Herbicide Carryover and Crop Rotation in Western Canada

Herbicide Application and Carryover Potential

Herbicides are applied for weed control in most agronomic crops. Understanding if they persist, and if so, the length of time they remain active in the soil, is important to preventing potential injury to subsequent crops.

Factors that influence potential herbicide carryover include:

• Soil related physiochemical properties of herbicide
• Rate, timing, and frequency of herbicide application
• Environmental conditions after application – particularly precipitation amount and timing
• Soil properties such as texture, pH, and Organic Matter
• Rotational crop sensitivity to herbicide.

Herbicide Physiochemical properties

The herbicide chemistry along with the rate of application impacts the potential for biologically active herbicide residues in the soil. Herbicides vary in their potential persistence and carryover and herbicides that remain active in the soil for long periods of time pose the greatest threat to rotational crops.¹ It is important to follow the label for the re-cropping restrictions following herbicide application as some herbicides may require a year or more before growing sensitive crops.

Rate, Timing, and Frequency of Application

The rate of herbicide application, seasonal timing, and repeated use of residual herbicides impact carryover. Misapplication, either by applying more than the labelled rate or overlapping spray patterns (such as in turn-around areas), increases the potential for herbicide carryover. Likewise, the later a herbicide with residual activity is applied in a growing season, the higher the potential risk of carryover.¹ Herbicide effect can also be cumulative in the soil. Residual herbicides that are similar in how they target related plant metabolic systems may build up in the soil and result in additive injury to subsequent crops.² Always follow label directions for sequential herbicide applications and follow the most restrictive re-cropping interval. For Bayer herbicides, Varro^® Herbicide or Velocity m3 herbicide may be used in a system with Olympus^® Herbicide as specified on their respective labels. These herbicides should not be used in the same year as any other Group 2 herbicides.

Environmental Conditions

Weather is the major factor influencing potential herbicide carryover. Microbial degradation is the primary means for dissipation of many herbicides. It takes adequate soil moisture and warm temperatures for soil microbes to degrade herbicide residues. Most herbicide degradation resulting from microbial activity occurs during the summer and early fall after the herbicide is applied. Under extremely dry conditions, the rate of herbicide degradation by soil microbes can be slowed enough to allow herbicides that would normally break down to persist into the next season. Microbial activity also decreases as soil temperatures decrease and little to no herbicide breakdown due to microbial activity occurs through the winter in Western Canada.³ Moisture during the winter is not likely to increase microbial activity enough to enhance the rate of herbicide degradation due to cold soil temperatures. Late spring or summer herbicide applications followed by dry summer and fall weather conditions, then cool temperatures that extend into early spring the following year can provide the perfect scenario for herbicide carryover problems. For Western Canada, risk of herbicide carryover is high when accumulated rainfall from June 1 to August 30 totals less than 125mm.⁴

Soil Properties

Soil microorganisms are responsible for most herbicide degradation. Therefore, good soil conditions for microbial growth are important. Warm temperatures, adequate moisture, good aeration, fertility, and a moderate pH work best for microbial herbicide breakdown.¹

Soil properties can influence herbicide persistence and bioavailability. Herbicide carryover occurs when herbicides become bioavailable to subsequent crops. In the Prairies, soils with low organic matter and low clay content are at an increased risk for rotational cropping injury due to herbicide carryover. Low organic matter soils have limited microbial populations and therefore reduced microbial herbicide degradation rates in the soil relative to high organic matter soils. Low clay content results in reduced herbicide adsorption to soil particles leaving herbicide available for plant uptake. Without sufficient rainfall during the warm growing season, the herbicide may not break down and what is not tightly bound to soil colloids (adsorbed) can be solubilized with sufficient rainfall, causing crop injury.³ Some herbicides with rotational cropping restrictions have labels which therefore specify minimum soil organic matter content and recommended use rates that vary based on soil clay content.

