Winter linseed weed control: strategies for grass and broad-leaved weed control

Abstract

The aim of the study, through four related field studies, was to investigate the efficacy and crop safety of herbicide strategies for winter linseed. The study used a range of sites (from North Yorkshire to Hampshire), soil types (clay loams to heavy clay soils), weed burdens and weed species, to study the effects on efficacy of weed control and linseed yields. the aims of the four studies were to evaluate a) the effect of grass-weed competition and timing of weed removal on winter linseed yields, b) the effect of pre-emergence herbicide on sensitivity of grass-weeds to post-emergence graminicide application, c) the effect of pre-emergence herbicides on sensitivity of broad-leaved weeds to post-emergence herbicide application and d) the effect of incorporation of Treflan (480 g/l trifluralin) on weed control and overwinter survival of winter linseed.

The critical period for grass-weed removal was found to be linked to the balance between crop and weed biomass. Yield potential was reduced, compared to herbicide-treated crops, where grass-weed biomass was allowed to develop to a level which equalled that of the linseed crop before removal. In practical terms this meant that in most cases, removal of grass-weeds could be delayed until March without compromising the yield potential of the crop. Treflan did provide a useful reduction in autumn grass-weed populations. Where effects were significant, grass weed populations were reduced by 26-27%. However, effects did not persist until spring. Relatively high doses of Laser (200 g/l cycloxydim) were required to control competitive brome (0.75 to 1.0 l/ha) and volunteer wheat (0.75+ l/ha) where herbicide application was delayed until tillering had started. There were few significant yield responses to grass-weed control, except at very high grass-weed populations (>75 black-grass plants/m²). The only occasion where Treflan appeared to complement Laser application was in suppression of black-grass heads.

In around 60% of cases, Treflan reduced the total broad-leaved weed flora present prior to spring herbicide application, with the greatest effect where sensitive weed species dominated the weed flora. Where herbicides had a good broad-spectrum of weed control, there were interactions between autumn and spring-applied products to give additional benefits to use individually. A yield response of only 0.2 t/ha was required to cover the cost of application of a full dose of Ally (20% w/w metsulfuron-methyl), and 0.3 t/ha the costs of a sequence of Treflan and Ally at full label dose rates. In contrast, Basagran (480 g/l bentazone) required a yield response of at least 0.5 t/ha to cover costs and it was difficult to achieve such yield responses in winter linseed crops. Applications of Ally in spring caused viable stunting and damage to winter linseed crops on a number of occasions. However effects were quickly out-grown by the start of flowering and yields were unaffected.

Where Treflan was incorporated into soil (the traditional method of application), winter linseed plant populations were always reduced and significant reductions were recorded three out of six site years. Where Treflan was applied as a surface treatment, plant populations were similar to, or higher than, those in untreated plots. However, weed control by Treflan was not significantly compromised by post-drilling application.