Weed resistance is an urgent problem that is rapidly growing and spreading around the world. Non-diversified weed control programs that recurrently use the same herbicide Mode-of-Action (MOA) actively select for resistant weeds that are no longer responsive to this MOA. These in turn reproduce, yielding resistant weed seeds. This ongoing evolutionary process results in a population shift towards more and more resistant weeds.
The U.S is the world’s dominant “hot-spot” with regards to weed resistance, but this issue is an emerging major global concern. According to estimations made in 2016, at that time, in the U.S alone, over 100 million acres (out of 250 million acres of cropland) were infested with herbicide resistance, and over 65% of U.S crop growers had resistant weeds present in their fields. Moreover, according to weed experts’ predictions, the amount of land infested with weed resistance in the U.S is expected to reach 164.5 million acres by 2020.
In the last 30 years no new commercial herbicide MOA has been introduced, and resistance already exists against 23 out of the 26 existing MOAs. Moreover, many cases of weeds with multiple resistance have been described, including a case in Australia of resistance against as much as 7 different MOAs.
As the presence of uncontrolled weeds is devastating for crops due to competition for resources (water, nutrients and light), new strategies for overcoming weed resistance are essential for maintaining food security.
Palmer Amaranth, a summer annual weed, was ranked as the most troublesome weed in the US in surveys published in 2016 and 2017 (Weed Science Society of America). Palmer amaranth is a very aggressive weed, with a rapid growth rate that can reach 2 to 3 inches per day. It is a very prolific seed producer, which can produce up to a million seeds per plant. Palmer amaranth is a dioecious species (separate male and female plants) that reproduces by outcrossing. This property leads to high genetic variation, which allows it to readily adapt to new environmental conditions and thus quickly develop weed resistance. Due to its aggressiveness, when resistant it is very hard to control.
It has been reported that the costs of controlling resistant Palmer amaranth have increased farmer expenses dramatically, and in some cases even caused growers to abandon their fields. According to the USDA, Palmer Amaranth has been found to cause yield losses of up to 91% in corn and 79% in soybeans.
Palmer Amaranth has already evolved resistance to the following herbicides modes of action: ALS inhibitors, Photosystem II inhibitors, HPPD inhibitors, Microtubule inhibitors, PPO inhibitors, Synthetic auxins (dinitroanilines) and EPSPS inhibitors (glyphosate).
Waterhemp (Amaranthus Tuberculatus)
Waterhemp was ranked as number 4 and 5 in surveys of the most troublesome weeds in the US (Weed Science Society of America), published in 2016 and 2017, respectively. Waterhemp is a summer annual weed that is characterized by a rapid growth rate that can reach up to 1 inch per day, and can produce up to a million seeds under ideal growing conditions. It was demonstrated that season-long competition by waterhemp reduced soybean yields by 44%.
Waterhemp is dioecious and accordingly reproduces by outcrossing, increasing its population’s genetic diversity and its chances for resistance development and spreading.
Waterhemp resistance is a major problem. In 2017 it was published that in Iowa, 100% of the waterhemp population is resistant to at least one herbicide MOA, and 69% of its population is resistant to three herbicide MOAs in parallel. The resistance levels that were reported are astonishing: 100% for ALS inhibitors, 97% for PSII inhibitors, 98% for EPSPS, 83% for PPO inhibitors and 72% for HPPD inhibitors. Management of multiple herbicide resistances is a significant challenge for farmers.
Rigid Ryegrass (Lolium Rigidum)
Rigid ryegrass is a widely distributed and troublesome weed in many countries. In Australia it has been classified as the most economically damaging weed. It can reach very high densities (>100 plants/sqm) and produce up to 45000 seeds/sqm. It was shown that an infestation of 200 plants/sqm can cause a 20-50% yield loss in wheat. It is a wind-pollinated out-crosser with a high genetic variability, enabling it to adapt to different environments quickly. Rigid ryegrass was the first example of a weed demonstrating multiple resistance to many herbicide families and is currently resistant to 13 different MOAs. These include ACCase inhibitors, ALS inhibitors, DOXP inhibitors, Microtubule inhibitors, Mitosis inhibitors, Long chain fatty acid inhibitors, Lipid Inhibitors, Carotenoid biosynthesis, Photosystem II inhibitors, EPSP synthase inhibitors, PSI Electron Diverter, Lipid Inhibitors and PPO inhibitors.