This past year, herbicide-resistant corn varieties made up about 11 percent of the corn planted in the United States. This compares to 75 percent of soybeans and 58 percent of cotton acreage.
"In Georgia, we’re probably planting about 85 percent Roundup Ready cotton and 85 percent Roundup Ready soybeans. For several reasons, we haven’t seen a similar acreage explosion in herbicide-resistant corn," says Eric Protsko, University of Georgia Extension weed scientist.
As the price of glyphosate continues to fall, the use of Roundup Ready systems becomes more economical in row crops, says Protsko. "The Roundup Ready corn hybrids are getting better, and that’s been a big limiting factor in the adoption of this technology in corn production. Roundup Ready corn is much more tolerant than cotton. In corn, you can spray over-the-top up to the V-8 stage or 30 inches tall. Of course, we’d like to see you get in earlier than that, but you have this flexibility with Roundup Ready corn," he says.
Growers should keep in mind, he adds, that there is an $18-per-bag technology cost for Roundup Ready corn seed. "You need to consider this as part of your weed control costs," he says.
Protsko also reminds growers that a few weeds will be missed in a Roundup Ready system. "We don’t seem to be able to get good, consistent control of Florida pusley or morningglory with glyphosate. Even with a Roundup Ready system, we’ll still need atrazine to give us some broad-spectrum and residual control of these troublesome weeds."
The Liberty Link herbicide-resistant corn system has been made more attractive this year by a drop in price, notes Protsko.
"The biggest drawback to Liberty Link has been that it is very expensive. Last year, Liberty herbicide was about $85 per gallon. This year, however, the price has dropped to about $60 per gallon. Liberty is in a class of herbicide somewhere between paraquat and Roundup.
"It doesn’t move as much as Roundup, but it moves a little bit more than paraquat. It’s a broad-spectrum herbicide that generally will give you control over many weed species. It can be weak on pigweed. If you have a pigweed problem, you need to be very timely in your applications. You should be able to control a 1 or 2-inch tall pigweed."
Even with the Liberty Link system, growers still will need atrazine in the system to pick up missed weeds, says Protsko. A pre-mix of Liberty and atrazine — Liberty ATZ — is available, he adds.
"Like the Roundup Ready hybrids, you can go up to a larger stage on corn — up to 24 inches — and there’s no seed premium with the Liberty Link system."
There are several important factors to consider as more growers move towards herbicide-resistant corn technology, says Protsko, including hybrid performance, weed shifts, weed resistance and public acceptance of the technology.
"Another concern is the future of herbicide research and development if we continue to rely on a single chemistry. Most of the U.S. soybean acreage is planted in Roundup Ready varieties, but we need to be looking at other soybean compounds. As long as one chemistry dominates the market, there’s no need for a company to invest money into soybeans because they probably won’t receive a return on their investment. That might hurt us in the long run if we continue to rely on one product for weed control."
Protsko also is concerned about the effect herbicide-resistant systems might have on timely applications. "Even though we have a wider window of application in corn, we don’t necessarily need to wait that long. We need to make timely applications to get the best weed control and to prevent yield loss. We might tend to become a little lazy if we adopt this technology."
Yield data from this past year shows that herbicide-resistant corn hybrids are improving, he says, and now are better suited to Coastal Plain growing conditions.
Weed shifts, says Protsko, already are occurring. "We’ve seen it in several south Georgia counties. Whenever we put pressure on weed populations, something is going to happen, and management is affected. Tropical spider wort is an emerging weed pest that can cause real problems on your farm. This weed grows well in a wide range of conditions, and we don’t know a lot about how to control it.
"We typically don’t have a big problem with tropical spider wort in corn until we get nearer to or following harvest. In other crops, we see it throughout the growing season."
The problem of resistant weeds is here, he says, and it was caused by using a single chemistry. "We are developing problems around the world and in the United States due to our reliance on glyphosate. In the United States, horseweed or mare’s tail has been found to be resistant in Delaware, Tennessee and possibly Alabama. We need to know that this will happen if we continue to rely on a single chemistry."
There are obvious benefits to the Roundup Ready and Liberty Link systems, says Protsko. They are convenient to use and generally are regarded as being environmentally friendly. Drift and contamination problems are minimal, he adds.
The top three weed problems in Georgia corn production are Texas panicum, sicklepod and morningglory, says Protsko. The control of these weeds is complicated by growing practices in the Southeast, he says.
"We don’t use a lot of pre-emergence, soil-applied herbicides in the South. We see re-emergence from weeds like Texas Panicum throughout the year. We plant in March or April, and we get the corn off in July or August. So, we’re getting the crop off when weeds still are emerging."
Southern producers limit their inputs because they know their returns are limited, says Protsko. "Our average U.S. corn yield is 135 bushels per acre while the average yield in Georgia is 119 bushels per acre. We don’t have as much potential to make high yields, so we’re reluctant to invest in a pre-emergence herbicide."
There are several options for controlling Texas panicum in corn, he says. "One of the key options will be Prowl or Pendimax as a delayed pre-emergence or early postemergence treatment. The ‘Cadillac’ treatment has been Accent over-the-top. We also have Option this year, which is similar in activity to Accent. In addition, Roundup Ready and Liberty Link systems can be used to control Texas panicum."
Many growers are using post-directed or layby systems with Evict/paraquat, he says.
The cheapest option for sicklepod control, says Protsko, is atrazine or 2, 4-D. Cotton sometimes limits the use of 2, 4-D, he adds, but it is one of the best and least expensive herbicides to use in corn.
The soil-applied material Python also is good on sicklepod, he says, as are Roundup and Liberty.
"Morningglory is a hard one to answer. No matter which rate we use, atrazine won’t last, and it probably won’t be there at harvest. We’ll probably need to use two applications of something. We have products like atrazine and 2, 4-D. Roundup isn’t as good on morningglory as Liberty or other materials. But it can be very effective if used twice. It won’t give you residual control."
Growers need to use the maximum amount of atrazine possible in a field, he says. "One system to consider for controlling morningglory is making an initial application of 2, 4-D, if you can, and following that with a later application of atrazine. We can go as late as 12-inch corn with atrazine, and that’ll help us extend the residual effect."