Harvest time in willow and poplar affects the yield, quality, nutrient removal and regrowth when harvesting green biomass for feed

Trees grown in outdoor animal farming systems may provide green biomass as a potential feed source, remove nutrients through uptake in biomass and, hence, reduce the risk of nutrient loss. We investigated the effects of harvest time during the growing season in three field experiments with willow and one with poplar, harvested either during the previous winter or one year earlier. Green biomass including shoots and leaves was either harvested 24th June or 24th September 2021. Regrowth in the subsequent growing season was measured in November 2022 in two of the willow experiments. Biomass yield and removal of N and P with harvested biomass was measured.

The nutrient concentration in the biomass decreased significantly from June to September whereas the yield of dry matter (DM), N and P increased. For example, for the willow clone Tordis harvested the previous winter, the N concentration in DM decreased from 2.86% to 1.27% (corresponding to 17.9 and 7,9% crude protein in DM), whereas the DM yield increased from 0.9 to 8.5 Mg ha⁻¹, and removal of nutrients increased from 32 to 114 kg N ha⁻¹ and from 3.8 to 16.5 kg P ha⁻¹. Regrowth of willow was negatively affected by harvest during the growing season; total DM yield over a full harvest rotation was reduced 46–48% and 34–40% when harvesting in June or September compared to no harvest during the growing season. Optimal harvest strategy in willow and poplar may differ whether the purpose is feed production, nutrient removal or maximum DM production.