Herbicide carryover: Seven key considerations

The agronomic implications of drought remain top-of-mind this spring as the remnants of the previous season still linger in our fields. 

Because moisture after application is essential for herbicide degradation, fields that received insufficient rainfall during the 2021 season may be at risk for herbicide carryover. 

Jeremy German, agronomy manager with G-Mac’s AgTeam in Kindersley, Sask., is very familiar with these dry conditions.   

“We have a high risk for herbicide carryover in our area as we received just over three inches of rainfall between June and August south of Kindersley. That’s why we’re taking extra time this year to review cropping history with our farmer customers and make sure they are fully aware of potential carryover issues,” says German. 

With seeding just around the corner, now is the time to double-check application records, environmental data, and herbicide recropping restrictions.  

To make sure you understand the risk of crop injury due to herbicide carryover, Sarah Anderson, agronomy manager with Saskatchewan Pulse Growers, shares the seven key points you need to consider:  

1. Microbial activity regulates most herbicide breakdown.  

Soil moisture and temperature conditions dictate microbial activity. Soil microbial activity is highest in moist soils and increases with soil temperature (up to 30°C). These criteria need to occur simultaneously, and moisture received late in the fall or early spring is unlikely to spark sufficient microbial activity due to low soil temperature.  

2. Herbicide degradation takes time.  

Although rainfall accumulation between June 1 and August 31 typically guides recropping intervals, the clock starts once the herbicide is applied. Early moisture conditions are unlikely to mitigate herbicide carryover risk if it stays dry after application as the carryover risk increases the later the herbicide was applied.   

3. Degradation differs by soil characteristics and active ingredients. 

Soil organic matter, texture classification, pH and soil zone also influence how herbicides breakdown and inform guidelines for recropping intervals, but it is not a one-size-fits-all situation. Active ingredients belonging to the same herbicide group also have variable persistence across different soil characteristics.   

4. Refer to manufacturer guidelines.  

Some herbicides are known to pose a greater carryover risk than others and have recropping restrictions under various soil and environmental conditions expressed on their label; however, many herbicide products have not been tested under conditions reflective of the environmental extremes of the 2021 season, so some chemical manufacturers have issued additional advisories warning of recropping risks in response to heat and drought conditions.  

5. Extra caution with imidazoline herbicides. 

Imidazoline herbicides are highly dependent on microbial degradation, and because they maintain herbicidal activity at very low doses, even limited residues can cause damage to sensitive crops. Solo®, Odyssey®, Viper®, Davai®, Python®, Quasar® and Ares™ herbicides are commonly sprayed on pulses and CL® tolerant crops and are all examples of herbicides that contain imazamox, imazethapyr, imazapyr or a combination. Be aware of potentially sensitive recropping options such as durum wheat, canary seed, and canola. 


6. There is no substitution for on-farm records.  

Predictive maps provide a great starting point, but herbicide carryover risk should be assessed at the individual field level. Details of herbicide application, soil characteristics, previous crop and herbicide history, and localized precipitation data all need to be factored into recropping decisions.  

7. Crop rotation is the best tool to mitigate the risk of herbicide carryover injury.   

Vigorous crop establishment and growth may help buffer against added stress of herbicide carryover, but agronomic practice alone will not safeguard against injury. Substitution of highly sensitive crops for lower-risk alternatives that still have agronomic suitability is recommended.  

Featuring: Jeremy German, agronomy manager - G-Mac’s AgTeam