium concentrations increased from 0.321 to 0.342 and then to 0.449 mg/kg over the same periods. Plasma and whole-blood selenium concentrations were positively correlated, whereas hair selenium concentration showed no consistent association with the blood-based biomarkers. Despite the winter improvement, more than half of the animals remained below the 100 µg/L whole-blood selenium threshold used to indicate inadequate selenium status in both winter and spring.

No significant association was found between total soil and pasture grass selenium concentrations; the bioaccumulation factor of approximately 0.05 indicated limited soil-to-plant selenium transfer. Plant selenium concentration, however, was associated with certain soil properties and vegetation composition.

Our findings identified soil-to-plant transfer as a potential bottleneck in selenium supply; soil selenium availability did not ensure adequate selenium concentrations in the vegetation.. Although blood selenium concentrations improved during the winter feeding period, this did not indicate resolution of inadequate selenium status at the herd level. The different spring patterns observed for plasma and whole-blood selenium suggest that recent changes in selenium supply and longer-term selenium status may be reflected differently by the two matrices. Therefore, their combined assessment, timed according to the feeding period, may support the detection of persistent inadequate selenium supply and the appropriate timing of targeted selenium supplementation. Thus, this study linked the assessment of selenium deficiency with the investigation of soil-to-plant transfer and the temporal windows represented by different biomarkers.