Despite control options

Growers still singing `johnsongrass blues' If farmers have more herbicide options for controlling problem weeds than ever before, then why is johnsongrass still so prevalent in much of the Mid-South and Southeast?

That was the question raised at one of the tour stops at this year's University of Tennessee Milan Experiment Station No-Till Field Day. Tom Mueller, associate professor of plant and soil sciences at UT, gave producers some answers.

Mueller said that johnsongrass can now be easily controlled in the major row crops. Select, Fusilade and other postemergence herbicides control johnsongrass in soybeans and cotton. Accent and Beacon provide control in corn, including rhizome johnsongrass in no-till corn.

"Additionally, Roundup Ready crops provide an excellent means of controlling both seedling and rhizome johnsongrass with postemergence applications of glyphosate," he noted. "So, why is it still prevalent in so many fields? Why do some farmers still have the johnsongrass blues?

Once johnsongrass goes to seed, dormancy mechanisms allow the seed to remain viable for more than five years, says Mueller. So, even if no johnsongrass was present for five years, on the sixth year, johnsongrass plants could germinate from that seed.

"Johnsongrass seed does not always germinate even when conditions are favorable," he said. "In theory, 50 percent of the seed should germinate the next spring, but the reality may be closer to 20 percent.

"There's a lot of truth to the old saying that one year's weeds means seven years of seeding."

In some areas, johnsongrass plants still go to seed in ditches and waterways, along roadsides and railroad tracks or other non-cropped areas, creating a seed bank for the weed even when farmers are controlling it a few feet away.

"These areas can be a source of seed for movement by wind, water (in flood conditions) or wildlife," says Mueller. "Birds can eat the seeds and, in turn, spread them to fields. Johnsongrass is a prolific seed producer, and one clump of johnsongrass can produce more than 80,000 seeds."

Rhizome johnsongrass is a vigorous competitor in no-till systems, and it emerges earlier (when soil temperature reaches 55 degrees) and grows faster than corn or other crops, he notes.

Seedling johnsongrass begins to germinate when soil temperatures reach 65 degrees and is initially less vigorous than plants rising from rhizomes. "In most johnsongrass-infested fields, the majority of plants come from seeds, although the large clumps are probably from rhizomes."

One of the biggest problems posed by johnsongrass for corn growers is its serving as a host for potentially devastating diseases such as the maize dwarf mosaic virus. Aphids can transfer such viruses from johnsongrass to sensitive corn varieties, resulting in significant yield loss.

Mueller and Darren Robinson, an assistant professor at the University of Tennessee, showed growers johnsongrass plots that had been sprayed with Roundup Ultra two, four and six weeks prior to the field day.

While the plots sprayed two weeks earlier contained no vegetation, the four-week-old plots contained several johnsongrass plants. Those sprayed six weeks were completely covered with the weed.

"With herbicides like Roundup that have no residual activity, we're suggesting that you make two applications," said Mueller. "One to get the first flush of rhizome johnsongrass and the second to take out the grasses that emerge from seed."

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