Weed Management

Jed Colquhoun

Persistence of herbicides used for grass control in wheat: will I be able to plant grass seed this fall?

Wheat production in the Willamette Valley has rebounded in acreage in the past 2 years in response to a more lucrative wheat price and low grass seed prices. Many growers, however, have expressed interest in planting perennial ryegrass after wheat this fall as the perennial ryegrass market stages a partial recovery. On the other side of the Cascades, many wheat production areas are suffering from a multi-year drought. With this trend in mind, it is worth revisiting herbicide persistence in soil. More specifically, some of the herbicides that are used for grass control in wheat have rotational restrictions that may limit the planting of other crops following wheat.

Three factors affect herbicide persistence (defined as the length of time a herbicide remains active in the soil): climate, soil factors, and herbicide characteristics.

Climatic conditions that include soil moisture, temperature, and sunlight affect herbicide breakdown. In general, herbicide persistence is shorter in warm, moist soils that favor microbial activity and chemical breakdown. In eastern areas of the Pacific Northwest, poor soil moisture during recent droughts has led to several herbicide carryover issues into sensitive crops. The role of sunlight in chemical breakdown is minimal for most herbicides.

Soil factors that affect herbicide persistence include the physical classification of the soil, soil pH, and microbial populations. Soil texture and organic matter play a role in herbicide persistence, with high organic matter, fine-textured soils leading to longer soil persistence than low organic matter, sandy soils. Soils high in clay content provide more binding sites for herbicides than sandy soils. Herbicide breakdown by chemical degradation or microbes is sensitive to soil pH. Microbial populations play an important role in herbicide degradation. Microbial activity is dependant on several factors, including soil pH, temperature, moisture, oxygen, and nutrient supply.

The physical characteristics of herbicides affect their soil persistence. Herbicide solubility affects leaching and the ability of herbicides to be moved through the soil profile to where they won’t affect subsequent crops. This dispersion of herbicides is dependent on soil moisture and is minimal in droughty conditions. Additionally, microbial degradation of herbicides is often dictated by the chemical structure of the herbicide.

Avoiding herbicide persistence in rotational crops requires several steps:

1. Most importantly, check the rotational restrictions on the herbicide label prior to herbicide application while keeping in mind the short-term cropping plans for the land. The rotational restriction for selected grass herbicides that are used in wheat are listed in Table 1. Some broadleaf herbicides, such as Harmony, Ally, Peak, Glean, and Express, are soil-persistent and have rotational restrictions. Please consult labels for further information.
2. Avoid use of persistent herbicides late in the cropping season. Dry summers in the Pacific Northwest can prevent adequate microbial degradation from late-season applications.
3. Avoid use of two or more persistent herbicides from the same mode of action in a cropping season. The persistence of herbicides is often additive when involving the same mode of action.
4. Tillage can be used to disperse herbicide concentrations near the soil surface and to encourage microbial breakdown, but are not a replacement for the rotational restriction on the label.
5. If in doubt, consider planting a test strip in the questionable field, or conduct a bioassay with the next crop using soil from the field prior to planting.

Table 1. Crop rotation restrictions for selected herbicides used for grass control in wheat. Rotational crops listed include those commonly grown in Oregon and are not inclusive of all those listed on the herbicide label. As always, consult the label for directions prior to use of any herbicide.

^a Rotational restrictions vary by label. Please consult Karmex or Direx label prior to use.
^b Finesse should only be used on soils with pH between 5.0 and 7.9.
^c bio = bioassay. Field bioassay must be conducted before planting next crop.
^d 16 months if pH > 6.6.
^e fs = following season.
^f Rotational restrictions valid when pH < 7.5 and precipitation > 18 inches. Conduct field bioassay if pH or precipitation is outside of this range.
^g For all 3 month rotational restrictions, conduct field bioassay before planting next crop.
^h 22 months if soil pH > 6.5 and precipitation < 24 inches.

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