Methods of assessing and describing the joint action of materials as herbicides are reviewed, and the shortcomings of the terms synergism and antagonism are discussed. Some of the lack of agreement on their meaning and evaluation is attributed to failure to define or recognize appropriate models to represent absence of synergism (and antagonism). General analytical and graphical procedures are presented, and for two simple models, the additive-dose model (ADM) and the multiplicative-survival model (MSM), the dose-response curves and isobols are compared. Three examples from recent literature are reworked.

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An oil soluble formulation of the dodecyl (51% w/w) and the tetradecyl (13% w/w) amine salts of (2,4-dichlorophenoxy)acetic acid (2,4-D) was diluted with diesel oil and applied in homogeneous sprays of 100, 200, and 400-μ. droplets to seedlings of sunflower (Helianthus annuus L. ‘Peredovik’). At each droplet size all combinations of five dosage levels and five spray volumes were compared. For the same applied dosage the amount of stem curvature increased as droplet-size was decreased. When compared with 100-μ droplets, approximately three times and six times as much active ingredient was needed to produce the same effect with 200-μ and 400-μ droplets, respectively. For a fixed droplet-size, the effective dosage could be increased equally well by increasing the concentration or by increasing the number of droplets per unit area.

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