W.A.R. : Weeds and Resistance
by Don "Moe" Moeller, Sales Agronomist

Weed resistance to herbicides is not a new phenomenon but, for many of us, resistance is not taken seriously until the popular and successful herbicide everyone uses starts to fail. Then we take for granted that a new and even better replacement will be introduced and the problem will be taken care of. Tomorrow, if needed, that replacement may not be available!

Weed specialist Joe DiTomaso defines weed resistance to herbicide as, "The inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type." In a plant, resistance may be naturally occurring or it may occur by mutation. But to date, no evidence has been presented to demonstrate herbicide induced mutation. Resistance is most likely caused by selection with the herbicide providing the selection pressure. Hence, susceptible plants are killed while herbicide resistant plants survive to reproduce without competition from the susceptible plants. In this scenario, the appearance of herbicide resistance in a field is most likely an example of weed evolution, influenced by the amount of genetic variation in the weed.

The many genetic variations Mother Nature has given most weed species help them survive under a variety of environmental conditions. When those species are placed under pressure (herbicide pressure in this case), those that survive this pressure are weeds that develop resistance. In this example, it could be resistance to a popular herbicide that everyone is using today.

Since Mother Nature has given weeds many genetic variations, it is possible that weed resistance could be absolute if a weed species is put under high herbicide pressure. That's why planned and proper management
of herbicides is essential. Here are three very important strategies to help prevent herbicide resistance:

• Rotate herbicides yearly, not by brand, but by type and mode of action.
• Use the proper herbicide rate and application for the herbicide chosen.
• Vary the time when the herbicide is applied.

Cutting back on the herbicide rate is asking for trouble! This vastly increases the chance for weed escapes, poorer herbicide coverage or survival by weeds that need a higher herbicide rate to be killed. Time of application is also important in relation to plant size, humidity, temperature and crop canopy.

Herbicide rotation is the most important contributing factor to prevent or delay resistance in weeds. Cost and ease of use often are the main reasons the same herbicide is used year after year, but poor rotation practices will most likely limit or end the effective use of a good herbicide. The once popular ALS herbicides are a good and recent example of weed resistance to herbicide. When ALS herbicides were used extensively without rotating them with herbicides having different modes of action, ALS weed resistance quickly developed.

Could this happen again to the popular glyphosate herbicide programs we currently use? In the article "Weeds to Watch for in Roundup-Ready Soybeans," Dr. Steven Knezevic, from the University of Nebraska, lists weed species in Nebraska that he feels are shifting toward resistance. He bases this list on phone calls made to extension offices and on questions from producers, crop consultants and agronomists. The shifting species he lists are: marestail, morning glory (common and ivy leaf), wild buckwheat, Pennsylvania smartweed, Venice mallow, yellow sweetclover, field bindweed, waterhemp, kochia, Russian thistle, primrose species and volunteer Roundup-Ready corn. Dr. Knezevic writes that if these species are not controlled, their seeds could be a major problem in the future and the cost and frustrations of good weed control will become much higher.

Remember, a management practice as simple as yearly herbicide rotation could help prolong the effectiveness, life cycle and cost benefits of today's popular and widely used herbicides.

To comment or for more information, send an Email to: d.moeller@hoegemeyer.com.