Johnsongrass (Sorghum halepense) is an invasive grass that can be found in croplands, pastures, hay fields, and roadsides across Missouri. In Missouri, this is the only grass that is listed as a noxious weed. Johnsongrass is a perennial that can reach up to 8 feet in height and has multiple, erect stems (Figure 1). The ability of the plant to produce both seeds and rhizomes allows the plant to spread easily and contributes to its hardiness. Rhizomes are root-like structures from which new plants can grow. These are typically found within the top 10 inches of soil and are white or cream in color with a reddish-orange outer layer (Figure 2). Seeds of the plants have a purple hue and are approximately 3 to 5 mm in length. A single plant can produce up to 80,000 seeds per year. At maturity, seeds easily shatter from the panicle and can remain dormant in the soil for over 10 years. These characteristics make Johnsongrass troublesome and tough to control.

Figure 1 Johnsongrass growing in a pasture field.

Figure 2 Rhizomes of Johnsongrass are white or cream colored with a reddish-orange outer layer.

In 2024, many growers reported reduced control of Johnsongrass following post-emergent herbicide applications. Currently, there are Johnsongrass biotypes that are resistant to four different herbicide sites of action in the United States (Table 1). The increasing spread of weeds with herbicide resistance remains a growing concern in Missouri and especially in Johnsongrass populations.

Table 1 Herbicides that Johnsongrass biotypes are currently resistant to in the United States.

Following the 2024 growing season, a screening was performed on sixteen Missouri Johnsongrass populations to determine the likelihood of herbicide resistance. Populations were collected from corn or soybean fields around the state that reported difficulty in controlling this weed (Figure 3). Seed from each of these populations were planted in the greenhouse, and three post-emergent herbicides were sprayed on seedlings that were 4 to 6 inches in height to determine the likelihood of herbicide resistance.

Figure 3 Locations where Johnsongrass populations were collected across Missouri.

The results from this work are shown in Table 2. The good news is that only two of the populations showed evidence of herbicide resistance. Both Johnsongrass populations were from Dekalb county and both exhibited a decreased response to glyphosate. However, clethodim and nicosulfuron did prove effective in the control of all populations.

Table 2 Control of Missouri Johnsongrass populations with clethodim (Select Max), nicosulfuron (Accent Q), and glyphosate (Roundup Powermax 3). Red highlighted cells indicate significantly lower control.

One of the potential reasons for the inability to properly control Johnsongrass in corn and soybean is that in-season applications are not typically an ideal time for the application of herbicides to Johnsongrass. This isn't to say that we cannot control seedling Johnsongrass with herbicide applications made in the spring or early summer, or even achieve some level of rhizome Johnsongrass control. But herbicide applications made in-crop to vegetative-stage Johnsongrass do not generally provide as good of control of rhizome Johnsongrass as applications of these same herbicides made in early fall. Some of the highest levels of rhizome Johnsongrass control achieved in Missouri is after corn or soybean harvest on Johnsongrass regrowth. At this time, the net flow of carbohydrates and photosynthates within the plant are moving downward toward the rhizomes therefore systemic herbicide applications can provide more effective.

Management of Johnsongrass requires an integrated approach including the use of both herbicides and cultural control techniques. Collectively, these techniques should focus on the prevention of seed and rhizome production. Based on this survey, herbicide resistance in Missouri Johnsongrass populations seems to be relatively low. Reports of reduced weed control could be due to several factors including the hardiness of the plant's rhizomes and timing of herbicide applications. Using management practices that weaken, kill, and stop rhizomes from spreading-along with rotating and mixing herbicide modes of action-will improve control and slow the spread of herbicide resistant weed species.