With resistance to glyphosate increasing in weed species throughout Ohio, the importance of using pre-emergence products as part of an integrated herbicide program is becoming more evident.

Jeff Stachler, an Ohio State University Extension weed specialist, said that pre-emergence herbicide applications can drastically improve the control of lambsquarter, giant ragweed and marestail (horseweed) in corn and soybean fields, but their effectiveness is contingent upon how they are incorporated into management practices.

“Many growers don’t use pre-emergence herbicides in their Roundup Ready soybean fields and some of them don’t in their corn crop. They’ve gotten used to only using glyphosate for post-emergence weed control and over-simplified their weed control by reducing the number of applications to save time and costs,” said Stachler, who also holds a research appointment with the Ohio Agricultural Research and Development Center. “However, it’s getting to the point where growers can no longer use just glyphosate to get effective control all the time. Glyphosate-resistance marestail is already abundant in Ohio, and we believe that some populations of giant ragweed and lambsquarter are evolving to have a low level of resistance to glyphosate, all of which is being attributed to the intensive usage of this particular herbicide.”

Marestail, giant ragweed and lambsquarter remain some of the most challenging weeds to control for several reasons:

• They are some of the first weeds to emerge in the spring, and marestail grows quickly in size, making proper burndown treatments a must to control them.
• They can emerge well into the growing season, giant ragweed most often, which makes it difficult to time a single post-emergence application.
• They become more difficult to control with increasing size and age.
• Their increasing resistance to herbicides, both glyphosate and ALS (acetolactate synthase), reduces the number of control options.

Pre-emergence herbicides are meant to be applied prior to weed and crop emergence. The benefits of pre-emergence herbicide applications go beyond getting a head start on weed control. The management practice maintains yield at a minimum, and in many cases, improves yield; allows for greater flexibility of post-emergence herbicides; reduces early season competition between weeds and the crop; and aids in more efficient nitrogen management in corn, which can save money.

“The key to capitalizing on these benefits is more timely weed control, something that growers, both in no-till and conventional tillage production systems, really need to focus on,” said Stachler.

Stachler said that conventional tillage Roundup Ready soybean growers have usually only had to apply glyphosate in a single application to obtain effective weed control. However, for the last two to three years more conventional tillage soybean growers are having difficulty controlling some key weed species, especially lambsquarter and giant ragweed.

“Growers practicing conventional tillage, especially for soybeans, need to use pre-emergence herbicides, because these growers are simply having a harder time controlling weeds with glyphosate,” he said. “Growers who omit pre-plant burndown treatments in no-tillage make applications when weeds are large and old, and use rates too low for the weed size and age, placing themselves at risk for control failures.”

No-till soybean growers are also at risk for ineffective weed control. Stachler said they tend to practice delayed burndown, where they plant soybeans and then let the weeds grow to about 12 inches to 24 inches tall before they make a one-time spray application intended to last the entire season.

“This scenario is exacerbating the problem of glyphosate resistance more than any other practice, and it’s got to stop,” said Stachler. “No-till growers should be spraying when weeds are much smaller and younger.”

The best method for controlling marestail, giant ragweed and lambsquarter is to incorporate pre-emergence herbicides into other herbicide programs, ones that make use of several herbicide application timings and a diversity of herbicides to compensate for existing or developing resistance issues. Such management techniques include pre-plant burndown treatments in no-tillage, use of residual herbicides in conventional and no-tillage, and proper management of post-emergence herbicide applications, including glyphosate. Depending on the field situation, the following management tips for effective weed control may be useful to corn and soybean growers.

For marestail control in Roundup Ready soybeans:

