Marestail: Big Potential Populations After 2015’s Warm Fall

looking-aheadThe warm fall conditions we experienced in 2015 could mean more marestail pressure this spring, say weed scientists in parts of the Midwest and South where the resistant weed proliferates.

It’s still too early, of course, to accurately say how marestail will stack up. If February weather turns bitterly cold, that might trigger higher mortality rates. But warm conditions in the fall of 2015 gave the weed a big headstart for 2016.

“We had a very warm fall in Indiana, even into December and we only got into winter temperatures around the start of the year,” notes Travis Legleiter, a Purdue University weed scientist. “It would be difficult to say how much marestail did emerge in November and December, but conditions certainly would have been there for it.”

The warm fall gave growers more time to make burndown applications or do fall tillage, which helped mitigate marestail in places. But anybody who waited into November to address winter weeds may have been caught by rainfall that stayed with us well into December.

“Where growers didn’t or couldn’t make a fall burndown application, marestail got established,” says Legleiter. “That gives it the ability to gain size much faster in the spring compared to what we’d normally expect when fall temperatures are more seasonally cool.”

But Legleiter doesn’t count on winter weather knocking out significant numbers of fall-established marestail in Indiana. “Winter is just part of the weed’s lifecycle, and it goes dormant in the winter, anyway,” he adds.

Where growers couldn’t make a fall burndown spray, it will be imperative to make the spring burndown when plants are still in the rosette stage, he says. “Even if the rosette is still pretty big, we can control it. But once it starts to bolt and grows upright, control can fall off quickly.”

“Before you plant soybeans, we want a residual on the ground ahead of beans emerging.”

Timing means hitting a window sometime between when marestail begins actively growing again and before bolting starts and the weeds are less susceptible. The earlier a grower can start spraying, the better, Legleiter emphasizes. “You can’t do this based on the calendar,” he says. “Keep checking and make a reasonable determination.”

If the burndown doesn’t work or it’s too late to spray, “tillage is always an option” for many Indiana growers, he says. “But before you plant soybeans, we want a residual on the ground ahead of beans emerging.”

Marestail also made an early, vigorous start in parts of the South. In areas with rolling ground and a need to preserve winter cover, fall burndown and tillage generally aren’t an option, so any marestail that made its start in the fall has at least some chance of making it into spring.

“We tend to have a very high mortality rate in Tennessee (due to winter weather), especially in the last couple of winters,” points out Larry Steckel, Tennessee Extension Weed Specialist. “On the other hand, marestail can be more of a challenge if more of it emerged in the fall and then you have a warmer winter. It’s better established, with a deeper taproot going into the spring. We had 70 degree days into December, so we do have some marestail that had good early conditions.”

If enough cold weather develops in February, that could work against the marestail. “If that doesn’t happen, we’ll probably have to be more aggressive with burndown applications in March and April,” he adds. “That probably means repeat applications, maybe Roundup and dicamba early, and then a second treatment with something else to try to finish off the bigger ones that got past the dicamba.”

Marestail, he says, can emerge in Tennessee as early as August, so some plants that started early had a long, favorable stretch last fall to become part of the landscape.

“Once it bolts (in the spring), it’s difficult to deal with, and we get into these three-way tankmixes with something like 2,4D, Sharpen and Roundup or dicamba, Sharpen and Roundup.

More acreage appears to have been cover cropped in the fall, and that will help with marestail control this spring, Steckel says. “Marestail isn’t very competitive if you have cover crops out there, and that warm December was just what cover crops needed to take off.”

Keep your Coke can handy

Plenty of marestail also is waiting for farmers in north Alabama, notes Larry Walker, who operates Walker Cotton Technical Services, a firm that covers the region’s Tennessee River Valley.

Besides the warm fall weather, plenty of rain fell, so we have a generous population of marestail,” says Walker. “With our rolling ground, we stay away from fall tillage or fall burndown applications, and everything depends on how well we time our spring burndown applications – usually in February or March – with dicamba, 2,4D or that combination, plus residuals.”

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As marestail became resistant to Roundup, its physiology also seemed to change, Walker adds. In the past, it only emerged in the winter. Now, though, it can come up in any month of the year, given the right conditions, Walker says.The common goal in his area is to hit marestail before it’s any taller than a can of Coca-Cola. “Nobody has a 12-inch ruler with them,” says Walker. “But we generally always have a Coke can, and we’ve found that we need to treat before marestail gets taller than the can, which is in the range of 5 to 6 inches. If you spray after that, control will be less than satisfactory.”

“You’ll see generous and continuous flushes at times,” he continues. “At the end of May in fields that haven’t been sprayed you’ll find marestail from over knee high to just emerging. Because of that, if you just put out Roundup and atrazine, you won’t kill it. We have to pay attention to herbicides that will do a good job in both the burndown phase and as residuals.”

PPO-type materials have done well on marestail, Walker says, but overreliance on that chemistry will almost certainly trigger PPO resistance in marestail. Every time Walker makes a spring herbicide recommendation, he keeps that in mind. As much as possible, he tries to “mix it up,” bringing in different herbicide families and including older materials to diversify the program.

“And we’ve got to use full rates,” he emphasizes. “Our standard rate of dicamba at one time was 8 ounces an acre, but that’s on the low side now. We now go with 10 to 12 ounces an acre. You get one chance to gain control, and skimping on 2 to 4 ounces an acre doesn’t make sense.”