Cool periods in May and June 2012 played havoc with some herbicide applications, leading to injury in normally tolerant crops. Examples from last season include group 1, 2 and/or 4 injury in wheat and barley and group 2, 9 and/or 10 injury in canola.
Crop tolerance to certain herbicides or herbicide groups allows for selective, post-emergent weed control. Tolerance is achieved by metabolism or deactivation of herbicide molecules within the plant. The process of deactivation requires that plants are actively growing and not stressed.
All crops have a minimum temperature above which plants are able to carry out normal functions for growth (e.g. photosynthesis, metabolism, etc.). This temperature is often referred to as ‘base temperature’ and is used to calculate heat unit accumulation (e.g. GDDs) for individual crops (Table 1). Base temperature is not the same as freezing point.
Table 1. Base Temperatures for Select Manitoba Crops
|Crop||Base Temperature (°C)*|
|Canola||0 – 5|
|Cereals||0 – 4.5|
|Corn||6 – 10|
|Potato||5 – 7|
|Soybean, beans||5 – 8|
|Sunflower||5 – 8|
It’s important to note that, while we use common base temperatures for certain crops, actual base temperature depends on crop variety, growth stage and cold-hardening.
Given the above, herbicides should be applied when day time temperature is ≥ 10°C to ensure optimal crop tolerance. This will also result in better weed control. MAFRI also recommends producers:
- Aim for day time temperatures of ≥ 10°C on the day of application and for 2 days after application
- Apply herbicides earlier in the day if night time temperature is expected to be below 10°C
- Refer to the product label for specific instructions regarding herbicide application and environmental conditions
Submitted by: Jeanette Gaultier, MAFRI Pesticides – Minor Use & Regulatory Specialist