Soil pH is another factor that can affect herbicide persistence and bioavailability. For example, high soil pH can lead to greater carryover of triazine and sulfonylurea herbicides but shorter persistence of imidazolinone herbicides.⁵ Risk of rotational cropping phytotoxicity due to pyrasulfotole carryover increases as soil pH increases.

Rotational Crop Sensitivity to Herbicide

To avoid the possibility of injury to subsequent crops after an application of the recommended rate of Bayer herbicides, follow the crops and replanting interval which appear on the label (provided here). If tank mixing, follow the most restrictive replanting interval of the tank mixture components. Only the crops with a prescribed interval provided below have been field tested and may be safely planted in accordance with the given interval. Other crops have not been tested and should not be planted the year following application of the herbicides listed below.

Table 1. Replanting Intervals for Bayer Herbicides Most Commonly Used in Western Canada Cereal Crops

* Manitoba only

** Field peas may be grown the year following application in all Black, Grey-Wooded and Dark Brown soil zones. DO NOT plant field peas the year following in the Brown soil zone where organic matter content is below 2.5% and where soil pH is above 7.5.

Herbicide Carryover Symptoms

Though soil conditions may be conducive for herbicide persistence, herbicide carryover symptoms may not appear until sufficient precipitation washes persisting herbicide residues into soil solution while sensitive crop roots are present.

Some Group 2 herbicides used in cereal crops carry rotational cropping restrictions and can persist when weather conditions are dry in the year of application and may cause injury in sensitive crops the following year. This injury affects the growing point after crop emergence. This injury can result in complete plant death (Figures 1 and 2).

Thiencarbazone-methyl is the Group 2 active ingredient in Varro and Velocity M3 herbicides. Propoxycarbazone is the Group 2 active ingredient in Olympus herbicide. Bayer has tested these products under a wide range of environmental conditions and supports these products within the rotational cropping restrictions outlined on their respective labels (see above). Please ensure that these rotational cropping labels are followed.

Figure 1. Canola impacted by imazamox carryover. Photo courtesy of Lyndsey Friesen.

Figure 2. Canola stand impacted by imazamox carryover. Photo courtesy of Lyndsey Friesen.

HPPD inhibitor herbicides are labeled for use in a range of crops including cereals corn and are part of several pre-mix herbicide products. Pyrasulfotole is an HPPD inhibitor that is included in Velocity m3 Herbicide, Tundra^® Herbicide, Infinity^® Herbicide, and Infinity^® FX Herbicide. Under normal conditions these herbicides have a low risk of carryover, but dry conditions during the application season have been shown to increase the carryover potential of these herbicides in sensitive crops. These herbicides are degraded primarily by soil microbes. Low soil moisture, cool temperatures, and extreme soil pH can inhibit microbial degradation. HPPD carryover injury may first be noticed in spray overlap areas where elevated application rates increased the risk for carryover. Phytotoxicity can be very visual (white colored leaves) initially, but these symptoms clearly dissipate over time.³ Bayer has tested these products under a wide range of environmental conditions and supports these products within the rotational cropping restrictions outlined on their respective labels. Please ensure that these rotational cropping labels are followed.

Figure 3. Pyrasulfotole injury to canola over time.
From left to right images taken: June 7, June 12, June 20, and June 28. Photos courtesy of Jessica Runge.

How to Manage Risk of Herbicide Carryover

Soil residual herbicides are an important component of effective weed management. Herbicide carryover issues generally do not occur within labelled parameters under normal environmental conditions. However, rotational cropping plans may need to be changed in fields where carryover could occur due to abnormal environmental conditions. Good rotational crop planning (for example, use of less sensitive rotational crops) and effective herbicide application recordkeeping is necessary to help minimize potential carryover to rotational crops.

Labelled crop rotational intervals can vary between herbicides within the same group as well as with local environmental conditions and use practices. Product users should consult individual product labels for specific recommendations and precautions. Information on herbicide performance under unique local conditions and soil types should be obtained from local experts.