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Herbicide Residue Injury on Canola
Residue Injury Intro - Herbicides - Crop Production - Canola - Plant & Soil Sciences
INTRODUCTION
Canola injury can occur from exposure to low soil concentrations of several Group 2 herbicides. In recent years, slower breakdown of residual herbicides due to drought has caused crop injury in subsequent crops. Herbicide carryover can cause crop injury ranging from minimal to complete crop loss. Injury problems have arisen where normal breakdown of herbicides has been affected by factors such as drought, low organic matter and pH.
OBJECTIVES AND METHODS
This section is intended to:
1) show canola injury symptoms associated with Group 2 herbicide residues, and
2) assist in differentiating between symptoms due to residue versus other problems.
Photographs were generated from field plots and bioassays. Field plots were sprayed with reduced rates of Group 2 herbicides prior to planting the crop to simulate herbicide carryover.
FACTORS AFFECTING HERBICIDE CARRYOVER
FIELD HISTORY
Residual herbicide injury can only occur in fields with a history of Group 2 herbicide application. Not all of the Group 2 herbicides have residual action and only some affect canola. Refer to specifi c herbicide labels for restrictions on recropping to canola.
SOIL CHARACTERISTICS
Interactions between soil factors are complex and may either decrease herbicide decomposition or increase the herbicide residue available to the crop.
ORGANIC MATTER AND SOIL TEXTURE
As organic matter decreases, microbial degradation of the herbicide decreases, increasing potential carryover. Soils with low clay content have decreased adsorption of residual herbicides, thereby increasing potential carryover. Therefore, potential for injury on subsequent canola crops increases as organic matter decreases, and clay content decreases.
pH
Soil pH affects herbicide decomposition and availability to the subsequent canola crop (Table 1).
| Table 1. A general guideline to soil characteristics that increase carrryover for Group 2 herbicides. For information on specific products, consult the herbicide label. | ||
| Chemical Family | pH | Organic Matter |
| Imizadolinone | <6.0 | low |
| Sulfonyurea | >7.5 | low |
| Sulfonyylaminocarbonyltriazolinones | >7.5 | low |
CLIMATE
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 favourable pH or organic matter conditions.
Temperature
When microbial decomposition is an important mechanism (e.g. imidazolinones), decomposition is reduced by cool soil temperatures.
Carryover can have considerable field variation in acreage affected and severity of plant injury. Uneven plant stands can affect crop maturity and weed pressure. |
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Symptoms can vary in a small area. All plants in this picture emerged at the same time. 1. Unaffected |
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