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Keywords:

  • 2,4-D;
  • corncockle;
  • competition;
  • dose–response;
  • ED50;
  • pesticide

Summary

  • 1
    The estimation of dose–response relationships is an integral and legally necessary part of the routine regulatory process for herbicides. For each herbicide, plants of both target and non-target species are exposed to different levels of the chemical, and effects on mortality and performance expressed as LD50 and ED50 values (respectively, the dose at which 50% of plants die and the dose at which plants show a 50% response to herbicide). Thereafter, LD50 and ED50 values may provide comparative information between herbicides and species. There is little published on the effects of plant density on dose–response relationships.
  • 2
    We used the herbicide 2,4-D amine and Agrostemma githago as a model system to investigate the effects of plant density on the dose–response relationship (ED50) between 2,4-D and A. githago biomass measured as fresh weight.
  • 3
    Plants were grown in a controlled environment room at both low and high densities (two and 64 plants per pot) for 2 weeks, sprayed with 2,4-D using a precision sprayer, and then harvested after a further 2 weeks. The ED50 values were significantly greater when the plants were grown at high density (616 g active ingredient ha−1) than those grown at low density (42 g active ingredient ha−1); a 15-fold difference.
  • 4
    Mathematically, it was shown that a simple multiplicative relationship exists between ED50 and dose received when plants are grown at different densities, all other conditions being equal.
  • 5
    To explore the underlying effects of competitive processes arising from different densities, we experimentally investigated three phases where competition could be influencing the response of plants: (i) before spraying, (ii) at the time of spraying and (iii) after spraying. We used a sequential series of experiments to determine which phase was contributing most to the observed difference between ED50 values obtained from plants grown at low and high density.
  • 6
    It was shown that the competition between plants after spraying was the most likely phase to have contributed to the observed difference in ED50 values between the two densities.
  • 7
    These results demonstrate that trials used in the pesticide regulatory process ought to test not only different doses of pesticide but also different densities of plants (both crop and weed).