Weed Management in Sweet Corn

Even though more products are now available, weed control in sweet corn can still be a challenge on certain weed species (see Table 1).  However, over the past several years, some newer herbicide products have been labeled for use in sweet corn that could provide effective control of problem weeds. Historically, weed control in sweet corn has primarily been limited to soil-applied materials.  Currently, some newer preemergence and postemergence herbicides are available. Products such as Acuron, Armezon Pro, Anthem Maxx, Liberty, Restraint, Revulin Q, Shieldex, Solstice, Verdict, and Zidua SC can now be used in sweet corn production. These products generally provide effective weed control and exhibit good crop safety in field corn. However, some research has been conducted with them in sweet corn in Pennsylvania and the Mid-Atlantic region.

Initial results of these studies have found that:

1. Under heavy weed pressure, a full rate of residual herbicide followed by a postemergence application was needed for consistent weed control.
2. The newer herbicides (Acuron, Revulin Q, Solstice, Liberty, and Armezon Pro) performed comparable to Lumax, Accent, and Impact in terms of crop safety and yield.
3. Across two years and two locations, a trend was observed for more sweet corn injury and a negative effect on yield with Zidua and Verdict plus atrazine.

Also, with more weeds becoming herbicide-resistant, it is critical that growers use other effective modes of action to combat this problem. Some of these new products can help.

Newer GMO sweet corn varieties that are resistant to Roundup and Liberty are currently available for use. These varieties can be valuable since glyphosate and Liberty (glufosinate) provide broad-spectrum weed control with no soil residual issues that could interfere with rotational crops. Due to the increasing number of glyphosate-resistant weed species, Roundup Ready sweet corn varieties may be less attractive. However, treatments that include Liberty in LibertyLink variety may still have utility.

Weed control rating scale: 9 = 85–95%; 8 = 75–85%; 7 = 65–75%; 6 = 55–65%; N = no activity

Glyphosate-Resistant Marestail/Horseweed

Glyphosate-resistant marestail (Conyza canadensis), sometimes called horseweed, is a persistent problem in most crops throughout the state, including agronomic and horticultural row crops, orchards, vineyards, and many other areas such as roadsides, non-crop areas, and other natural or idle areas.

Marestail has received increased attention due to the identification of glyphosate and ALS-resistant populations around the country.  Marestail is a member of the aster or sunflower family.  It starts out as a small rosette, and as it grows upright, its hairy leaves whorl around a central stem. Mature plants become 2 to 5 feet tall, producing numerous small daisy-like flowers. Marestail is a prolific seed producer (>100,000 seeds/plant), and the seeds are wind-dispersed much like dandelion. Marestail has traditionally been considered to have a winter annual life cycle. It typically germinates in the fall and overwinters as a rosette, then bolts and sets seed by summer. However, there is another biotype of marestail that can also be found in our region. This one doesn’t germinate until early spring and completes its lifecycle by late summer. The two different lifecycles can cause problems when managing marestail, especially when trying to grow crops planted either earlier or later in the growing season. This, too, can pose problems when selecting burndown and residual herbicide programs depending on the cropping system. A few things to consider when managing marestail: first, its seeds are very small, and they do not tolerate tillage. If the seed can be buried at least a quarter of an inch, germination is drastically decreased. Also, once marestail gets to be taller than 6 inches, it is difficult to control with herbicides. Herbicide applications in the fall or early spring, when it is in the rosette stage, are best.

The number of herbicides that are effective on marestail is rather limited in vegetable production systems. However, certain sweet corn herbicides can provide control of marestail. Here are some suggestions: In no-till sweet corn, paraquat plus a triazine herbicide and glufosinate plus atrazine applied burndown for control of emerged seedlings and residual control of glyphosate- and ALS-resistant horseweed. 2,4-D and Sharpen can be used in the burndown program, but it is best to wait 7-14 days before planting; otherwise, crop injury can occur. If using atrazine alone or in premix products such as Bicep, Lumax, Acuron, etc, it will provide good residual control of marestail but does not control it if it's emerged. 2,4-D is an effective postemergence herbicide if it is applied before marestail reaches 4 inches tall. Otherwise, for small, emerged marestail, foliar-applied HPPD inhibitors (e.g., Callisto) plus atrazine are effective. Liberty (flufosinate) is very effective on marestail postemergence, so consider a LibertyLink variety if it’s a problem.

