Causes of uneven desiccation of flax by glyphosate: poor penetration of the canopy and variable spray deposition

Flax (Linum usitatissimum) cv. Hera was grown at densities of c. 2000, 1500 and 1000 plants m^-2sprayed with glyphosate (1·44 kg a.e. ha^-1) and red dye (1·5%). Dye deposition was measured and the mean total was 402 , 503 and 602 μg per plant or 691, 761 and 761 μg g^-1DW for high, intermediate and low densities respectively. Variation between plants was large and significant differences were found only in deposition on the lower stem, this being about two-fold higher for plants grown at 1000 m^-2than 2000 m^-2. However spray penetration through the crop canopy was poor and only c. 4% of the total deposit was intercepted by the lower stem. There was no detectable dye deposition on the lower stem of some plants in stands of all densities. The minimum deposits of glyphosate required for desiccation were assessed by applying precise doses, using a microsyringe, to plants grown under controlled environmental conditions. A total dose of 40 μg applied to the capsules, the upper stem plus leaves and the lower stem in the approximate ratio found for dye deposition on field grown plants, was required for successful desiccation. The minimum amount of glyphosate deposited in the field experiment, calculated assuming a linear relationship between dye and glyphosate deposition, was only 5 μg per plant. Position of application was also important. Application of 6 μg glyphosate to the lower stem caused desiccation of the whole plant whereas in applications to the upper stem and leaves 64 μg was required to have a similar effect. These experiments indicate that uneven desiccation of flax crops can be accounted for by both lack of uniformity of spray deposition and poor penetration of spray through the canopy to the lower stem, where glyphosate would be most active.