Frequently Asked Questions about Herbicide Residues in 2005

• What is residual herbicide?
• What is my risk of herbicide carryover in the 2005 season?
• How do I prevent crop injury from residual herbicide carryover?
• How can I give my crop the best chance to tolerate or recover from a herbicide residue that I didn’t expect, given the previous herbicide’s options?

What is a residual herbicide?

A: 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.

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What is my risk of residual herbicide carryover in the 2005 season?

A: Recropping recommendations for residual herbicides can be found in the 2005 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 particularly dry seasons, such as those of 2001, 2002 and 2003. Drought conditions, that saw the soil surface dry to less than the permanent wilting-point, caused microbial activity and the resulting herbicide breakdown to stop for large portions of the season. This resulted in a greater than expected risk of injury and yield loss from herbicide residue.

By comparison, the 2004 season had 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 result, producers should be conservative when deciding which crops to grow in 2005 following residual herbicide applications in 2004, and stick with those crops that are recommended on the product label and in the Guide to Crop Protection. More importantly, producers should remain conservative with crop selection on fields that had previous applications of a herbicide with multi-year restrictions, since breakdown conditions were poor in some areas in 2003. Good field records are important to refer back to in this situation.

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How do I prevent crop injury from residual herbicide carryover?

A: 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 back 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.

How can I give my crop the best chance to tolerate or recover from a herbicide residue that I didn’t expect, given the previous herbicide’s recropping options?

A: 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, 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.