Herbicide Resistance Basics
The principles of herbicide resistance are straightforward – as the same herbicides are used year after year, there is intense selection pressure in weed populations for resistance traits. Once these traits are developed, they can be passed freely amongst weed populations of the same species. It is well understood that resistance traits can rapidly spread through nearby geographic areas through pollination or natural or human driven seed diffusion. The same principle applies to human health and the development antimicrobial resistance which has rendered antibiotics, antimalarials, and HIV antiretroviral drugs ineffective in contexts where they are relied upon as the sole means of control for various disease-causing pathogens.
Since the mid-1990s, widespread use of herbicide-tolerant corn, soy, canola, and cotton varieties have led to the development herbicide resistant weed populations in areas across North America where these crops are grown. The highly effective in-season weed control that herbicide-tolerant crop varieties offer, coupled with tight crop rotations have led to the development of resistance to commonly used herbicides such as Round-Up, Liberty, and Xtend. It is possible for weed species populations to develop resistance to multiple herbicides. Once resistance traits are widespread, some herbicides (or combinations of herbicides) can be rendered partially or wholly ineffective. Once herbicide-resistant traits are developed, it is almost impossible to eliminate them.
Non-herbicide weed control strategies (minus tillage)
Fortunately, herbicide resistance can be countered with a wide range of measures. While not all measures discussed in this post are universally practical, any one of them can help to counter herbicide resistance if implemented as part of cash crop production systems. Importantly, these measures do not rely upon tillage. While tillage is a highly effective non-herbicide based weed control measure, the costs of relying upon regular tillage practices are too high (both in monetary cost and soil function).
a) Crop diversification:
Diverse cash crop rotations allow for the use of a wide range herbicide chemistries. This makes it less likely that resistance to a single herbicide can develop since this helps to reduce repeated weed population exposure to the same herbicides.
Multiple cover crops grown over the course of spring, summer, and fall seasons can help to suppress weed populations by outcompeting weed species. Common and fast-growing cover crops include winter rye and buckwheat. Growers would be forced to give up one growing season in order to take full advantage of cover crop weed suppression. However, cover crops can be harvested for seed or grazed in order to partially offset the costs of a lost cash cropping season. An added benefit of skipping a cash cropping year is the ability to break cash crop insect pest and disease cycles.
b) Equipment-based weed control:
A number of recently developed equipment-based weed control methods are available now or are nearly ready for wider adoption. Some many consider the costs of these equipment-based methods to be too high – but there is no doubting their effectiveness.
Flame weeding: This practice is growing in popularity amongst organic row crop farmers and flame weeding could have a place in non-organic row crop production – especially in areas with significant populations of herbicide resistant weeds. Electrical weeding: Electrical weeding has the potential to be a more common practice within the next 3-4 years. Large manufacturers such as Case IH now have electrical weeders on the market.
Weed seed mills: Not commonly used, but weed seed mills have shown promise with initial testing and adoption in Canada, US, and Australia. They are attached below the straw handling systems of a combine in order to not interfere with normal straw distribution. Various tests have demonstrated that weed seed mills can destroy between 90 and 98% of weed seeds sent through a combine.
Controlled traffic farming and chaff lining: In scenarios where controlled traffic farming is used, combine straw handling systems can be set to leave straw (and accompanying weed seeds) in tramlines. This significantly reduces the chances of weed seed germination.