Foliar Selenium Biofortification of Multispecies Grass–Legume Mixtures: Effects of Application Dose, Sward Height, and Seed Mixture

Selenium biofortification of permanent grasslands represents a promising strategy for enhancing the nutritional quality of forage for livestock. This study evaluated the effectiveness of foliar selenium biofortification in multispecies grass–legume mixtures as affected by application rate, timing, and seed mixture. The study was conducted over three consecutive growing seasons (2023–2025), with a total of 648 aboveground herbage samples collected for analysis. Three multispecies mixtures were treated with four rates of a Se-containing formulation (0.0, 0.5, 1.0, and 1.5 L ha−1) applied at two different sward heights (10 and 15 cm). Selenium concentration was determined in the plant dry matter, and treatment effects were assessed using non-parametric statistical analyses. The application rate was the only factor that significantly affected Se concentration in the plant material. Se concentration increased progressively as the application rate increased. Mean Se concentrations were 0.035 mg kg−1 at 0.0 L ha−1, 0.077 mg kg−1 at 0.5 L ha−1, 0.086 mg kg−1 at 1.0 L ha−1, and 0.115 mg kg−1 at 1.5 L ha−1, corresponding to increases of 120.0%, 145.7%, and 228.6%, respectively, relative to the control. In contrast, neither the plant growth stage at application nor the species composition of the mixtures significantly affected Se accumulation (p > 0.05). The results demonstrate that foliar application of Se is an effective strategy for increasing Se concentration in multispecies grass–legume mixtures. Among the factors examined, the application rate showed the most pronounced effect on the effectiveness of Se biofortification, whereas no statistically significant effects were detected for application timing or seed mixture under the conditions of the present study. However, the lack of a statistically significant effect of seed mixture should not be interpreted as evidence that mixture composition is irrelevant to Se biofortification. Differences in species composition may influence Se uptake, translocation, assimilation, and accumulation through species-specific physiological responses and interactions among species within the sward. These aspects warrant further investigation, particularly through a detailed assessment of the actual species composition of the sward during the experimental period. Overall, the findings indicate that foliar Se biofortification may represent a flexible and practical approach for producing Se-enriched forage under diverse cultivation conditions.