Herbicide resistance is one of the largest problems in the field of weed science today. This problem is something that drives research and extension efforts in the program. This page includes confirmed and suspected cases of herbicide resistant weeds in Virginia.
The Big Three
In Virginia, there are three major herbicide resistant weeds that are a cause for concern for growers: Palmer amaranth, common ragweed, and horseweed/marestail. The maps below highlight in red where herbicide resistant weeds have been reported. It is likely that these weeds are resistant to glyphosate (WSSA group 9) and ALS-inhibiting (WSSA group 2) herbicides.
Italian ryegrass is another weed causing concern in Virginia. It is resistant to ALS-inhibiting herbicides and widespread across the eastern part of the state.
Common waterhemp has been found in Virginia recently. Like the big three listed above, this weed is suspected to be resistant to glyphosate and ALS-inhibiting herbicides.
Resent research has shown evidence that common lambsquarters is resistant to glyphosate in the mid-Atlantic. Reduced control with glyphosate has been noted in field studies, as well. This research is ongoing and more results will be reported soon.
The list below is of confirmed cases of herbicide resistance found in Virginia. This list was taken from the International Survey of Herbicide Resistant Weeds and last updated on March 16, 2020.
Species |
Common Name |
First Reported |
Mode-of-Action |
Amaranthus hybridus |
Smooth Pigweed |
1976 |
Photosystem II inhibitors (5) |
1994 |
ALS inhibitors (2) |
||
Chenopodium album |
Common Lambsquarters |
1979 |
Photosystem II inhibitors (5) |
Lolium perenne ssp. multiflorum |
Italian Ryegrass |
1993 |
ACCase inhibitors (1) |
Amaranthus retroflexus |
Redroot Pigweed |
1993 |
Photosystem II inhibitors (5) |
Sorghum halepense |
Johnsongrass |
1995 |
ACCase inhibitors (1) |
Poa annua |
Annual Bluegrass |
2001 |
Photosystem II inhibitors (5) |
Sorghum bicolor |
Shattercane |
2003 |
ALS inhibitors (2) |
Conyza canadensis |
Horseweed |
2005 |
EPSP synthase inhibitors (9) |
Stellaria media |
Common Chickweed |
2008 |
ALS inhibitors (2) |
Amaranthus palmeri |
Palmer Amaranth |
2011 |
EPSP synthase inhibitor (9) |
Eleusine indica |
Goosegrass |
2015 |
PPO inhibitors (14) |
What do growers in Virginia think of herbicide resistance?
Surveys were conducted in 2015 to learn more about the spread of herbicide resistance and what growers are doing to mitigate resistance. From 97 responses, representing approximately 10% of Virginia’s corn, soybean, and small grains acreage, we see that many growers in the Commonwealth are combating herbicide resistance through changing herbicide practices such as rotating and introducing new modes of action and reintroducing tillage.
