Every farmer wants a "silver bullet" herbicide chemistry to control the toughest weeds in their fields. But University of Nebraska Extension researchers told producers at weed resistance meetings this spring that they basically shouldn't hold their breath in waiting. Researchers are making it clear that resistance issues can only be remedied by rotating crops and chemistries, and through an understanding of the biological lives of the six resistant weeds found in Nebraska--waterhemp, marestail, giant ragweed, Palmer amaranth, kochia and shattercane.
While glyphosate is blamed for resistance, weeds have been overcoming chemistry for years. For instance, populations of waterhemp exist that are resistant to HPPD-inhibitors, glyphosate, 2,4-D, and ALS-inhibitors.
According to Amit Jhala, UNL Extension weed scientist, "Herbicide resistance is the acquired ability of a weed population to survive herbicide application that previously was known to control the plants." Resistance responds to selective pressure, so by using a single herbicide repeatedly producers are actually helping select for resistance in the population.
"Some resistant weeds can survive at the labeled rate of herbicide application," Jhala said. "Others can survive 100 times the recommended rate of herbicide."
Resistance can be spread through pollen flow, with pollen grains traveling up to 1,000 feet or more. It can also spread through human activity such as on tillage equipment or machinery, or by animals and water.
There are some very tough weeds to deal with. Waterhemp, which is dioecious or has separate male and female plants, can hybridize with Palmer amaranth or redroot pigweed. So, a single resistant plant can cause problems, Jhala said. Giant ragweed is a prolific pollen producer, producing up to 10 million pollen grains daily, or one billion during its life cycle, he explained. It can also hybridize with common ragweed. Kochia, which causes the most problems in western Nebraska, has a deep taproot and produces 14,000 seeds per plant.
"Once a plant develops resistance, it isn't going away," said UNL Extension integrated weed management specialist, Stevan Knezevic. "Some weeds have several types of resistance."
Since the 1980s, as more herbicides have been utilized around the world, the number of cases of resistant weeds has risen.
The U.S. has the greatest number of resistant weeds because farmers have widely adopted the use of herbicides in crop production, according to Lowell Sandell, UNL Extension weed science educator. "Most problem weeds are perennials, but most resistant weeds are annuals," he said. Australia and Canada also have high numbers of resistant weeds. "It's not an easy game," Sandell said. "So even if you don't have resistance yet, develop a proactive program" of rotating crops and herbicide chemistries.
Follow-up scouting is crucial. Fields should be scouted periodically after herbicide treatments to identify weeds that have exhibited regrowth.
New University of Nebraska Extension weed management guides for 2013 have additional information to help producers deal with resistance issues. Weed scientists in 16 north central universities, including UNL, are distributing Corn and Soybean Herbicide Charts to farmers that provide information on specific herbicides and their mode of action and site of action, to help farmers rotate chemistries in their corn and soybean fields. A table available on page 8 in the new weed guides is excerpted from these charts. The table identifies herbicides by mode of action, site of action, number of resistant weed species in the U.S., chemical family and active ingredient. The complete chart also lists product and trade name examples of each herbicide, as well as component herbicides in premixes.
For more information on herbicide resistance issues, contact Knezevic at 402-584-3808, Jhala at 402-472-1534, or Sandell at 402-472-1527