Herbicide-resistant weeds are mostly referred to as superweeds. Weeds that have acquired some characteristics through evolutionary process, that make them more difficult to manage under fields due to repeated use of similar management practices like chemical and physical controls of these wild plants.
Either a wild plant which has become genetically modified due to accidental pollination by genetically modified plants and that is extremely resistant to pests and herbicides or these plants have become resistant to one or more herbicidal mechanisms of action after their repeated use in the area with the absence of more diversified weed control measures.
History
Prior to the era of selective herbicides and GMOs (Genetically Modified Organisms), farmers used to control weeds with hand weeding and tillage practices. Initially, Roundup, having specific ingredient namely “glyphosate” well known for its performance under fields trails was introduced as a non-selective herbicide.
Once Roundup became a selective herbicide in 1990, sales started as Monsanto introduced Roundup Ready crops that were genetically modified to tolerate the chemical. The introduction of glyphosate-resistant crops allowed farmers to spray their fields to kill other wild species while leaving their crops safe.
Soybeans were the first Roundup Ready crops that was genetically modified and made available for commercial use in 1996. Today, Roundup Ready crops account for about 90 percent of soybeans and 70 percent of corn grown in the United States.
However, Bacillus thuringiensis (BT) cotton is the only genetically modified crop in Pakistan and almost every cotton grower prefers BT variety because it is less susceptible to insect, pest and diseases. Seeds of genetically modified crops are mostly sterile and notoriously nicknamed as terminator seeds.
Each year farmers purchase latest strains from international seed selling agencies to continue using glyphosate herbicides, which is not economical and feasible too.
The development of glyphosate-resistant wild plants has led farmers to turn to older herbicides such as dicamba and 2,4-Dichlorophenoxyacetic acid, an ingredient used in Agent Orange, the notorious Vietnam War era defoliant, resulting in the evolution of weed species that are resistant to multiple weed killer chemicals.
Both dicamba and 2,4-D are volatile chemicals that evaporate and can drift far away from their targets, especially in warmer weather, posing a significant public health issues to nearby rural and urban communities.
Causes
- Monoculture
Growing of same crops year after year on the same piece of land has always helped wild plants to get flourished and gain correspondence with the prevailing set of environmental conditions. Most pests, including many weeds, prefer some specific crops over others which allow them to become best adapted to prevailing set of field conditions.
Ultimately it helps them to compete with main crop during early growth period to flourish and multiply over time. These extended populations of weeds increase the likelihood, when a particular herbicide is used, of the existence and selection of rare individual weeds resistant to that herbicide.
- Overdependence on a single chemical herbicide
When farmers use same chemical herbicide every year for controlling wild plants especially perennial weeds, these starts getting resistance against herbicides more quickly and their control becomes more difficult for the farmers. In such condition often, they are forced to spray heavier doses of chemical herbicides even twice or thrice of recommended dose.
- Neglect of other weed control measures
There could be many other ways of controlling weeds like physical, biological and integrated weed management. Agricultural techniques like crop rotation can prevent weeds without complete dependence on herbicides and it can also reduce the chances of developing resistance. The convenience of Roundup Ready system encourages farmers to abandon a range of practices that had been part of their weed control strategy.
Table: List of countries and their super weeds
Country | Species | Common name | First Year | Site of action |
Philippines | Sphenoclea zeylanica | Gooseweed | 1983 | Synthetic Auxins (O/4) |
Australia | Poa annua | Annual Bluegrass | 2017 | ALS inhibitors (B/2) |
China | Amaranthus retroflexus | Redroot Pigweed | Multiple Resistance: 2 Sites of Action ALS inhibitors (B/2) PPO inhibitors (E/14) | |
Pakistan | Phalaris minor | Little canary grass | 2015 | ACCase inhibitors (A/1) |
United States | Lolium perenne ssp. multiflorum | Italian Ryegrass | 2016 | Multiple Resistance: 4 Sites of Action ACCase inhibitors (A/1) ALS inhibitors (B/2) PSI Electron Diverter (D/22) |
Here are some issue resolving strategies:
Herbicide use could be reduced up to 90 percent while maintaining or sometimes increasing farmers’ yields and net income. Following are some agronomic practices to prevent superweeds.
- Use of cover crops as well as artificial and natural mulches
- Relay on minimum or conservation tillage
- Inclusion of those crops in rotation which are less susceptible to superweeds
- Sowing of crops on ridges also helps preventing superweeds
- Biological weed control could be a better choice
- Chemical control should be the last option
- Try to adopt integrated weed management practice
- Taking advantage of the weed-suppressive chemicals (allelochemicals) produced by some crops and crop varieties
- Use of compost, Animal manure and previous crop residues rather than synthetic fertilizers can help to control some weeds
- Organic farming has a potential to avoid chemicals. It will help preventing superweeds significantly
Authors: Muhammad Hamza Latif*, Fareeha Athar, Ahmad Zunair Zaman and Ayesha Mustafa
Department of Agronomy, University of Agriculture, Faisalabad.