• Apply burndown herbicides when weeds are small, less than four inches tall.
• Most effective pre-plant burndown treatments include: 2,4-D ester plus glyphosate plus one of the following: Canopy, SynchronyXP, FirstRate, Amplify, or Gangster; 2,4-D ester plus glyphosate; or a combination of 2,4-D ester plus Gramoxone (at least 0.64 lb ai/A) plus Sencor. Where the soybeans have been planted and 2,4-D ester cannot be used, apply a combination of glyphosate plus one of the following: Canopy, SynchronyXP, FirstRate, Amplify, or Gangster.
• Use a glyphosate rate of at least 1.5 lbs acid equivalent per acre when 2,4-D ester is not in the burndown mixture and when applying post-emergence, especially after bolting.
• Burndown treatments applied before about May 10 should include residual herbicides to control later-emerging marestail. Most effective options where ALS resistance is known or suspected include Valor, Gangster, Sencor (minimum of 8 oz/A), and Domain. The following are also effective residual control options where the grower knows the marestail is not ALS-resistant: Canopy, SynchronyXP, FirstRate, Amplify, and Python.
• Where a post-emergence herbicide treatment is being applied in Roundup Ready soybeans to control marestail that have survived a prior treatment of glyphosate, apply the maximum-labeled rate of glyphosate and consider the addition of Classic, FirstRate, or Amplify.

For marestail control in corn, any pre-emergence product that contains atrazine is usually effective. Lumax and Luxar are the most broad-spectrum products available in corn. Follow the above burndown recommendations in corn as well.

For lambsquarter control in Roundup Ready soybeans:

• Most effective pre-plant burndown results from a combination of glyphosate plus 2,4-D ester.
• Most pre-emergence soybean herbicides provide effective control of lambsquarters, often for the entire growing season. Effective pre-emergence herbicides include: Command, Canopy, SynchronyXP, Python, Gangster, Valor, FirstRate, pendimethalin, Scepter, Sencor (except triazine-resistant), and Domain (except triazine-resistant).
• Make post-emergence glyphosate applications when lambsquarters are less than six inches tall, and use a glyphosate rate of at least 1.1 lb ae/A. Use a rate of 1.5 lb ae/A for plants more than eight inches tall. 
• Avoid making post-emergence applications during periods of adverse environmental conditions, such as low temperatures, extended cloudy periods, and drought.
• Make a second post-emergence glyphosate application as necessary to control plants that survive an initial glyphosate application. Use the maximum-labeled rate in these follow-up applications. Maximum amount of glyphosate that can be applied post-emergence in one season is 2.25 lb ae/A in Roundup Ready soybeans and Roundup Ready 2 corn. So, if you use 1.1 lbs ae/A in the first application, use 1.1 lbs again in the second application.

For pre-emergence control of lambsquarter in corn, any product that contains atrazine is usually effective, unless the lambsquarters are triazine resistant. Lumax and Luxar are the most broad-spectrum products available.

For giant ragweed control in Roundup Ready soybeans:

• Most effective pre-plant burndown results from a combination of glyphosate plus 2,4-D ester. 
• Use a pre-emergence herbicide with activity on giant ragweed, which can reduce the giant ragweed population and suppress growth of plants that emerge in the first few weeks after soybean planting. This should minimize the risk of yield loss from early-emerging ragweed and allow for more flexibility in post-emergence application timing. Pre-emergence herbicides with activity on giant ragweed include: Canopy, SynchronyXP, Scepter, FirstRate, Amplify, and Gangster. Note that the giant ragweed activity is based on the ALS-inhibiting component of these herbicides, so control will be reduced in ALS-resistant populations, which can be frequent. None of these herbicides will control moderate to heavy giant ragweed populations for the entire season, even if the population is sensitive to ALS-inhibiting herbicides.
• The most effective post-emergence strategy is to apply at least 1.1 lb ae/A of glyphosate in Roundup Ready soybeans initially when plants are about six to 10 inches tall, and apply again at the same rate approximately three weeks later. Use a rate of 1.5 lb ae/A when plants are more than 10 inches tall, and make a follow-up application as necessary at the rate of 0.75 lb ae/A. Applications to plants more than 12 inches tall can put producers at significant risk of soybean yield loss, especially in dense populations, and large plants that have developed a low level of resistance may be difficult to control with even two post-emergence applications.
• Make a second post-emergence glyphosate application as necessary to control plants that survive an initial glyphosate application. Use the maximum labeled rate in these follow-up applications. Maximum amount of glyphosate that can be applied post-emergence in one season is 2.25 lb ae/A. 

For giant ragweed control in corn, any product that contains atrazine can also be effective, unless the ragweed is triazine resistant. Lumax and Luxar are the most effective broad-spectrum products available and Hornet and Balance can provide effective early season control. Several post-emergence herbicides can effectively control giant ragweed and should be considered in a tank mix with glyphosate in Roundup Ready corn.