Broad versus narrow‐spectrum herbicides and the future of mixtures

The weed flora of cereals is subject to changes in husbandry methods such as cultivations and crop rotations. The herbicide chemist inevitably has to aim at a moving target. Herbicides themselves apply a selection pressure to the weed flora and when newly introduced often give exceptional results. Continued use allows the less‐susceptible species to become dominant and herbicide mixtures may prove necessary. The concept of mixtures is not new; the phenoxy‐acid herbicides were being formulated together in the 1960s. More recently bromoxynil and ioxynil have been added to assist in the control of more difficult species like mayweeds and parsley piert. Grass weeds, which have been encouraged by minimum‐cultivation techniques, can now be controlled, in addition to broad‐leaved weeds, by suitable mixtures. Completing the weed spectrum is the most obvious reason for mixing two compounds in a formulation or in the spray tank but synergistic interaction, combination of different modes of action, simple convenience and lengthening a product's commercial life may all be involved. Despite the very large choice of products for use in UK cereal crops there is not a single herbicide that could not be replaced by more effective compounds, safer to the crop, more flexible in application criteria, and with generally more reliable action. The chances of new compound discovery are decreasing and the fluctuations in the weed flora make it difficult to aim at specificity. The development of resistant biotypes within species seems to happen rather slowly and compounds with less specificity or in mixture with others having different sites of action would almost certainly help to avoid the problem.