The weed-crop connection
Weeds have existed since recorded history, but the tools to control them are relatively new. Before World War II, farmers’ most common weed control was tilling with tractors or horses or applying an inorganic salt to the soil which made it hard to grow anything, crops included.
During World War II, a systemic herbicide called 2,4-dichlorophenoxyacetic acid (2,4-D) was secretly developed as biowarfare, originally intended to destroy potatoes in Germany. Another grass herbicide was being developed to kill rice in Japan.
“The plan was to starve Germany and Japan in to surrender, but 2,4-D was never used for that purpose,” DiTomaso says.
Instead, within a year of the war’s end, 2,4-D was commercially released as an herbicide to control broadleaf weeds, and its affect on agriculture was huge and immediate.
“Synthetic herbicides revolutionized agriculture,” DiTomaso says. “With weeds under control, crops were more robust and yields really took off. Chemical companies developed more and more products, and weed science research exploded.”
In 1940, the estimated market for herbicides in the United States was $1.5 to $2 million. By 1962, American farmers treated over 70 million acres at a total cost of $271 million.
The number of cultivated acres continues to grow and farmers continue to fight their perennial battle with weeds. Of all the pests that threaten crops – weeds, diseases, insects, mites, nematodes and others – weeds are by far the most destructive. Some 65 percent of all pest control time and money is devoted to weeds, as opposed to 35 percent for all other pests.
“You hear about catastrophic pathogens that can wipe out a crop, but those aren’t annual concerns,” Lanini says. “If growers didn’t control weeds every year, they would lose 100 percent of their crop.”
How’s that for economic incentive?
Like any other plant, weeds need water, nutrients and light to grow. The first or biggest or greediest plant to occupy a site has a competitive advantage over the others so the key is to knock down gluttonous weeds before they have a chance to rob crops of the resources they need to thrive. There are many ways to do it – organic and otherwise – and the weeders spend their days testing the efficiency of them all, the better to provide growers an unbiased list of pros and cons. No two fields are alike – and in fact, weed conditions aren’t even consistent within the same field – so the UC research is invaluable for helping growers make economically and environmentally sound decisions to meet their specific needs.
Let’s take a look at some of the current research on organic and non-organic means of fighting weeds in agricultural settings, starting first with non-organic means.
In 1973, Monsanto released the weedkiller Roundup, the trade name for glyphosate, which quickly became little short of a miracle chemical for farmers. It kills a broad spectrum of weeds, is easy and safe to use and binds to soil quickly to become biologically unavailable thus reducing its environmental impact.
Sales really took off in the late 1990s when Monsanto developed Roundup Ready crops genetically modified to tolerate the herbicide, allowing farmers to spray their fields to kill the weeds and leave the crop unharmed. Today, Roundup Ready crops account for about 90 percent of the soybeans and 70 percent of the corn and cotton grown in the United States.
Glyphosate is popular in nonagricultural settings, as well, such home landscapes, parks and schoolyards. And now that it’s off patent, with many generic versions available, glyphosate is cheaper than ever to use.
“It’s such a good product that people are over using it,” Specialist Hanson says.
As Robbins warned, that’s a problem. Just as heavy use of antibiotics helped create drug-resistant germs, widespread use of glyphosate is giving rise to a tenacious crop of resistant weeds.
“Using a single herbicide over a number of years, you’re selecting for plants that aren’t controlled by that material,” explains Specialist Lanini. “Genetic selection means the few weeds that survive Roundup treatment will produce similarly resistant offspring. You build up tolerant populations.”
Roundup Ready crops can add to the problem, he says, because even the crops themselves become weeds.
“For example, if we plant a field with Roundup Ready cotton one year, and then plant Roundup Ready corn there the next year, any volunteer cotton that comes up with the corn will be a Roundup-resistant weed. How do you control that?”
That’s exactly the question Lanini and his colleagues have been charged to answer. Last year the U.S. Department of Agriculture Deputy Secretary Kathleen Merrigan came to the department to personally deliver a nearly $500,000 USDA grant to Associate Professor Marie Jasieniuk and her team for their project “Evolution, Impacts and Management of Glyphosate-Resistant Weeds”.
Jasieniuk’s lab is researching how resistance to glyphosate evolved and spread in the weedy species Italian ryegrass (Lolium multiflorum), rigid ryegrass (Lolium rigidum), hairy fleabane (Conyza bonariensis) and horseweed (Conyza canadensis). Studies include determining the genetic basis of resistance, and identifying the origins, pathways, and mechanisms of spread of glyphosate resistance across the agricultural landscape of California. Jasieniuk’s results, together with the research and extension activities conducted by the other members of the glyphosate-resistant weeds team will ultimately be used to develop economical and environmentally sound strategies for managing glyphosate-resistant weeds and for preventing the further evolution and spread of glyphosate resistance.
The other members of the glyphosate-resistant team are Lanini and Hanson, Weed Scientist Anil Shrestha and Agricultural Economist Srinivasa Konduru from California State University at Fresno; and Farm Advisors Kurt Hembree from Fresno County and Steven Wright from Tulare County Cooperative Extension.
In presenting the USDA grant, Merrigan said she was impressed with both the “outstanding science” under way at UC Davis and the integral role Cooperative Extension plays in projects such as this.
Lanini estimates that glyphosate-resistant weeds now affect 5,000 acres of California farmland – less than one percent of California’s 25,365,000 acres – but, like other scientists, he wants to ensure glyphosate remains viable.
“Roundup has been an awesome product, and I’d like to see it remain that way as long as possible,” he says.
Lanini and Hanson focus on the management aspect of glyphosate resistance in weeds – encouraging growers to not repeatedly expose a weed population to the same select agent – and conducting research to find alternate control methods.
“We’re always testing new herbicides, giving people unbiased information on their strengths and weaknesses, which is really important to growers,” Hanson says.
When you spray herbicides for weed control, there is always the danger of them drifting and damaging nearby crops. And that raises vexing questions for growers and weed scientists.
“We make a lot of field calls, sometimes with a pathologist, helping growers figure out the source of crop injury,” Hanson says. “A lot of crop disorders get blamed on herbicide drift - sometimes rightly, sometimes wrongly - and by the time we see the symptoms it can be hard to identify the cause. Is it disease? Insects? Herbicide drift? Frost? Heat? Drought?”
The questions are sparking new research – forensic weed science.
“We try to recreate the symptoms in controlled settings,” Hanson says, “which allows us to more quickly and accurately identify the source of those symptoms in the field.”
