Aggressive weed sending roots into Illinois fields

2014-04-10T07:00:00Z Aggressive weed sending roots into Illinois fieldsBy Amber Selman, Illinois Farmer Today Illinois Farmer Today

BELLEVILLE — Farmers wear a lot of hats and with the increase in glyphosate resistance, weed expert has become a higher priority.

However, instead of knowing everything about every weed, many farmers are relying more on local experts to help them make the best management decisions for their operations.

When it comes to Palmer amaranth, it is vital for farmers to enlist the help of these experts.

“Palmer amaranth has become a big problem in the last four to five years because we have unknowingly selected populations that are glyphosate resistant,” says Ron Krause, researcher and farm manager at the Southern Illinois University-Carbondale Belleville Research Center.

This can be a dangerous weed if not controlled properly. Palmer amaranth grows rapidly, almost two to three inches a day. It comes from the desert Southwest so the plants have a very long tap root to help dig deep for their water source.

This origin also helps Palmer adapt to various conditions. With a limited time to germinate and grow in the desert-like conditions, it takes advantages of the good growing conditions in the Midwest.

“This weed is very adaptable. It has been found as far south as Georgia and as far north as Canada,” Krause notes.

“Palmer amaranth is probably more widespread in Illinois than what farmers may think. In 2014, Palmer will likely be found in almost every county.”

Not only is the adaptability and large footprint of Palmer amaranth a concern, but each plant, males and females, can produce up to 1 million seeds per plant.

In addition, seeds can travel as far as 1,000 feet or almost a quarter of a mile. This becomes a big worry when it comes to resistance.

Palmer amaranth has been in Illinois for many years. But, in recent years it has become an even bigger problem because farmers have seen growth in a resistant population.

“Ten years ago, Palmer wasn’t glyphosate resistant, but there is always a biotype that has resistance and can be selected for prevalence,” Krause explains.

There are two main reasons for this growth.

Because this weed can be easily confused as waterhemp in the early stages, many farmers take a pass with glyphosate to control the weed.

Second, when Roundup Ready soybeans became available, many farmers stopped using soil herbicides.

Economics at the time just made more sense to use the herbicide that would kill everything — except those resistant biotypes.

“Fortunately, we never got into that pattern with corn,” notes the St. Clair County researcher. “So, the best way to start and prevent the resistant strain of Palmer amaranth is to begin in a corn year of rotation.”

That is only step one.

During a 12-year study on weeds such as Palmer amaranth, Krause tested for the best approach to controlling tough weeds.

On a corn-soybean rotation he tested three herbicide programs: a pre-emergence only, a pre- and post-emergence herbicide program and a post-emergence only.

“We saw consistent weed control with the pre- and post-emergence herbicide program,” Krause says. “We saw that 99 percent of the weeds were not a problem.”

He also notes the trial showed signs of trouble with a post-emergence-only program.

“Weeds would get too tall to spray, and the weather may delay the perfect time to spray the weeds,” Krause says.

A mid-season herbicide also may be necessary because Palmer amaranth can germinate from May to August. This is where narrow rows also come into play.

By establishing overlaying canopies, it reduces the chance of late-season germination.

Along with crop rotation, narrow rows and a pre- and post-emergence herbicide program, using multiple modes of action is another addition when combating this species.

“If we can manage this weed correctly, it won’t become resistant to any herbicides and will be easily managed in the future,” Krause notes.

“But, farmers need to realize that resistance is real and is not being fabricated by anyone source. It is here and that is why management of Palmer amaranth and other weeds is essential.”

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