My grandfather always figured that if a little medicine was good for you, doubling the dose must be twice as good. Some farmers are starting to think that same logic might apply to Roundup Ready crops.

Following the overwhelming adaptation of Roundup Ready soybeans, growers are now looking at planting both corn and beans that are tolerant to Roundup, Touchdown or other glyphosate products on the market.

“The farmers I hear from consistently say they like planting Roundup Ready corn and soybeans because of the easy management,” says Monsanto certified crop adviser Murray Ellis, Panora, IA. “They don't have to worry about what weeds are in the fields or what herbicide to use for their crop rotation. And, it reduces their concern about product drift within the field.”

It's that management aspect of Roundup Ready crops that convinced Onawa, IA, farmer Jim Johnston to rotate the herbicide-tolerant crops. “Each time you get bigger, you inherit a new set of management problems,” says the 5,000-acre farmer, realtor and land manager.

“Roundup Ready crops widen the window we have for chemical application. With some of our heavy, river-bottom gumbo soils, there are only a few days when you can get on them to spray,” he says. “That sometimes hurts us with products that have to be applied within a narrow window of time.”

Although he's tried different herbicide programs, Johnston has a favorite for both corn and soybeans. “My preferred program for corn is a half-rate of Harness Extra followed by a full rate of Roundup,” he says. “I've also tried two applications of Roundup, but I wasn't satisfied with the results.”

On non-Roundup Ready corn, Johnston uses Harness Extra with either Marksman or Callisto. “It's the Cadillac program and costs $20-24, compared with the Roundup program that runs about $17,” he says. “I don't want to ever be completely Roundup Ready corn. I use it where woolly cupgrass is a problem and on our problems soils. But I am concerned about resistance becoming a problem if we use too much of it.”

Johnston plants 100% Roundup Ready soybeans and puts down a trifurilan product ahead of planting, followed by a single application of glyphosate. “We get satisfactory weed control for about $16/acre with that herbicide combination,” he says. Volunteer herbicide-tolerant corn gets a shot of Select in Johnston's fields.

“I've got fields where I've used Roundup Ready crops for three years, and so far we haven't seen any problems,” he says. “I do use a full rate of glyphosate to make sure the weeds die and don't develop resistant strains.”

Johnston's concern about potential weed resistance is music to weed scientists' ears. “We look at glyphosate a lot like atrazine. If it's used correctly, we won't get widespread resistance, although tolerant weeds will increase,” says Brady Kappler, weed science educator at the University of Nebraska.

“If farmers use glyphosate exclusively, we're going to see weed shifts to some of the more tolerant species such as morningglory, waterhemp and winter annuals like henbit and pennycress,” he says. “In the past, herbicides that had some residual into the fall provided control of the germination of those winter annual weeds.”

Kappler suggests that farmers use a pre-emergent product to buy some flexibility. “You can probably get by with a ½ to ¾ rate in corn and beans, followed by a single application of glyphosate,” he says. “If you don't use a pre-emergent product and get delayed in your glyphosate spraying, you've already lost yield.”

University of Missouri Regional Agronomist Don Null worries that some farmers are abusing Roundup Ready crops which will lead to resistance. “They know that soybeans can take quite a bit of competition early without losing yield. So, they don't use a burndown application and then no-till into weedy fields and wait several weeks to spray,” he says. “They're trying to control big weeds and it's really an abuse of the system.”

Both Kappler and Null have conducted research that shows it doesn't make any difference which glyphosate product you use for weed control in Roundup Ready crops. “You can use any of the glyphosate products that are readily available on the market, at full rate, and it won't affect your weed control,” Null says. “Some do require additives, but that information is on the label.”