Quackgrass (<i>Elytrigia repens</i>) management in flax (<i>Linum usitatissimum</i>)

Field studies were conducted at the Agriculture and Agri-Food Canada, Research Centre, Morden, Manitoba from 1992 to 1996 to evaluate quackgrass management in flax with several graminicides applied at annual and perennial grass rates, and with preharvest glyphosate applied after postemergence graminicides. All graminicides at both application rates reduced quackgrass biomass. However, at annual grass rates, clethodim and sethoxydim were sometimes less effective at controlling quackgrass than quizalofop and fluazifop-P. There was some evidence that annual-rate applications of sethoxydim can result in increased quackgrass regrowth in the second year. Flax seed yields were not affected by the use of graminicides at annual grass rates compared with perennial grass rates, whereas untreated (weedy checks) flax had yield losses of up to 67% compared with weed-free plots. The use of preharvest glyphosate reduced second-year regrowth of quackgrass by half or more, compared with no preharvest glyphosate treatment. Where preharvest glyphosate had been applied, the weedy checks tended to have markedly less quackgrass regrowth than where graminicide treatments had been applied in the spring. This suggests that use of spring-applied graminicides may reduce the effectiveness of preharvest glyphosate to control quackgrass. However, use of a postemergence graminicide is often necessary to control annual grasses as well as quackgrass because flax yield is seriously affected by uncontrolled weed growth. A second experiment was conducted to compare the efficacy of liquid and dry formulations of glyphosate applied with water and 28-0-0 fertilizer as carriers. The three glyphosate formulations applied at 0.9 kg a.e. ha −1 were equally effective at reducing quackgrass biomass in flax without yield loss, but at 0.45 kg a.e. ha −1 , the liquid formulation was sometimes less effective. There was no consistent evidence that the use of liquid urea/ammonium nitrate fertilizer as a carrier, rather than water, affects the performance of either liquid or dry glyphosate formulations. Key words: Quackgrass, flax, graminicides, glyphosate, preharvest application

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