Cover crops (e.g., cereal rye) can help suppress marestail growth during the winter and spring months, allowing for fewer and smaller marestail, making the burndown herbicide program more effective. Also, if using a cereal cover crop, 2,4-D or dicamba can be applied in the fall to control small marestail seedlings, or 2,4-D could be applied in the spring when the rye is less than a foot tall to obtain control of marestail. Then, the rye can continue to grow before it is terminated and/or rolled down before sweet corn planting.

For those using a tilled seedbed, marestail usually is not an issue in this setting since the tillage process controls existing seedlings and buries the seeds deep enough to manage them for that growing season. However, using effective residual herbicides can ensure none is established while the crop grows.

Other Issues in Sweet Corn Production

As more producers use no-till farming techniques for vegetable production, herbicide programs play a key role in effective weed management. Yet many growers want to move to the next level and produce sweet corn in a no-till setting without using long residual herbicides such as atrazine. Atrazine continues to be a very effective yet economical herbicide for broadleaf weed control in sweet corn.  Over half of the herbicides labeled for use in sweet corn contain atrazine or recommend atrazine as a tank-mix partner. Pennsylvania producers likely use atrazine on a high percentage of the sweet corn acres. Despite its wide acceptance by producers, atrazine use in crop production systems is controversial for various reasons, including environmental issues and resistant weeds. In addition to these concerns, atrazine can cause problems with rotational crops, especially vegetables, and cover crops after sweet corn production. Many growers have inquired about herbicide programs that do not contain atrazine to potentially alleviate carryover problems with successional crops. Other herbicides such as mesotrione (Callisto), topramezone (Impact/Armezon), and pyroxasulfone (Zidua) also potentially leave residues, causing injury to rotational crops. However, these can vary depending on use rates, soil types, rainfall, and other environmental conditions.

Atrazine does improve control of certain weed species (as is well documented through various research) and is still a very effective yet economical herbicide for broadleaf weed control in sweet corn, including no-till systems. However, depending on the weed species present, reducing the rate of atrazine or eliminating it could be possible if there are concerns about carryover to rotational crops, especially vegetables, and cover crops following field or sweet corn production.  Problems with atrazine residues causing injury to rotational crops vary depending on use rates, soil types, rainfall, and other environmental conditions. However, simply replacing atrazine with another product, such as an HPPD- or PPO-inhibiting herbicide (Acuron, Callisto, Impact/Armezon, Laudis, Verdict) will not necessarily eliminate the aforementioned concerns.

Several of these types of products have stringent crop rotation restrictions as well. Only a few herbicides have short rotations for a multitude of crops. Liberty can be a good fit for sweet corn production in a LibertyLink sweet corn system. Roundup Ready varieties can also have a good fit. However, limited variety options, cost of these technologies (e.g., seed tech fees), resistant weed species (esp. glyphosate), and customer apprehension about these genetically modified varieties may limit their widespread use.

Postemergence herbicides should only be used in sequence after a soil-applied herbicide. Total-post weed control is not recommended because sweet corn seedlings are very non-competitive with weeds, and weather conditions that prevent postemergence herbicide application may delay weed control until it is too late to prevent loss. Having a soil-applied herbicide down improves overall weed control, provides additional herbicide modes of action for resistance management, and provides some insurance in case postemergence herbicides cannot be sprayed on time. In previous Penn State research, a two-pass system provided more effective weed control overall compared to a single application timing, especially in no-till systems. Spray the post-treatment when weeds are small (<3 inches tall). For best results, fields with heavy populations of annual grasses (foxtail, crabgrass, panicum) will require a PRE followed by POST herbicide program for consistent control.

Depending on the program, common ragweed may require a two-pass program for adequate control. Also, control of annual morning glory, Palmer pigweed, and waterhemp are a few species that could be problematic depending on which herbicide program is used. Palmer amaranth and waterhemp are becoming a problem in various parts of PA. These noxious pigweeds are very aggressive and can be difficult to control in certain cropping systems. There are certain herbicides in sweet corn that provide control of Palmer and waterhemp, including atrazine, acetochlor products, Lumax, Zidua, Callisto, Impact/Armezon, Laudis, Liberty 280, 2,4-D, and a few others. Again, two-pass systems work best and are usually necessary with Palmer amaranth/waterhemp since it has a long germination period. Control of these weeds after sweet corn harvest may be essential to stop seed production and additional spread.