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Seedling growth inhibitor herbicides

IV. Seedling growth inhibitors (SOA 3, SOA 8, SOA 15)

The seedling growth inhibitors include the following herbicide families:

  • Dinitroaniline
  • Chloroacetamide
  • Isoxazoline
  • Thiocarbamate
  • Benzofuran

These herbicides have little or no foliar activity and are applied preplant incorporated or preemergence to control weeds effectively. In some uses, these herbicides are applied after crop emergence but before weed emergence (lay-by).

In general, seedling growth inhibitors are most effective on small-seeded broadleaf and grass weeds. Large-seeded broadleaf and grass weeds generally survive normal dosages, because their root and shoot rapidly grow through the herbicide treated zone in soil.

Seedling growth inhibitors interfere with new plant growth, thereby reducing the seedlings' ability to develop normally in the soil. Plants can take up these herbicides after germinating until the seedling emerges from the soil. Therefore, these herbicides are effective only on seedling annual or perennial weeds.

Plants that have emerged from the soil uninjured are likely to remain unaffected.

Seedling growth inhibitors are active at two main sites: the developing shoot and the root. Much more is known about the action of seedling root-inhibitor herbicides than seedling shoot-inhibitor herbicides.

Root inhibitors

The root inhibitors are mitotic poisons that interrupt cell division, which inhibits shoot elongation and lateral root formation. Uptake is through developing roots and shoots of emerging seedlings but are not readily translocated. Thus, herbicide injury is confined primarily to plant roots and shoots. Selectivity may be based on metabolism, herbicide placement and type of emergence of grass species.

The emerging shoot is the primary site of absorption on grass species. Injury symptoms on grass species include short, swollen coleoptiles. Injured broadleaf plants often have swollen hypocotyls. Both grasses and broadleaves may have short stubby secondary roots. Thus, affected plants may be stunted and exhibit nutrient deficiency or drought symptoms because of poorly developed root systems.

Shoot inhibitors

Shoot-inhibiting herbicides are absorbed from the soil solution or vapor phase through roots and emerging shoots but are translocated only in the xylem. The primary site of absorption and action is the emerging shoot and growing point with thiocarbamates and roots (broadleaf species) and emerging shoots (grass species) with chloroacetamides. Present evidence suggests that these herbicides can affect multiple sites within a plant, primarily interfering with fatty acid and lipid biosynthesis.

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Reviewed in 2018

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