Poet William Cowper didn’t know it, but his 1785 poem, “The Task,” contained a line that aptly fits 21st century farming. “Variety is the spice of life,” penned Cowper, and variety certainly is key when finding herbicides to combat ever-evolving weeds.
Herbicide resistant weeds have been plaguing producers since 1960 when groundsel developed a resistance to Triazine herbicides, followed by prickly lettuce in 1987 when it developed a resistance to the ALS class of weed killers.
Most recently common ragweed, marestail, giant ragweed, kochia, common waterhemp, and Palmer amaranth have developed resistance to glyphosate in Nebraska. Weed specialists in Nebraska estimate more than 5 million acres of fields are infested with at least one glyphosate-resistant weed.
A recent survey by Iowa State University (ISU) shows giant ragweed, common water hemp and marestail have developed glyphosate resistance in that state. The survey noted 77 percent of respondents have made changes to their herbicide program, up from 52 percent in 2013.
With six species in Kansas also resistant to various herbicides including kochia, marestail, two types of mustard, Palmer amaranth, common waterhemp and redroot pigweed, management of glyphosate-resistant weeds is now the greatest challenge for many producers and land managers in the three states.
In selecting an herbicide spray program, it is not enough that growers use different herbicides. They also must ensure they are using effective herbicides with different sites of action, noted Sarah Sivits, cropping systems Extension educator for Dawson, Buffalo, and Hall counties in Nebraska.
This can provide the diversity needed to help slow the further development of herbicide resistance, she said, noting more than 90 percent of corn and soybean fields in Nebraska receive at least one herbicide application every year.
“It is important to be able to differentiate between site of action and mode of action,” she said. “Site of Action (SOA) refers to the exact site or exact enzyme disrupted by herbicide, while Mode of Action (MOA) is what the herbicide does to the weed, such as a growth regulator or pigment inhibitor.
“Resistance occurs because of repeated use of herbicides and use of herbicides belonging to the same group or MOA over time. Now we are seeing weeds with multiple herbicide resistance and those weeds are no longer controlled by two or more herbicides with different sites of action at label rate (different chemistries); over-reliance or continuous use of these chemistries over time,” Sivits said.
Because of this evolution of herbicide-resistant weeds, a program called, “I Will Take Action” has been developed, encouraging growers to carefully select their herbicides, considering which herbicides belong to which site-of-action group as classified by the Weed Science Society of America. A numbering system has been developed to help growers select their herbicide program using different sites of action. Growers don’t need to remember which herbicide belongs to which site of action, but rather can reference an easy-to-use herbicide classification chart developed by Take Action Against Herbicide Resistant Weeds.
Take Action is an industry-wide partnership between university weed scientists, major herbicide providers and corn, cotton, sorghum, soybean and wheat organizations to help growers manage herbicide-resistant weeds.
At ISU, Meaghan Anderson, a field agronomist in East Central Iowa, and Bob Hartzler, ISU professor of agronomy and Extension weed specialist, noted in a series of articles on the problem, that the slow development of new herbicides versus the fast evolution of herbicide resistant weeds means it's important to maximize the usefulness of every herbicide application.
A new herbicide site of action (or herbicide group number) for use in corn and soybean production has not been discovered since the early 1980s. According to Ian Heap with www.weedscience.org, since the 1980s, the confirmed number of unique cases of herbicide resistance globally is increasing at a rate of about 12 discoveries per year.
Herbicide groups are a relatively new way of determining the site of action of the myriad herbicides on the market. Each site of action has been assigned a number, and most herbicide labels prominently display this group number on the product label. If a product includes two different sites of action, the label will have two different group numbers listed. Keeping track of the herbicide group numbers is the simplest way for farmers to keep track of the different sites of action they’re using in their herbicide program.
Things to consider when determining whether an herbicide is effective against your target weed include whether the herbicide is labeled to control the weed and whether your target weed is resistant to the herbicide group.
According to Anderson and Hartzler, it is equally important for producers to be sure they using an effective rate to manage weeds. To manage weed pests and delay resistance problems as long as possible, Extension educators in all three states encourage crop producers to adhere to Integrated Pest Management or IPM practices.
Noted Sivits, “Number one, identify the pest, evaluate damage, determine needs for control, consider control options, select the best combination of options, and monitor selection.”
Proper weed identification is key, she said, because a number of weeds look similar when small, but react very different to sites of action found in different herbicides.
Weed and crop size is also important, said Sivits, as smaller is best for control and some restrictions come into play on herbicide applications for the crop depending on its’ size.
Understanding emergence patterns (fall, spring or summer emergence) and field history all provide important perspectives on what type of herbicides to apply.
“It used to be that one pass across the field in the spring would be enough to control weeds well into the summer,” Sivits said. “Now multiple applications are usually needed to achieve control. Producers need to think about a spring plan that includes burndown, pre-emergent, early post-emergent plus residual, and late post-emergent applications.”
Rotating herbicides to avoid resistance issues over many years is also a wise option to pursue, Sivits said. Separating herbicides between corn and soybeans is also an important measure to curtail resistance possibilities, noted Extension personnel.
Multiple herbicide-resistant crops (Xtend Beans for example) are useful tools in producer’s toolboxes, but weed specialists caution producers not to over-rely on these herbicides or they can end up with multiple herbicide resistance issues.
Additional tips for producers to consider this spring, according to Sivits, are using weed-free seeds to plant, preventing weed seed production by scouting, using residual herbicides, selecting herbicides based on multiple MOA, being sure to apply at label rate and proper weed stage.
“Because the next ‘silver bullet’ is not coming,” Anderson and Hartzler noted in their series, “we need to protect existing herbicide technologies by using other weed management tactics to prolong herbicide efficacy.”
These include preventing new weed species or new resistant biotypes from getting established in crop fields. Sivits suggested cleaning equipment to prevent spread of seed and maintaining field borders.
“Talk to your neighbors to help keep things clean,” she said.
Cultural tactics, such as altered planting dates, narrow row spacing, increased soybean seeding rates and cover crops, provide opportunities to enhance crop competition or reduce competitiveness of weeds. In fields with intense weed pressure, delayed planting allows early emerging weeds to be controlled prior to crop planting, noted the ISU specialists.
Tillage is yet another option, but does not come without a downside. It increases erosion risks, reduces soil health and requires significant time and money to implement. However, Anderson and Hartzler noted in their series that tillage moves newly produced weed seed from the soil surface deeper into the soil profile: “Waterhemp seeds germinate best when they are within the upper one-half inch of soil due to their small seed. After a failure of weed control, deep tillage is an option to bury seed produced by those weeds. An Arkansas study found that deep tillage alone resulted in an 81 percent decrease in Palmer amaranth emergence over the two following years when compared to no tillage.”
For more information on weed management in Nebraska, including efficacies of various herbicide products against specific weed species, see the Nebraska Extension publication, 2018 Guide For Weed, Disease and Insect Management in Nebraska. It's available in print and in a digital format from the UNL online Marketplace.
Source: Midwest Messenger |