Canola injury can occur from exposure to low soil concentrations of some herbicides, in particular several Group 2 herbicides, plus some Group 4, 5 and 14 herbicides. Herbicide carryover can cause crop injury ranging from minimal to complete crop loss. Increased acreage of canola and new herbicide options in other rotation crops have created more opportunities for carryover damage. Injury problems have typically arisen where normal breakdown of herbicides has been inhibited by factors such as drought, low organic matter and pH, sometimes in conjunction with increased frequency of use of residual herbicides in the rotation.
Factors that affect herbicide carryover are:
Field history. Residual herbicide injury can only occur in fields with a history of Group 2, 4, 5 or 14 herbicide applications. Not all of the Group 2, 4, 5 or 14 herbicides have residual action and only some affect canola. Refer to specific herbicide labels for restrictions on re-cropping to canola. Alberta guide. Manitoba guide. Saskatchewan guide.
Soil characteristics. Interactions between soil factors are complex and may either slow the rate of herbicide breakdown or increase the availability of any remaining herbicide residue to the crop.
Organic matter and soil texture. As organic matter decreases, microbial degradation of the herbicide decreases, increasing potential carryover and decreased adsorption to buffer the impacts of the herbicide on the follow crop. Soils with low clay content have decreased adsorption of residual herbicides, thereby increasing potential availability of the herbicide to sensitive plant roots when a significant rainfall occurs. Therefore, potential for injury on subsequent canola crops increases as organic matter decreases, and clay content decreases (except where soluble herbicides are leached by lots of rainfall).
pH. Soil pH affects herbicide decomposition and availability to the subsequent canola crop. For some herbicides, low pH increases carryover risk. For others, it’s high pH. See this table from the Canola Encyclopedia.
Drought. Under drought conditions, microbial and hydrolytic breakdown of herbicides is decreased and adsorption of herbicide to soil particles is increased. The influence of drought on soil may override any previously favorable pH or organic matter conditions.
Temperature. When microbial decomposition is an important mechanism (e.g. most herbicides), decomposition is reduced by cool soil temperatures. Herbicides that are broken down via chemical hydrolysis will also slow as chemical reactions occur more slowly at lower temperatures.
For photos and symptoms of herbicide carryover damage:
Canola Encyclopedia section on Group 2 residue symptoms
CCC booklet Recognizing herbicide residue and drift injury in canola