Silflower Seed and Biomass Responses to Plant Density and Nitrogen Fertilization

Silflower (Silphium integrifolium Michx.) is a potential perennial oilseed crop that can provide ecosystem services; however, its seed, oil, and biomass yield potential are unknown. Our objectives were to determine the effects of planting density and N fertilizer on silflower seed, oil, and biomass yields and seed yield components for 3 yr after establishment. Experiments tested six planting densities on two soil types, and five N rates and two planting densities on one soil type. Averaged over years and soil types, increasing planting density from 11,954 to 53,792 plants ha⁻¹ increased biomass yield (BY), but decreased seed heads plant⁻¹ and seeds plant⁻¹. Planting density had no effect on seed yield (SY) or oil yield (OY), other seed related variables, or harvest index. On a sandy soil, SY and OY increased linearly with N rates from 0 to 180 kg ha⁻¹ in the year after seeding but were not affected by N fertilization rates in subsequent years. At 180 kg N ha⁻¹, SY, OY, and BY were 1,651 kg ha⁻¹, 381 kg ha⁻¹, and 10.5 Mg ha⁻¹, respectively, in the year following seeding but declined thereafter. Over both experiments SY and OY were positively correlated (r = .99) and together associated with seed heads plant⁻¹, seeds head⁻¹, and seeds plant⁻¹. Relative to annual oilseed crops, seed and oil yields for this new perennial species are low. Germplasm improvement for seed and oil yield and agronomic research to maintain yields as stands age are critical to develop this new perennial oilseed crop.

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