The dynamics of selenium in dairy agroecosystems

Environmental selenium deficiency and toxicity can result in Keshan disease and selenosis, respectively. Due to dietary preferences, milk is considered a primary source of selenium, where, in controlled environments milk selenium concentration reflects the selenium provided in fodder to lactating cows. However, the movement of selenium through agroecosystems is not well understood. Therefore, the aim of this current study was to investigate variables that are responsible for transfer of selenium from soil to milk. Investigated parameters include spatial variability, soil selenium status, season, herd diet and husbandry of cattle. Farm-based sample collections were carried out, where soil, grass, silage and milk were collected over forty-eight geographically spaced locations over Northern Ireland during both summer and winter. Selenium concentrations were determined using ICP-MS. Median selenium concentrations for soil (0.46 mg/kg DM), grass (0.06 mg/kg DM) silage (0.03 mg/kg DM) and milk (0.23 mg/kg DM). Results showed that soil selenium concentrations were significantly affected by the pH and organic matter composition of soil. Additionally, a statistically significant relationships between soil and grass selenium concentrations were also found. Despite these strong environmental relationships with grass, these relationships were not reflected in the milk selenium concentrations, indicating a disconnect between the selenium concentration in the agricultural environment and the milk from which it is derived. Selenium was higher in milk in winter months as compared to summer, related to supplemental feed practices over the winter when cows are housed. This study highlighted that cattle feed supplementation of feed with selenium, an essential element, was an effective way of enhancing dietary exposure through a commonly consumed foodstuffs derived from dairy.