by Sally Colby

Farmers have planned crops and ordered seed for 2019, but many haven’t considered a weed management program for those crops.

Dr. Bryan Young, weed biology and Extension specialist at Purdue University, said dealing with weeds today is different than in the past, with new invasive species and the increasing issue of resistance to available herbicides.

“It’s important to know what you’re looking at and what you want to control with management practices,” said Young, listing waterhemp, marestail and Palmer amaranth as top challenges for soybean and corn growers. “Farmers have long-term plans for managing crops and soil, but they often lack plans for weed management.”

Young said the ag industry has failed to adequately deal with herbicide resistance. “We’ve ignored it,” he said. “We’ve been planning for this year, maybe a little for next year, but in general, we manage weeds on an annual basis. We made it through harvest and we’ll think about what we’ll invest on weeds later.”

Although most farmers plan crops one or two years ahead, Young suggested thinking about the year 2025. “How are you going to manage weeds then, and how are you going to get there?” he asked. “If you don’t take steps now, you can’t do it overnight.” Young added that farmers will always be at the mercy of products available for use in a given year.

One goal in managing difficult weeds is not allowing them to go to seed. “I don’t care if velvetleaf, lambsquarter or common ragweed goes to seed,” said Young, “but one of the biggest long-term problems is marestail, and control isn’t going to happen with one herbicide application. It’s going to start with knock-down, then another product to clean it up.” Young added that while many farms aren’t yet dealing with Palmer amaranth, it’s important to manage crop fields as if it were already present.

Most farmers make weed management changes only after a weed species shows herbicide resistance or tolerance. “Tolerance means the plant has always been that way,” said Young. “If you spray 2,4-D on corn, the corn is tolerant. Resistance is a genetic change in the plant’s response to the herbicide. There can be various levels of tolerance in a field.”

Young believes resistance issues are not caught early enough, and said evidence of resistance is clear when a weed takes over a major portion of planted acreage. “Historically, weeds will drive herbicide use,” he said. “The ‘next’ herbicide is what we adopted to kill weeds. We had a need and started using Roundup. In the 1990s, we had certain weeds Pursue would kill, so we started using Pursue. Today, herbicide resistance isn’t driving herbicide use as much as our overall strategies.”

Young said resistant weeds drive farmers to select herbicides and herbicide strategies. Unfortunately, farmers have fallen into thinking that herbicide companies will come up with the next new product to solve current weed problems. However, between 1999 and today, no new MoAs (Mode of Action) have been introduced for corn and soybean production.

“We’re in a 20-year drought,” said Young, referring to the lack of new MoAs. “There are several reasons we don’t have any new herbicide MoAs. One is devalued markets and no incentive to develop an herbicide and compete against an herbicide that could kill all weed species and be sold at $3 per acre. Also, the hurdles to get a product registered keep changing.” Young said it may take longer (past 2025) before a new MoA is developed for corn and soybeans because there’s no incentive to do so until absolutely necessary.

However, Young is optimistic and believes there may be new technology by 2025. “There’s a lot more research activity looking at the future use of PPO inhibiting herbicides like Flexstar and Valor, and developing crops for those chemistries than finding new modes of action,” he said.

Several companies are developing interference RNA technology for insect management, and there’s potential for using the same technology in weed management. However, such technology is years away. “Essentially, you make the plant susceptible to Roundup again,” said Young. “But these technologies are high cost and unlikely to be widely adopted by most farmers until prices [for such methods] drop.”

Although farmers often believe they don’t have effective herbicides, Young disagreed. “We have the right herbicides, but have the weeds adapted to how we use herbicides?” he said. “I think they did, and it led to resistance.” Young supported his point with the fact that although there’s no confirmed resistance to Liberty, farmers are applying it at twice the rate as they did 15 years ago.

What should farmers do? Cover crops are a proven, effective weed management option. A thick stand of cereal rye, terminated in spring, can suppress marestail. “For summer annuals, there are inconsistent results,” said Young. “It depends on how good the cover crop is. For winter annuals, cereal rye has proven beneficial, with 90 percent control of marestail.”

Cover crops combined with herbicide programs will likely be the best management option in the future. “If we do anything other than cover crops, there’s going to be an increase in cost of production,” said Young. “Paying $40 to $60 an acre for soybean weed management that includes Palmer amaranth would be reasonable, but beans are going to cost more. Cover crops are a good supplement to an herbicide program, but they aren’t going to replace herbicides.”

Palmer amaranth, one of the biggest challenges in crop production, won’t be gone in 2025, and Young said neighboring properties will likely have more of it, increasing the chance of it appearing on your farm. “If we’re still using herbicides in 2025, the challenge will be being smarter about how we use them,” he said, adding corn and soybean varieties change every year while weed management stagnates. “There aren’t that many herbicides, and they haven’t changed since the 1990s.”

For now, weed management is a matter of selecting the right herbicides and using them correctly. “People like the simplicity of what an herbicide provides,” said Young. “Spray it and forget it. But can we spray and forget it in 2025? We need plans A, B and C to make sure we get the correct mode of action – pre-plant burn down and residual as well as post-application. We need to extract as much activity out of every herbicide in the program to make sure we don’t get more herbicide resistance. We have to make sure we’re applying the best rate of herbicide on the smallest weed possible, which means pre-application. At time of application, weeds should be no taller than the depth of the crop that’s planted.”

The bottom line remains – the herbicides in use today will likely be the herbicides available in 2025. The challenge will be making those effective and determining which chemistry will be most effective on different fields or even within fields, depending on weed species present.