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Floodwaters likely helped spread resistant weed seeds to brand new areas.

P.J. Griekspoor, Editor

November 5, 2019

4 Min Read
Resistant Palmer amaranth overtakes a field of soybeans
WEED WOES: Resistant Palmer amaranth, shown here, can overtake a field of soybeans. A scientist with Syngenta warns that producers could face higher pressure from weeds in 2020 because of the extremely compressed season in 2019 and widespread flooding that spread seed to new areas. P.J. Griekspoor

It would be hard to find a farmer who isn’t relieved to be leaving the weather challenges of 2019 behind and looking toward a better year in 2020.

However, agronomists and plant pathologists are warning that 2019’s challenges could spill over into next year’s growing season, especially where problematic weeds, pests and disease are concerned.

Big numbers of prevent pant acres saw heavy growth of weeds and prolonged wet weather made it hard for many farmers to make the herbicide applications they needed to prevent weed escapes. At the same time, floodwaters likely carried in new weed seeds so scouts in 2020 could see weeds they are not used to dealing with popping up in fields.

Floodwaters also provide a path for the soybean cyst nematodes to move into new fields. Add in early winter weather and snow in many areas, and you have unharvested crops in the field that can harbor disease pathogens and insects.

Just in case that’s not enough depressing thoughts to spoil the holidays, there’s also more and more weed species that have developed resistance to both glyphosate and PPO-type herbicides, and those populations were probably spread by both extreme wind events and widespread flooding.

All of those factors increase the need for careful decision-making ahead of the 2020 planting season, according to Dane Bowers, technical product lead of herbicides with Syngenta.

“The most problematic weeds we are seeing are those that have developed herbicide resistance — especially water hemp and Palmer amaranth, which have expanded dramatically. Those weeds are resistant to both glyphosate and PPO type herbicides and that makes control very difficult.

“We also have problems with giant ragweed, marestail and kochia. The spread of these weeds means we not only need to use the herbicide options that we have available, but we also need to use other cultural practices including managing the weed seed bank and looking at tillage options or cover crops.”

Bowers says producers should prepare to face problems in 2020 because of the severely compressed season in 2019 that resulted in producers not being able to use all of the best management practices that they had planned.

“Many growers were put in a situation where they had to abandon their weed management pan and adjust on the fly. The weather just wreaked havoc,” he says.

Acuron herbicide

Growers planting corn in 2020 will be able to use Syngenta’s corn herbicide, Acuron, which has three sites of action.

“The data from up to 18 replicated Syngenta and university trials for comparison showed 5 to 15 more bushels per acre than competitors when applied preemergence at full labeled rates,” he said.

Bowers says there is often a temptation during cycles of low commodity prices to skip or reduce herbicide application to save money. But in the end, protecting yields will more than pay for the cost of herbicide.

Bowers recommends a two-pass program using half the labeled application of Acuron before crop emergence and then coming back over the top to kill any weeds that emerge. This will provide a longer period of residual control.

“Weed seed has a long period of dormancy. When grown in the absence of crop competition, a waterhemp plant can produce up to one million seeds. In a crop, that number might be 200,000 to 300,000 seeds. Huge numbers either way,” he said. “I recommend that growers who notice an escape here and there take action to manually remove that weed so it doesn’t add to the seed bank.”

For soybeans, Bowers recommends growers consider Tavium Plus VaporGrip Technology herbicide, the only dicamba product with two active ingredients.

“Tavium has good residual control, but I think growers should plan a two-pass system of strong preemergence herbicide followed by Tavium early post-emergence. The goal is to prevent water hemp and Palmer amaranth from emerging. Dicamba has been a really good tool to manage resistant weeds and we certainly don’t want to see those weeds becoming resistant to it. Tavium is unique in that it is the first and only premix residual dicamba herbicide, so in addition to dicamba there is S-metolachlor premixed for convenience.”

He said Tavium is also labeled for cotton.

Mechanical weed seed management

Moving forward, Bowers says producers should consider adding mechanical harvest weed seed management in addition to herbicides. He says a machine invented in Australia has become popular there. It pulls behind the combine to collect chaff and weed seed, then grinds the weed seed into dust. Some growers have also used machinery that creates narrow windrows of chaff and weed seed and burn those windrows.

“Now, there is an integral unit available that is mounted on the combine to destroy weed seed and it has proven effective at reducing the seed bank in field testing,” he says.

He says that reducing the number of weed seed in the soil is critical because seed have a very long lifespan, with some that can be viable for a decade or more.

One more legacy of the weather in 2019 was the movement of pests such as soybean cyst nematode, which can float on water and are likely to have been deposited in new fields by flooding.

He suggested soil sampling on a wide basis to re-establish geographical distribution and population levels of SCN, which causes extensive root damage and makes plants more vulnerable to sudden death syndrome.

About the Author(s)

P.J. Griekspoor

Editor, Kansas Farmer

Phyllis Jacobs "P.J." Griekspoor, editor of Kansas Farmer, joined Farm Progress in 2008 after 18 years with the Wichita Eagle as a metro editor, page designer, copy desk chief and reporter, covering agriculture and agribusiness, oil and gas, biofuels and the bioeconomy, transportation, small business, military affairs, weather, and general aviation.

She came to Wichita in 1990 from Fayetteville, N.C., where she was copy desk chief of the Fayetteville Observer for three years. She also worked at the Pioneer Press in St. Paul, Minn. (1980-87), the Mankato Free Press in Mankato, Minn. (1972-80) and the Kirksville Daily Express in Kirksville, Mo. (1966-70).

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