Many commercial growers spray chemicals to rid crops of weeds, but few actively consider the possibility of herbicide resistance. Yet the first known reported case of herbicide resistance occurred back in 1957 when a spreading dayflower spotted in a sugarcane field became resistant to a synthetic auxin herbicide, according to the Weed Science Society of America (WSSA). This spreading dayflower withstood five times the usual herbicide treatment!
Since 1957, WSSA reports that 250 species of weeds now have resistance to 160 different herbicides. That spans 23 of the 26 known herbicide mechanisms of action. It’s not just a concern in the U.S. either; in fact, 86 crops in 66 countries are affected. It’s a global problem.
“Given all the media attention paid to glyphosate, you would think it would have the greatest number of resistant weed species,” said Mississippi State University weed scientist David Shaw in a WSSA press release. “Though there are currently 35 weed species resistant to the amino acid synthesis inhibitor glyphosate, there are four times as many weed species resistant to acetolactate synthase (ALS) inhibitors and three times as many resistant to photosystem (PS) II inhibitors.”
According to WSSA, scientific research has shown that resistant weeds may evolve whenever a single approach to weed management is used repeatedly. This likely means the lack of crop rotation, a necessity for good weed control. Growers are advised to practice an integrated approach to weed management using different cultural and chemical controls. But these controls can get expensive.
“Although diversification is critical to crop sustainability, it can be difficult to make a decision to spend more on integrated weed control strategies,” University of Georgia weed scientist Stanley Culpepper said in the WSSA release.
He suggested high costs are not the only concern of alternative measures in an integrated weed management plan. Some growers believe newer chemicals will magically appear in the near future to solve this magical mystery of herbicide resistance.
“It would be naive to think we are going to spray our way out of resistance problems,” Culpepper said. “Although herbicides are a critical component for large-scale weed management, it is paramount that we surround these herbicides with diverse weed control methods in order to preserve their usefulness – not sit back and wait for something better to come along.”
Besides, as mentioned earlier, herbicide resistance has been around a while and likely isn’t going anywhere. Shaw said resistance issues aren’t limited to a single herbicide and were present before “glyphosate-resistant, genetically engineered crops were even introduced.”
Without a doubt, growers are wise to practice an integrated weed management plan that includes different cultural and chemical practices. “Farmers around the world will be responsible for providing a balanced plate for more than 9 billion people by 2050,” said Monsanto’s Chief Technology Officer Robert Fraley in a company news release. “To do it, we will need to effectively double the food supply. Science and innovation play an important role in meeting this challenge.”
So who knows, growers may someday – sooner than later – see other chemicals or innovations to combat herbicide resistance while feeding a larger population worldwide. Monsanto and other chemical companies will need to ramp up their research and development departments so growers can raise more crops on their existing farmland and reduce or maybe even eliminate weed, insect and disease issues so they can harvest and feed the world.
Part of that research and development may focus on plant breeding, plant biotechnology and crop protection, according to Monsanto.
Key reminders for good weed control, according to Bradley Majek of Rutgers University:
- Control weeds before the vegetable crop is planted.
- Identify weeds present in your field.
- Control any established perennials before the crop is planted.
- Apply cultural, mechanical and chemical weed control practices.
- Research and choose herbicides that control the weeds present in your field.
- Follow label directions and apply the right rate.
- Avoid crop injury when incorporating or spraying herbicides.
Herbicide application reminders
Until Monsanto and other company leaders solve the food and herbicide resistance problem, universities remind them about good weed control. For instance, the University of California reminds growers that preemergence herbicides should be soil-applied and mechanically mixed with the soil. They also should irrigate the soil at this time before any weeds appear. These preemergence herbicides also reduce any weed seeds that may germinate. However, do not apply them to dry seed. Remember, to reduce weeds, control them before the vegetable crop is planted.
As another reminder, growers spray postemergence herbicides on the weed plant foliage, according to the University of California. Leaves and stems can absorb the herbicides to reach the target area of attack. As a warning, make sure plants are not under stress so the mode of action can work properly, reaching its destination and test to see if the soil is moist enough (not too wet) for the chemicals to work as labeled.