Harvest interval and row spacing of SRC willow influence yield and nutrient content

Abstract This study investigated effects of planting density and harvest interval on SRC willow over six growth years. In a double row system with 0.74 m plant distance and 0.75 m row spacing within double rows, we compared normal (1.5 m) and large (2.62 m) double row spacing, corresponding to 1.2 and 0.8 plants m −2 . Over two three-year harvest rotations with two cultivars, large row spacing reduced mean annual dry matter (DM) yield significantly from 7.0 to 5.3 Mg ha −1  y −1 (23%). Row spacing did not interact with cultivar or harvest rotation. Harvest intervals of one, two or three years were compared for one cultivar on two sites over six years. DM yield generally increased from first to later harvest rotations. Mean annual DM yield increased significantly with increased harvest interval, with 4.9, 6.6 and 8.0 Mg ha −1  y −1 for one-year, two-year and three-year intervals, respectively. This corresponds to a yield reduction of 39 and 17% for annual and biennial harvest, respectively, compared to triennial harvest. The yield for annual harvest may have been affected by weed competition but also browsing. Across shoot ages from one to three years, the concentration of N, P and K in DM decreased non-linearly with a reduction of 53, 58 and 59%, respectively, when DM yield increased from 1.5 to 40.6 Mg ha −1 . Across data from this and 7 published studies, N concentration reached a constant level above 12 Mg ha −1 DM yield. This has implications for optimization of biomass quality and quantification of nutrient removal.