Improving the efficacy of herbicides for the control of Rumex obtusifolius L. in Irish grassland using aspects of Integrated Weed Management

Abstract

Docks are a troublesome weed of conventional grassland and can reduce grass dry matter (DM) production by up to 50%. Conventional farmers largely rely on herbicides to manage docks but often report disappointing results. With imminent EU legislation likely to restrict future herbicide use, there is an impetus to develop new dock management strategies. Three dock management experiments were conducted on a conventionally managed grassland farm (52°21N, 7°18W). Experiment I compared herbicides licensed for use in new leys (NLH) with herbicides licensed for use in established grassland (EGH). Experiment II examined the role of soil test Potassium (K) concentrations (Morgan’s solution; Na acetate + acetic acid, pH 4.8; STK) on competitiveness of docks in grassland. Experiment III combined data from the two field studies to develop control thresholds of seedling docks in new leys. Dock numbers were measured over seven years (2010–2015) and herbage production over five years (2012–2016). There was a 3.4-fold increase (due to clonal propagation) and strong correlation (R2=0.9; P<0.001) between dock m2 in 2013 and 2015. Each year (2013 to 2016), there were strong inverse relationships (r >-0.776; P<0.001) between dock and grass herbage production. NLH gave more (P<0.001) effective and enduring control than EGH, which varied in their effectiveness and showed recovery (P<0.001). STK influenced juvenile dock numbers (P<0.05). Lower STK values reduced (P<0.01) dock root and herbage production with an associated increase (P<0.05) in grass herbage production. Thus, STK levels can be manipulated as an alternative dock management strategy. Dock m-2 in 2010 accounted for 36% of the variation in the economic value of herbage produced from plots between 2012 and 2016. On this basis and considering the cost of NLH application, we recommend NLH’s should be applied to avoid population densities >1.0 docks m-2 in new leys.