2,4-D resistant waterhemp found in Nebraska
A waterhemp population from southeast Nebraska has been confirmed to be resistant to 2,4-D. The resistant population is believed to be limited to a few fields.
UNL Research on Suspected Population
In 2009 we received a report of a warm season grass field with a waterhemp population that was no longer being controlled by 2,4-D. Seed from this field was collected in 2009 and 2010 and greenhouse dose response trials were conducted to determine if the population was herbicide resistant.
When compared to a susceptible population, the suspect population was 10-fold more tolerant to 2,4-D. In the greenhouse, a dose of 5 fl oz/ac of 2,4-D (Lo-vol ester, 3.8 lb ae/gal) reduced growth of the susceptible population by 50%. In contrast, 54 fl oz/ac were required to reduce the growth of the suspect population by 50%. In a 2011 study conducted in the field where seed from the suspect population was collected, plants were treated with 2,4-D doses of 0.25 to 64 qt/ac (64 times a typical use rate). At 28 days after treatment (DAT), plants treated with 64 qt/ac were stunted compared to untreated plants and showed injury symptoms characteristic of 2,4-D, but were recovering. At 84 DAT, individual plants that had been treated with 64 qt/ac recovered sufficiently to produce seed.
We believe these results warrant labeling this population as “2,4-D resistant.” This is the sixth herbicide mechanism-of-action (Synthetic auxins) to which waterhemp has developed resistance in the United States, and the third reported in Nebraska.
Waterhemp is the predominant pigweed (Amaranthus) species in eastern and south central Nebraska fields and is problematic throughout much of the Corn Belt. It is well adapted to reduced tillage cropping systems that rely primarily on herbicides for weed control. Waterhemp has succeeded because it emerges from May through August, allowing late emerging plants to avoid herbicides.
Waterhemp is a dioecious species, which means that male and female flowers occur on separate plants. Because of this and the large number of seeds produced per female plant, genes favorable for survival under a particular selection pressure (such as repeated use of a specific herbicide) rapidly spread throughout a population. When a single herbicide is used repeatedly over many years, there is a high risk of herbicide resistant populations developing. Since 1993 waterhemp populations have been reported to have evolved resistance to atrazine (Photosystem II inhibitors), imazethapyr and chlorimuron (ALS-inhibitors), fomesafen and lactofen (PPO-inhibitors), glyphosate (Glycines), and mesotrione and tembotrione (HPPD-inhibitors).