New approaches needed for weed control

Antonio DiTommaso

“The diversity of our New York cropping systems means our growers cannot lay back,” said Antonio DiTommaso, a professor of weed ecology at Cornell University. Glyphosate-resistant horseweed seeds are now blowing around the state and no one is really immune. Other problematic weeds can move in on equipment or with inputs.

But it’s not just a bad news story. Tree fruit growers also have an advantage going into the era of herbicide-resistance, since weed pressure is a less immediate threat to perennial fruit systems.

“I think what the message should be to our tree fruit growers is we understand why your focus is on insect pests and diseases, but all around you are examples of people running into a lot of trouble. So here’s a great opportunity to be proactive,” he said. “They don’t want their backs to be against the wall like some of the field crop guys.”

That’s why two new weed scientists in New York, John Wallace in Cornell’s horticulture department and Bryan Brown at New York State’s IPM program both want to help growers adopt integrated weed management tools and reduce herbicide reliance.

Cultivation, mowing, mulch, weed fabric, cover crops, pre-emergent herbicides and even flamers are all viable tools to consider.

“We need to go beyond ‘how do we kill weeds’ and think about how do we design systems that are going to be profitable, productive and sustainable, and a diversified approach is going to improve long-term weed management,” Wallace said.

Looking forward by looking back

As glyphosate revolutionized weed control as a highly effective, easy-to-use, economical product, farmers across the country were able to move to no-till systems and leave nonchemical weed control methods in the domain of organic growers only.

“It worked magnificently, but we’ve worked it into the ground, and a very valuable tool is going by the wayside because the stewardship has not been what it should have been,” DiTommaso said. “We need to mix it up a little more, and I know it’s extra time and effort, but I’d rather have a product stay effective. That’s what integrated weed management is all about; it’s using our herbicides sparingly.”

John Wallace

There’s no silver-bullet herbicide modes of action in the pipeline, even for field crops, which drive herbicide research and development, Wallace said.

Instead of waiting for a new chemistry, Brown recommends taking a “many little hammers” approach and figuring out which weed control tools work best in your system. Growers who regularly mow down weeds are already doing this, even if they didn’t realize it.

“The idea is that using multiple different weed management tactics, rather than one big hammer of herbicide, such as cover crops, cultivation and just tweaking your production system to disadvantage weeds whenever possible, these can combine to have a big effect,” Brown said.

Growers don’t have to go all in with new management tactics either. Adding any one practice to disadvantage weeds can really improve the efficacy of an herbicide program, he said.

“Another key concept is no single tactic and no single herbicide is going to be 100 percent effective. It’s important to hedge your bets using different practices that disadvantage different weeds so no one type of weed will come to dominate,” he said.

New ideas and new research

One upside to all this is that it’s an exciting time to be a weed scientist, Wallace said, and herbicide-resistance has spurred lots of new research.

Cultivation is making a comeback, but with changes. New precision systems use cameras and robotic sensors to ensure cultivators move in and out of rows as they detect trunks or vines, Wallace said. He’s hoping to test these tools, mostly developed in Europe where herbicide regulations are tighter, in New York soon.

Bryan Brown

“A simpler innovation, while we are waiting for robots, is the use of our current cultivation tools in different setups and adjustments to make them more effective,” Brown said.

He’s currently experimenting with stacking tools that uproot, cut and bury weeds on a cultivator so a single pass can take a broader aim at a range of weed species. Tests in vegetable crops so far show efficacy can double or even triple. He hopes to test out similar strategies in tree fruit systems.

Decision-aid models are also in development that could make cultivation more effective. By using degree day models to predict weed seed germination, you could time cultivation for maximum impact, just like some insect models allow growers to spray at peak flight.

“If I wait another day or two for 30 percent more of my palmer amaranth to emerge, I’ll get more bang from cultivation. With the rise of resistance, we’re going to need this kind of precision,” DiTommaso said. “This is where weed science is going to catch up with our plant pathology and entomology colleagues.”

Wallace agrees that improved weed control is likely to come from better understanding of weed biology and ecology. But approaching management species by species like growers do insect pests won’t work because in many cases there are many different species present.

“Growers really need to think about managing weed communities,” he said. “More and more we think about the functional traits of weeds, not necessarily that they are grass or broadleaf, but what traits allow them to be adapted to our system.”

Overall, it’s important for growers who don’t have herbicide-resistance problems yet to start preparing for their arrival by experimenting with additional control methods, the weed scientists agreed. The good news is there is time to adapt, but not time to be complacent.

“I’m optimistic that because it’s such a widespread issue, it’s going to provide this opportunity for us to rethink weed management, and it’s going to breed better integration and control tactics,” Wallace said. •

—by Kate Prengaman