Herbicide resistance an increasing phenomenon

As the instances of weed resistance grow in number, farmers will face higher costs for controlling those weeds and perhaps even losses on crop yields.

That was the message from Darrel Daniels, a Syngenta regional agronomist based at Hartford, to attendees at the 2017 annual meeting of the Calumet County Forage Council. He noted that individual weed plants can produce hundreds of thousands or even a million seeds.

Incidence of resistance

Daniels described how resistance to certain herbicides by weeds that are prevalent in Wisconsin has generally come from the south and cited some of the measures that farmers have resorted to in order to cope with certain weeds, particularly Palmer amaranth. Those measures include burning both the seed-laden weed plants and the soybean residue from fields infested with resistant weeds.

About one-third of Wisconsin's 72 counties having at least one case of confirmed or suspected resistance by water hemp to glyphosate while three counties have some Palmer amaranth that's resistant to glyphosate, Daniels pointed out. Those confirmations have been made in laboratory tests at the University of Wisconsin in Madison or through a $50 tissue assay performed by the University of Illinois.

Through 2005, no Palmer amaranth resistant was detected in Arkansas but by 2010 it had spread to one half of the state's counties, Daniels reported. In Illinois, a great majority of the counties have confirmed cases of Palmer amaranth resistance, he added. Another problem in Illinois is that some water hemp has resistance to both glyphosate and PPO active ingredients, he observed.

Control measures

Controlling the two weeds in soybeans is assured only with the small portion of soybeans with the Liberty Link trait for herbicide tolerance, Daniels indicated. The introduction of RR Xtend soybeans, which tolerate dicamba herbicide, is not an answer because it is not very effective with water hemp or Palmer amaranth, he stated.

With water hemp being a major concern in Wisconsin, one way to control the spread of weed seeds is by a thorough cleaning of combines, Daniels advised. He acknowledged that this is a tough task, given that a combine is likely to contain 125 to 150 pounds of grain and bio-matter when shut down.

For the most part, however, the recommended and higher cost practice is to apply both pre-emergent and post-emergent herbicides to corn and soybeans and to have at least two different modes of action in a given year, Daniels stated. Because much of the water hemp doesn't germinate until early July, it is also important to use a Group 15 chemistry post-emergent herbicide with a residual, he explained.

Farmers who also grow alfalfa need to control weeds when corn and soybeans are being grown rather than trying to do so within an alfalfa stand, Daniels emphasized. For a burndown herbicide, he suggested gramoxone, noting that Syngenta has cut its price by 50 percent for 2017.

If there's one good point about water hemp and Palmer amaranth, it's that their seeds retain viability for only a few years compared to the much longer period for the more traditional field weeds such as ragweed an pigweed, Daniels noted.

Farmers and crop consultants who suspect the presence of resistant weeds, even after full rate application of the appropriate herbicide, can report this to their product supplier, whether Syngenta or another company, Daniels observed. He noted, however, that confirmatory testing would be referred to the Extension Service in Wisconsin or Illinois.