S-metolochlor Registration Improves Weed Management for Stevia

Author – Katie Chriest

If you’ve enjoyed Coke Zero, Lily’s chocolate, or numerous other sugar-free treats available today, you’ve already savored the sweetness of stevia. From yogurt to energy bars to ketchup to even toothpaste, stevia is the nonnutritive sweetener making myriad products more palatable.

Stevia plant
Stevia plant. Photo from Urban Farmer (ufseeds.com)

As more of us seek ways to avoid the pitfalls of sugar consumption, this appealing alternative, with a taste about 300 times sweeter than sucrose, is being widely embraced for its natural origins and versatility. In fact, as the Washington Post reported last April, “Sales of stevia sweetener — led by Truvia Natural Sweetener — have eclipsed sales of artificial sweeteners including aspartame, sucralose and saccharin. Last August, research firm Nielsen reported that stevia sales had grown 11.9 percent year over year, while artificial sweetener sales were down an average of 6.6 percent.”

Quite impressive, given that stevia’s sweetening extracts, steviol glycosides, have only been on the U.S. market since 2008.

Given all of that demand, increasing production has been essential. That’s why recent registration of S-metolachlor, ushered along by the IR-4 Project, is so encouraging.

Weed control is critical to this specialty crop’s success. “Stevia is a small transplant and it takes a little while for the roots to start expanding and get good top growth,” explains IR-4 Field Research Director Roger Batts, who first ran weed control trials with stevia back in 2011. If those young transplants are forced to compete with weeds, they’re a lot less likely to thrive, thus severely impacting yields. And while the herbicide clethodim, the active ingredient in Select Max plus several generics, was already approved for use with stevia, it only controls emerged grasses.

S-metolachlor (Dual Magnum), which recently received a North Carolina 24(c) Special Local Need (SLN) label, can also control yellow nutsedge, common purslane, annual grasses and broadleaf weeds. It was the first product submitted to the IR-4 Project to help support stevia growers.

“Growers didn’t have a lot of tools to control weeds until S-metolachlor got registered,” explains Dan Kunkel, IR-4’s senior associate director. “Stevia is giving growers a new and expanded market, and this is what IR-4 is doing to support those growers.”

“This is going to be very good for us,” says Batts. “The label is very flexible. You can apply it immediately after transplanting directly over the crop, giving it a chance to grow rapidly without weed competition. That gives you good residual weed control — I’ve seen up to four to five weeks in some of my trials. Up to 30 days prior to the first cutting, the label also allows you to drop nozzle spray it in the row middles and across the base of the plant, which gives you residual control of any weeds that might be coming up late in the season. It allows you to use it post-cutting, before it starts to regrow, broadcast over the top before your next set of growth starts. In the following year, you can broadcast spray it prior to green-up; and then you can go back again, drop nozzle in mid-season to control weeds.”

Batts adds that although other herbicides are coming along, clethodim and S-metolachlor are the only two currently labeled. “Other than that, you have to put a good dose of wraparound out there — wrap your hand around the weed and pull it up.”

The new S-metolachlor label, which is for a crop grown on less than 300,000 acres, should greatly increase stevia’s chances for success.

Stevia (Stevia rebaudiana Bertoni), native to Paraguay, is mainly produced in China, plus a few other Asian countries and parts of South America. In the U.S., production is primarily limited to North Carolina, California and Georgia, though Batts is aware of some being grown in South Carolina and north Florida. Total stevia acreage isn’t currently recorded.

The southeastern U.S. has particularly good potential for more stevia production. “In North and South Carolina, the tobacco and peanut industries have the infrastructure to handle stevia better than some of the other growers might,” Batts explains. “We have a lot of greenhouses to produce the transplants to go to the field, and drying capabilities, because the tobacco growers need both of those. The peanut growers also have drying facilities that with some adaptation can dry stevia leaf down. There have been reductions in acreage of tobacco, so some of the tobacco growers may need an alternative crop that makes use of equipment they already have.” Incidentally, stevia also works well as a rotation crop for tobacco.

There are a few kinks to work out, however. For one, stevia’s tropical heritage means it doesn’t overwinter in cooler climates. “We want to put stevia in perennial production — put it in one year and then get cuttings off of it for up to three years,” Batts says. But that’s occasionally been a challenge, even in the relatively mild North Carolina climate.

Nonetheless, Batts believes that interest in stevia will continue to grow. Hemp is a strong contender to take up some of the Carolina farmland no longer filled with tobacco, but stevia’s popularity is undeniable — even given a slight aftertaste that some consumers don’t prefer. And plant breeding and genetics work is on track to resolve that minor issue.

“Diseases can be a problem, now that you’re not in arid California, but in the humid Southeast,” Batts adds. He notes that David Shew at North Carolina State and Alyssa Koehler at the University of Delaware are on the case. “Through their work, we have projects going on to solve late-season foliar diseases,” he says.

Generally, bugs aren’t considered of real concern to stevia growers. Batts has seen a few insects eating on his crop, but seemingly not to a worrisome level.

Overall, stevia’s future looks bright. It’s a promising crop that produces an all-natural, calorie-free sweetness, finding mass appeal in countless highly sought-after products. And now, with S-metolachlor, growers have been given a new tool to ensure that their crop will be protected from the pressing problem of weeds.

Now that’s pretty sweet.


Katie Chriest is a freelance technical writer in Erie, Pennsylvania; kmc503@psu.edu.