Effects of springtime sodium selenate foliar application and NPKS fertilization on selenium concentrations and selenium species in forages across Oregon

Selenium (Se) concentrations in soils and plants are low in the Pacific Northwest, and consequently, in livestock consuming locally-grown forages. Applying sodium selenate with traditional nitrogen-phosphorus-potassium-sulfur (NPKS) fertilizers provides a cost-effective Se agronomic biofortification method for increasing forage Se concentrations and maintaining optimum health and productivity of livestock. This 2-year study was conducted across Oregon on four representative forage fields: orchardgrass (Dactylis glomerata L.) in Terrebonne (central Oregon), grass-clover mixture in Roseburg (southwestern Oregon), and both grass mixture and alfalfa (Medicago sativa L.) fields in Union (eastern Oregon). Using a split plot design with double-repeated measures (2017 and 2018 with first, second, third, and residual harvest cuts each year), we determined the effect of springtime Se foliar application rate (0, 45, or 90 g Se ha⁻¹ from sodium selenate; whole plot) and NPKS fertilization (none, NPK/PK fertilization, or NPKS/PKS fertilization; subplot; NPKS amounts adapted to meet forage and soil requirements) on total Se concentration and beneficial Se species in forages. Forage Se concentrations increased linearly with foliar sodium selenate application, doubling with each additional 45 g Se ha⁻¹ application (first cut median: 0.11, 2.06, and 4.15 mg kg⁻¹ dry matter, respectively). Nearly all incorporated Se was contained in the first two cuts (87 % and 9 % in first and second cuts, respectively) and most applied selenate was metabolized to selenomethionine. NPK/PK fertilization decreased Se concentrations in second and latter cut forages from Se-amended fields, whereas sulfur fertilization decreased forage Se concentrations from non-Se amended fields. Among forages, alfalfa had greater Se concentrations under low soil Se conditions, whereas grass-dominated forages had higher Se concentrations after Se-amendment. In conclusion, springtime sodium selenate foliar application is an effective management strategy to increase forage total Se concentrations and highly bioavailable selenomethionine concentrations across Oregon, and should be implemented to meet local forage and livestock requirements.