Effectiveness of dock (Rumex obtusifolius L.) control in new leys and established grassland following the application of herbicides

Docks are a widespread problem in intensively managed grassland, requiring repeated pesticide applications for control purposes. New European Union pesticide regulations aim to restrict pesticide use. This necessitates improved efficiency of pesticide use including the most effective time to apply them during the target weed's lifecycle. The experimental site (52 degrees 21 N, 7 degrees 18 W) was reseeded with perennial ryegrass in October 2009. A plot (5 m x 10 m) experiment was laid down in a randomized complete block design with nine treatments and eight replicates. Four herbicides licensed for use in new leys (NLH) were each applied in April 2010. Four herbicides licensed for use in established grassland (EGH) were each applied 2 years later in April 2012. The ninth treatment was an untreated control. Dock numbers were measured over 5 years (2010-2014) and herbage production over 3 years (2012-2014). Across all treatments, dock numbers were relatively low until the fourth year during which there was more than a three-fold increase due to clonal propagation. NLH gave more (p < .001) effective and enduring control than EGH. EGH varied in their effectiveness with (p < .001) recovery of individually marked docks that were 'apparently dead' following EGH application. In 2014 dock herbage dry-matter (DM) production (t ha(-1)) was 3.41 in the control compared with 0.55 for NLH and 1.38 for EGH. Across all treatments in 2014 grass DM production declined with increasing dock herbage DM production (t ha(-1)): grass = 11.17 - 1.047 x dock (R-2 = .73; p < .001). More eco-efficient control of docks was achieved by herbicide application to new leys.