Sorghum growers have long recognized the need for better grass control options, especially for post emergence control.

In 2021, Advanta Seeds introduced their igrowth® sorghum technology in the United States and a significant number of acres were planted. This summer, S&W Seed Company brought to market its Double Team™ sorghum. Joining these two technologies is Inzen™ sorghum from Corteva, whose seed has been limited, but supply is expected to increase substantially in the near future.

It now falls on growers to be good stewards of the technologies to ensure effectiveness of these new technologies for years to come. Through past experiences with other technologies, growers are now well aware of how quickly weeds can become resistant to any given herbicide when overused and when sound resistant management techniques are not utilized.

For these technologies to last, it is very important growers use sound stewardship practices to prevent the development of herbicide resistance in grasses

Resistance to Johnsongrass and shattercane are particularly concerning. Both of these grasses are closely related to grain sorghum and can potentially cross pollinate. While gene flow between sorghum and its weedy relatives does occur in nature at low frequencies, the risk can be greatly minimized by strictly following good stewardship practices.

Although igrowth® (SU) and Inzen™ (IMI) hybrids are very different, for stewardship purposes it is important to remember they are both tolerant to ALS herbicides (Group 2), although different subclasses.

When resistance develops in weed populations it is known that cross-resistance between SU and IMIs is possible, making the weeds resistant to both subclasses. For this reason, when considering stewardship practices, igrowth® and Inzen™ technologies should be considered to have the same mode of action.

Double Team™ hybrids are tolerant to the ACCase (Group 1) FOP herbicides. It is important to note Double Team™ hybrids are NOT tolerant to ACCase DIM herbicides. Examples of DIM herbicides that Double Team™ is not tolerant to are Select Max (clethodim) and Poast (sethoxydim).

Although there are several stewardship practices that are common for all herbicides and crops, such as using herbicides with multiple modes of action, the following are specific to sorghum and focused on preventing resistance developing in Johnsongrass and shattercane:

Specific Stewardship Guidelines for Herbicide Technology (HT) Sorghum

1. A Group 15 herbicide should be applied preemergence. These include S-metolachlor or metolachlor (Dual), acetochlor (Warrant) and dimethenamid (Outlook). Using one of the Group 15 herbicides provides a second mode of action to control grass that may be resistant.

2. Control grasses when they are small, preferably less than 4 inches tall. Grasses are much easier to control when small, which lessens the potential to escape control and produce seed.

3. Do not use if Johnsongrass or shattercane biotypes are present in the field that are known to be resistant to the herbicide technology being planned. These biotypes will not be controlled and will only get worse unless controlled by other weed control options.

4. If Johnsongrass or shattercane are present in the field following herbicide application, these plants must be controlled prior to flowering. It is very important Johnsongrass and shattercane plants are not flowering at the same time as the HT sorghum in order to prevent cross-pollination (Ohadi).

5. Manage Johnsongrass and shattercane growth in road ditches, fence rows and nearby places so flowering does not coincide with HT sorghum flowering. There is no set distance, but pollen from grain sorghum can travel hundreds of feet. For certified seed production, the Texas Department of Agriculture requires fields to be isolated from other sorghum fields, or off-type sorghum plants, by 660 feet to avoid cross pollination.

6. Control all volunteer sorghum/off-types in the following year prior to flowering. Not only does this prevent cross-pollination to nearby Johnsongrass and shattercane, but it also prevents the establishment of resistant wild sorghum in the field.

7. Scout for grass escapes. If resistance is suspected, treat the escaped grass with a herbicide with a different mode of action (or tank mixes) from that used in the initial application, and/or use nonchemical methods to achieve control where possible.

8. If grasses are not controlled as expected following herbicide treatment, contact the seed company or crop protection company immediately. Develop a strategy for controlling suspected resistant grass.

9. Utilization of a desiccant at the end of the season can help to control escapes and minimize viable grass seed production.

10. Consider tarping grain trucks. In trucking HT grain from the field following harvest, care should be taken to avoid spills along roadsides that could lead to wild HT sorghum the following year.

Crop Rotation Considerations

In the crop following igrowth® or Inzen™ sorghum, avoid solely depending on SU or IMI herbicides (both are Group 2 herbicides). Likewise, in the crop following Double Team™ sorghum, do not depend on ACCase herbicides (Group 1), especially FOPS, to control grass. If an ACCase herbicide is used, it should be from the DIM subclass. Examples are Select Max (clethodim) and Poast (sethoxydim).

REFERENCES

Ohadi, S., G. Hodnett, W. Rooney & M. Bagavathiannan. 2017. Gene flow and its consequences in sorghum spp., Critical Reviews in Plant Sciences, 36:5-6, 367-385, DOI: 10.1080/07352689.2018.1446813.

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This story originally appeared in the Summer 2022 Issue of Sorghum Grower magazine.