With tall, thick, untreated giant ragweeds growing in the background, University of Nebraska Extension weed scientist, Amit Jhala, told farmers that these weeds are not only massive, but they are also prolific pollen producers. Jhala was one of the UNL weed science presenters at the herbicide-resistant weed management field day held at David City recently, sponsored by UNL Extension and the Nebraska Soybean Board.
Growing up to 17 feet tall, a single giant ragweed plant can produce up to 10 million pollen grains daily, or more than a billion pollen grains during its life cycle, Jhala said. Because of this excessive pollen production from a single plant, pollen can be transported by wind and the plants can cross-pollinate, leading to greater genetic variation and diversity in ragweed plants in adjacent fields, and increasing the opportunities for the spread of herbicide resistance. "It has been able to spread resistance very fast," he said. According to Jhala, UNL researchers hope to study the travel of giant ragweed pollen in future trials.
Giant ragweed is survivor. Before glyphosate herbicide technology, ALS-inhibitors were a primary herbicide management tool in controlling the weed. By 1996, giant ragweed had developed resistance to ALS-inhibitors. It is no surprise, then, that it has developed resistance to glyphosate as well.
According to UNL Extension integrated weed management specialist, Stevan Knezevic, giant ragweed specimens at David City trials have been able to withstand extremely high doses of glyphosate, far beyond what is recommended on the label. Specimen plants even grew back last summer after high doses from a flaming machine.
Through studies at David City sites, researchers have learned that giant ragweed in Nebraska germinates primarily in April and May. This provides an opportunity for control through early cultivation or burndown herbicide treatments, said Jhala. Burndown treatments, particularly those utilizing 2,4-D formulations, have been successful in controlling more than 90% of the giant ragweed populations, he said.
Read more about UNL herbicide-resistant giant ragweed and waterhemp research in a future print article in Nebraska Farmer. You can contact Jhala at 402-472-1534 or email [email protected].