Controlling Herbicide Resistant Weeds in Sorghum

Controlling weeds and managing their resistance is a key factor is any farmer’s management system. Prepare yourself with both pre- and post-emergence tactics to found here to protect your crop this season.

Written by: Brent Bean

Overall, sorghum growers in the U.S. have fewer issues with herbicide resistant weeds than farmers who grow other crops—not because they have less resistant weeds, but because they never became dependent on glyphosate to achieve satisfactory weed control.

Even so, the resistant weed control practices that have been implemented for other crops should also be used in sorghum to not only control resistant weeds that are present but to also prevent weed resistance from developing or becoming worse.

These control practices include the use of multiple classes of herbicides applied in tank mixes and/or rotating the use of herbicide classes whenever possible. Growers should use a rate high enough to achieve optimum weed control results. Every effort should be made to control weed escapes to prevent weeds that may be resistant from producing seed and propagating the problem.

In sorghum, a successful pre-emergence weed control program is essential. This usually starts with the use of herbicides from two mode-of-action classes, the photosynthesis (PS) inhibitors and the fatty acid inhibitors.

Atrazine is the PS inhibitor most often used by sorghum growers and has been around since the 1960s. It has many advantages, including its effectiveness on several key broadleaf weed species, season-long soil residual and can be used both pre- and post-emergence.

Although weed resistance to atrazine is present in many fields, it remains a stalwart for weed control. In many fields, growers believe they have atrazine resistance, but what is often the case is that soil weed seed numbers, particularly palmer amaranth, have gotten so high that achieving 99 percent control still leaves enough emerged weeds to be an issue.

Because atrazine is inexpensive, it almost always pays to add it to any pre-emergence program. There are some regions where atrazine cannot be used and includes areas with soils classified as sand, sandy loam, loamy sand or where the pH is higher than 8.5. In these soils, atrazine can usually be applied early post-emergence.

The second class of herbicides, the fatty acid inhibitors or chloroacetamides, include the active ingredients metolachlor, acetachlor and dimethenamid, which are sold under the popular trade names of Dual Magnum, Warrant, Outlook and others. These have been around since the 1970s, but surprisingly the development of resistance to this mode-of-action class has remained relatively low compared to others.

When not restricted by soil texture or pH, it is best to use a pre-mix of one of these herbicides with atrazine. These are sold under the trade names of Bicep Magnum, Cinch ATZ, Degree Xtra, Fultime XLT and others. Dimethenamid (Outlook) is most often mixed in the field with atrazine rather than purchased as a premix.

The chloroacetamide herbicides generally give good control of a number of annual grass species but are not as effective on many broadleaf weeds. Fortunately, they do give relatively good control of pigweed species, including palmer amaranth. Their soil residual is relatively short and often only provides satisfactory weed control for 40-60 days. The use of these herbicides is seldom restricted by soil type.

For this reason, applying two-thirds of the herbicide treatment prior to emergence and the remainder early post-emergence can be a good practice to lengthen the time of control. This practice can be particularly beneficial if heavy rains occur soon after planting and leach the herbicide(s) below the soil seed zone, due to the fact that most weeds originate from seed in the top inch of soil.

Mesotrione is a HPPD herbicide that is labeled for use in sorghum and provides a third mode-of-action class. Mesotrione is sold in a three-way premix with atrazine and metolachlor under the trade names Lumax and Lexar. Until recently, these premixes were priced higher than what many sorghum growers were willing to pay, particularly in dryland areas where growing season precipitation is low. Recent price drops have made these more affordable. Some growers are mixing mesotrione sold by itself as Callisto or as a generic with atrazine and metolachlor. This three-way mix can be very effective on most weeds. Soil restrictions will be similar to those found for atrazine.

Saflufenacil (Sharpen) is a PPO mode-of-action herbicide that has gained popularity in sorghum in recent years. Saflufenacil is most often used in a premix sold as Verdict that contains dimethenamid. In order to improve the effectiveness of Verdict, most growers will add an additional 10 ounces of Outlook to the mix. One advantage of Verdict is that it has good post-activity of small emerged weeds that may be present at planting.

As is always the case, treating weeds when they are smaller achieves better control. To control any weeds that may have escaped the pre-emergence treatment, there are several post-emergence options. Common post-emergence options are atrazine, 2,4-D and dicamba (growth regulators), prosulfuron (ALS) that is sold as Peak and Huskie, which contains pyrasulfotole (HPPD) plus bromoxynil (PS inhibitor).

Atrazine plus crop soil should always be considered, especially if it was not used pre-emergence. Soil restrictions generally do not apply when atrazine is used as a post treatment. Dicamba and 2,4-D  should be the next option, but growers should be careful to follow the label and apply accordingly to prevent crop injury. Peak has some crop rotations that should be considered and is best applied with dicamba or atrazine. Also, many fields have ALS resistant weeds, particularly palmer amaranth, that may make Peak ineffective.

Huskie can be effective on most broadleaf weeds, but growers should expect some sorghum injury in the form of leaf burn and yellow flashing in the whorl. Sorghum will quickly grow out of these injury symptoms.

By combining these pre-emergence and post-emergence weed control strategies, growers can greatly enhance their ability to prevent weed resistance from developing or becoming worse.