Rapid Spread of Resistant Weeds
The spread of Palmer amaranth and waterhemp has been astonishing, and it continues at a rapid pace because those surviving are usually resistant to glyphosate and herbicides with other modes of action.
“A Palmer amaranth plant can produce a million seeds and is very competitive. It is a phenomenal species, and waterhemp is not far behind in seed production. Both are spread by seed and pollen. These two weeds have both male and female plants, and, therefore, are obligate outcrossers. Outcrossing allows genetic mixing and increased biodiversity of the species from which resistant individuals can be selected,” explained Glasgow.
U.S. TOPS RESISTANT WEEDS LIST
Late spring burndown herbicide application definitely requires a combination of different mode of action herbicides to counter any resistant weeds, especially with the size of weeds being treated. Weed species characteristics along with the use of one herbicide over and over leads to weed resistance.
The U.S. easily tops the list of countries with the most herbicide-resistant weeds at 144, according to the International Survey of Herbicide Resistant Weeds. Next most is Australia with 62 herbicide-resistant weeds followed by Canada.
All three countries have focused attention on herbicide resistance with university Extension or equivalent, commodity crop associations, the crop protection industry and others determined to identify weeds resistant to specific herbicides and then provide solutions to assist growers in combating resistant weeds while economically producing high-yield crops.
We hear much less about the e_ orts to assist growers, but rather more about banning herbicides, by the next level of countries with herbicide-resistant weeds: China, 34; France 35; Germany, 33; Spain, 33; and Brazil 31. And it wouldn’t be a farfetched assumption that some countries with many fewer herbicide-resistant weeds either haven’t been using as many modern intensive farming techniques, including herbicides, or really haven’t focused on identification of resistant weeds.
Worldwide, 24 weeds have been identified as resistant to glyphosate, and 14 of them are in one or more states of the U.S. Also worldwide, glyphosate ties for tenth for the number of weeds that are resistant to it. Atrazine leads the list with 64 resistant weed species identified worldwide.
Second internationally is imazethapyr with 39 resistant species. In descending order, the herbicides with number of species resistant are tribenuron-methyl, 35; imazamox, 33; chlorsulfuron, 31; simazine, 31; fenoxaprop-P-ethyl, 28; metsulfuron-methyl, 26; paraquat 26; bensulfuron-methyl, 24; _ uazifop-P-butyl, 24.