Herbicide resistance — evaluating hard-to-control weeds

The development of weed biotypes that are resistant to commonly used control measures is becoming an increasingly significant problem in California and around the world.

As of January 2010, there have been 194 confirmed reports of herbicide resistance affecting 19 different herbicide mode of action families (Heap 2010). Of these cases, about 60 percent are broadleaf and 40 percent are grass or sedge weeds. In the U.S., 128 resistant species have been reported (also 60 percent broadleaf weeds) and 15 herbicide families are affected. California has 21 individual cases of resistant weeds impacting 7 herbicide families; however, in contrast to the rest of the world, two-thirds of California’s resistant species are grasses and sedges due primarily to tremendous selection pressure for grass weeds in rice production.

When we consider what herbicide resistant weeds might become important in California in the future, we should focus on weeds that already have reports of resistance in other parts of the world.

The 10 most important herbicide resistant species in the world (Heap 2010) are all present in California and can be problematic weeds even without herbicide resistance (Table 1).

Table 1. Worst herbicide resistant weeds worldwide (based on number of infested sites)

	Present in CA	Resistance outside CA	Resistance in CA
Rigid ryegrass (Lolium rigidum)	√	8 MOA	√ glyphosate
Wild oat (Avena fatua)	√	6 MOA	√ difenzoquat
Redroot pigweed (Amaranthus retroflexus)	√	3 MOA
Common lambsquarters (Chenopodium album)	√	4 MOA
Green foxtail (Seteria viridis)	√	4 MOA
Barnyardgrass (Echinochloa crus-galli)	√	7 MOA	√ ACCase, thiocarbamate
Goosegrass (Eleusine indica)	√	4 MOA
Kochia (Kochia scoparia)	√	3 MOA
Horseweed (Conyza canadensis)	√	5 MOA	√ glyphosate, paraquat
Smooth pigweed (Amaranthus hybridus)		2 MOA

Four of those (three grasses and one broadleaf) are already known to be resistant to at least one herbicide in California. These 10 species have individual reports of resistance to many different modes of action (MOA) and we can expect that new cases may show up in this state over time. Additionally, other important weeds in these genera also are a likely risk for resistance in California (eg. Lolium spp., Amaranth spp., Echinocloa spp., and Conyza spp.).

A relatively recent trend in the development of herbicide resistant weeds has been the discovery of glyphosate-resistant weeds.

Although glyphosate (the active ingredient in Roundup, and others) has been used for nearly 40 years, no resistance was reported until 1996. However, since 1996, sixteen glyphosate-resistant biotypes have been reported around the world (Table 2).

 Table 2. Worldwide reports of glyphosate-resistant weeds

	Resistance USA	Resistance CA
Palmer amaranth (Amaranthus palmeri)	√
Common waterhemp (Amaranthus rudis)	√
Common ragweed (Ambrosia artemisifolia)	√
Giant ragweed (Ambrosia trifada)	√
Hairy fleabane (Conyza bonariensis)	√	√
Horseweed (Conyza canadensis)	√	√
Sourgrass (Digitaria insularis)
Junglerice (Echinochloa colona)
Goosegrass (Eleusine indica)
Wild poinsettia (Euphorbia heterophylla)
Italian ryegrass (Lolium multiflorum)	√	√
Rigid ryegrass (Lolium rigidum)	√	√
Ragweed parthinium (Parthinium hysterophorus)
Buckhorn plantain (Plantago lanceolata)
Johnsongrass (Sorghum halapense)	√
Liverseedgrass (Urochloa panicoides

Nine of these are already in the U.S. and four are known to be widespread in California. Similar to the earlier discussion, many of these species or close relatives are common throughout the state and are of great concern with regards to losing efficacy of this important broad-spectrum herbicide.

While this discussion is simply one guess as to which herbicide resistant weeds may become problems in California in the future, we can be certain that resistance is likely to remain a significant problem. Rapid and committed adoption of resistance management practices is critical for the preservation of important tools like glyphosate and other herbicides.

Source: Western Farm Press