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.
Herbicide use
- Ethalfluralin (Sonalan) for canola, chickpea, dry bean, field pea, lentil, potato, soybean and sunflower.
- Pendimethalin (Prowl/Prowl H2O) for chickpea, corn (pre-emergence only), dry bean, field pea, lentil, pastures, potato, rangeland, soybean, sunflower and wheat.
- Trifluralin (Treflan) for alfalfa, canola, chickpea, dry bean, field pea, flax, lentil, tame mustard, potato, soybean, sunflower and wheat.
Injury symptoms
Dinitroaniline residual in soil may cause sugarbeet to be severely stunted (Photo 29), with small leaves that are more erect than normal (Photo 30). The roots of damaged sugarbeet seedlings may turn brown and die, starting at the point where the root joins the hypocotyl, about 1 to 1.5 inches below the soil surface (Photo 31).
Plants with dead roots may die or they may survive by producing secondary roots from the hypocotyl. Identical symptoms on roots of seedling sugarbeet also can be caused by these herbicides:
- Imidazolinones
- Sulfonylureas
- Triazolopyrimidines
- N-Phenylphtalimides
They can also be caused by a fungal disease, Aphanomyces.
Affected plants will be smaller than unaffected plants. Plant regrowth by secondary root production would be prevented by drought in the surface two inches of soil and injured plants would die.
Imidazolinone, sulfonylurea, sulfonylamino carbonyltriazolinone, triazolopyrimidine and N-phenylphthalimide herbicides also can cause similar seedling root death.
Site of action
Tubulin protein involved in cell division.
Herbicide use
- Acetochlor (Harness, Warrant, Surpass) for corn, soybean and sugarbeet.
- S-metolachlor (Dual Magnum) for chickpea, corn, dry bean, field pea, lentil, potato, soybean and sugarbeet and sunflower.
- Dimethenamid-P (Outlook) for corn, dry bean, lentil, potato, soybean and sugarbeet.
Injury symptoms
Chloroacetamide herbicides cause no distinctive symptoms on sugarbeet. But over-application or extended periods of cool, wet weather shortly after planting may sometimes cause damage on sugarbeet.
Sugarbeet stand loss may occur because some plants may die before emergence. Emerged but injured sugarbeet may also be chlorotic in appearance and /or are stunted.
In photo 32, a combination of environment and repeat herbicide applications may have contributed to chlorotic, stunted and uneven growth, compared with the untreated area to the right. Sugarbeet along the row are not uniform; some are stunted and chlorotic, and other plants are normal in appearance (Photo 33).
Site of action
Specific sites or sites believed to inhibit very long chain fatty acid synthesis.
Herbicide use
Pyroxasulfone (Zidua) for corn, soybean, sunflower and wheat.
Injury symptoms
Same as chloroacetamides; see previous section (Photo 34).
Site of action
Specific sites or sites believed to inhibit very long chain fatty acid synthesis.
Herbicide use
- EPTC (Eptam) for alfalfa, dry bean, potato, sugarbeet and sunflower.
- Triallate (Far-Go) for barley, chick pea, field pea, lentil, sugarbeet, and wheat.
- Cycloate (Ro-Neet SB) for sugarbeet.
Injury symptoms
Thiocarbamates reduce the formation of epicuticular wax on leaves, which can cause leaves to fuse together rather than unfold normally (Photo 35). Affected plants may be stunted, leaves may be shortened and thickened, or true leaf development may be inhibited (Photo 36).
Some severely stunted plants may die, while others will start producing new leaves and will produce a nearly normal-sized root at harvest. Severely stunted plants may grow very little for two or more weeks after emergence and then make a complete or nearly complete recovery.
Site of action
Specific site or sites unknown; believed to have multiple sites of action.
Herbicide use
Ethofumesate (Nortron, Ethotron, Ethofumesate SC) for sugarbeet.
Injury symptoms
Soil residual injury symptoms
Benzofurans, like thiocarbamates, reduce the formation of epicuticular wax on leaves, which can cause leaves, especially leaves of cotyledon to two-leaf sugarbeet, to fuse together rather than unfold normally (Photo 37). This phenotype is most common in low-organic matter or course-textured soils (Photo 38). Affected plants may be stunted.
Postemergence injury symptoms
Ethofumesate soil applied decreases epicuticular wax and may increase injury from postemergence herbicide such as Betamix or from chloroacetamide herbicides by increasing foliar penetration.
Site of action
Specific site or sites not well understood; believed to inhibit biosynthesis of very long chain fatty acids.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Be sure that the area you wish to treat is listed on the label of the pesticide you intend to use. Remember, the label is the law.
Reviewed in 2018