Herbicide breakdown requires sufficient time under adequate moisture and soil temperature to support the growth of microbes that degrade herbicide molecules. Some herbicides are broken down quickly or are bound tightly to soil, preventing them from causing problems for crops that are planted the following season. Other herbicides take longer to decay, and as a result, persist into seasons following the year they were applied. These residues can injure sensitive crops that are seeded in following seasons. Herbicides that have restricted recropping options are considered residual herbicides.
Risk of residual herbicide carryover
Recropping recommendations for residual herbicides can be found in the Guide to Crop Protection and are taken from herbicide labels developed by the manufacturer. These recropping recommendations are developed to cover a range of conditions that might be considered normal for the Canadian Prairies. Occasionally, weather conditions can result in greater than expected herbicide residue remaining in the soil and may call the "normal" recropping recommendations into doubt. Many companies with residual herbicides have modified their labels to take unusually dry events into consideration, and further restrict the crops considered safe to seed following their residual herbicide.
Problems with greater than expected herbicide carryover can occur in seasons following exceptionally dry seasons, such as those of 2001, 2002 and 2003. Drought conditions, where the soil surface dries to less than the permanent wilting-point, cause microbial activity and the resulting herbicide breakdown to stop for large portions of the season. This results in a greater than expected risk of injury and yield loss from herbicide residue.
Very wet conditions or cool conditions can also result in the slowing of herbicide breakdown. Oxygen, required by the aerobic microbes that break down most herbicides, is not available in saturated soils, resulting in the stopping or slowing of breakdown of many herbicides in soil. Some herbicides, such as Group 3 herbicides, are broken down at an accelerated rate under oxygen limiting conditions and can result in a loss of weed control activity in saturated areas.
In the 2004 season there was ample rainfall, so that moisture would not have been a limiting factor for herbicide breakdown. Unfortunately, soil temperatures were very cool, which can also slow microbial growth which, in turn, slows herbicide breakdown. It is unknown whether ongoing moist but cool conditions will provide as much breakdown potential as "normal" summer conditions with warm temperatures interspersed with periodic rainfall. There is some evidence to suggest that microbial activity can be greater under wet/dry cycles than constant moist conditions.
As a result, producers should be conservative when deciding which crops to grow in years following residual herbicide applications when environmental conditions are not conducive to breakdown, and stick with those crops that are recommended on the product label and in the Guide to Crop Protection. It is also important for producers to keep good herbicide application records for each field, to prevent carryover concerns on some highly residual products. These products may have multi-year restrictions, and climatic conditions occurring in any of the years from application to the seeding of the sensitive crop can result in unexpected carryover and potential crop damage.
Preventing crop injury from residual herbicide carryover
Restrict crops following a residual herbicide to those that are recommended on the label of the residual herbicide. These are also listed on the herbicide product description pages in the Guide to Crop Protection, as well as a recropping chart in the front of the chapter on herbicides. Growing crops that fall outside of these recommendations greatly increases the risk of injury, yield loss and/or delayed maturity. Occasionally, because of extremely poor climatic conditions for herbicide breakdown during the year of application, even recommended recropping options can experience some risk of injury. This injury would not be expected under normal growing season conditions in the year that the residual herbicide was applied, or subsequent years for long-term residual products.
An unhealthy crop is less likely than a healthy crop to be able to endure the stress that herbicide residues can add. Therefore, to reduce the risk of injury or yield losses due to greater than expected herbicide residues, take measures to maximize crop health in fields with a history of residual herbicide. Measures such as shallow seeding with good seed to soil contact, adequate fertility including the use of starter phosphate fertilizer, and planting the healthiest seed available, will maximize the chance of producing a healthy crop that can hold its own against unexpected herbicide residue stress.
Soil testing is recommended to determine if there are any macro or micro-nutrient deficiencies that may be amplified by residual herbicide stress. Avoid the use of an in-crop herbicide that is in the same herbicide group as the residual herbicide you are concerned about. The combination of the in-crop and residual herbicide effect on the crop could result in injury in a situation where either herbicide influence alone would not.
This being said, good agronomics will only go so far. Even a healthy crop that is not recommended to follow a residual herbicide can be severely injured by those residues. Therefore, always stick to those crops that are recommended to be seeded following a particular herbicide.