Long-term herbicide-resistance management requires more than weed control aimed only at minimizing crop loss in any one season. It requires long-term strategies focused on delaying the evolution of herbicide resistance and reducing weed seed in your fields.
Effective herbicide-resistance management combines a variety of chemical and nonchemical management tactics to diversify selection pressure on weed populations and minimize spread of resistance genes.
It’s about giving your crops a competitive advantage against weeds. It’s about delaying the evolution of herbicide resistance and preserving herbicide technology.
Weed Seed-Free Fields
Herbicide-resistance management plans must begin before planting because control options become limited once the crop is planted and crops and weeds begin to emerge. Planting into weed-free fields helps prevent challenges later in the season.
- To keep crop fields as weed-free as possible, especially in conservation-tillage systems where preplant tillage is not feasible, residual herbicides can be used before or at planting (PRE).
- Consider tillage and cover crops to prevent weed emergence.
- Additional applications of residual herbicides in slow-growing or open-canopy crops, before the efficacy of the initial residual herbicide has dissipated, reduces selection pressure associated with sole reliance on POST herbicides.
Crops planted into established weeds are risky because even herbicide mixtures with multiple sites of action (SOA) may not provide effective control. Failure of preplant measures to control emerged weeds is devastating to crop yield and even more devastating if resistance evolves.
Once a dense crop canopy has formed, emergence of most weeds typically ceases. Once crops canopy, your focus should be on removing escapees and targeting weeds that emerge after crop harvest, if seed production is possible before a killing frost.
Weed management doesn’t end at harvest. Postharvest weed-seed production must be prevented to effectively manage soil seedbanks for the long term. In some weeds, such as Common Waterhemp, viable seed production can occur as early as seven days after pollination, meaning that these weeds are capable of producing seed after crop harvest, especially in southern climates.
For a production system to remain sustainable, the soil weed seedbank must be static or declining. An increasing seedbank is evidence of a weed that is escaping the current management regime through herbicide resistance or some other adaptation. Furthermore, the risk of resistance evolution is shown to be positively associated with the initial seedbank size; therefore, keeping the soil seedbank at low levels reduces the risk of future evolution of herbicide resistance.
The concept of what constitutes an acceptable level of weed-seed production must be abandoned in favor of a zero or near-zero threshold to slow the rate of herbicide-resistance evolution. Weed management programs must aim to eliminate weed-seed production from the most competitive, resistance-prone weeds in a field.
In particular, knowledge of the earliest time in the growing season when viable seed are produced is vital for timing late-season applications. Some nonherbicidal approaches to weed-seed prevention create additional management challenges. For example, weeds must be removed mechanically or physically before seed maturity.
After crop harvest, producers often allow weeds to grow uncontrolled. Such lapses in weed management can lead to increases in the soil seedbank if sufficient time elapses between harvest and a weed-killing frost, even if a high level of weed control was achieved during crop production. Post-harvest weed-seed production must be prevented to effectively manage soil seedbanks for the long term. In some weeds, such as Common Waterhemp, viable seed production can occur as early as seven days after pollination, meaning that these weeds are capable of producing seed after crop harvest, especially in southern climates.
Field Borders, Fence Rows, Creek Beds and Ditches
They won’t impact your yields, but left unmanaged, field borders, fence rows, creek beds and ditches provide breeding grounds for weeds, including herbicide-resistant weeds. Allowing weed-seed production in field borders can have long-term effects on your fields.
Troublesome weeds like Waterhemp, Palmer amaranth and Giant Ragweed thrive in these areas.
Perennial weeds often appear first in these areas. Left unmanaged, they can infest your fields.
You can prevent weed infestations by planting and maintaining dense grass cover in field borders.
After all the work you’ve done to plant your crops and keep them weed-free all the way through harvest... why risk it all with contaminated equipment? Equipment can transport weed seeds between fields. This is especially a concern at harvest when mature seeds from one field can contaminate another.
To prevent weeds from spreading, clean any equipment used in contaminated fields before using it in other fields.