Kansas Herbicide Activity on Old World Bluestems Herbicide Activity on Old World Bluestems

Abstract Two main species of old world bluestems (OWB), yellow bluestem (Bothriochloa ischaemum) and Caucasian bluestem (Bothriochloa bladhii), have encroached on rangelands, pastures, and road right-of-ways in Kansas. Patches of these OWB have been shown to reduce species diversity and abundance at multiple trophic levels, and pose a long-term threat to native plant, insect, rodent, and grassland bird populations. These OWB species are utilized by cattle early in the growing season, directly following prescribed burns, and during droughts when other forages lack water uptake and may go dormant. However, these OWB species mature more quickly than native grass species and quickly form stem tissue, thus losing palatability to grazing animals rather rapidly compared to native grass species. In native pastures where OWB has invaded, native species may become overutilized because animals avoid the OWB. This weakens the native grasses and allows OWB to have a competitive advantage for moisture and nutrient resources. Over time, patches of OWB have expanded from 2.5 to 3.5 times their original patch size in 9 years, or a 15% annually compounded patch growth rate, when left uncontrolled. Several herbicides have been analyzed for OWB control, but glyphosate and imazapyr are the two herbicides that in the past have been shown to reduce OWB abundance most effectively in pastures. However, other herbicides with new label information for use in different forms of grassland or recreation areas may also have some activity on OWB and provide control. This study was performed to test several alternative herbicides with no known prior history of evaluation for control of OWB.


Introduction
Two main species of old world bluestems (OWB), yellow bluestem (Bothriochloa ischaemum) and Caucasian bluestem (Bothriochloa bladhii), have encroached on rangelands, pastures, and road right-of-ways in Kansas. Patches of these OWB have been shown to reduce species diversity and abundance at multiple trophic levels, and pose a long-term threat to native plant, insect, rodent, and grassland bird populations. These OWB species are utilized by cattle early in the growing season, directly following prescribed burns, and during droughts when other forages lack water uptake and may go dormant. However, these OWB species mature more quickly than native grass species and quickly form stem tissue, thus losing palatability to grazing animals rather rapidly compared to native grass species. In native pastures where OWB has invaded, native species may become overutilized because animals avoid the OWB. This weakens the native grasses and allows OWB to have a competitive advantage for moisture and nutrient resources. Over time, patches of OWB have expanded from 2.5 to 3.5 times their original patch size in 9 years, or a 15% annually compounded patch growth rate, when left uncontrolled. Several herbicides have been analyzed for OWB control, but glyphosate and imazapyr are the two herbicides that in the past have been shown to reduce OWB abundance most effectively in pastures. However, other herbicides with new label information for use in different forms of grassland or recreation areas may also have some activity on OWB and provide control. This study was performed to test several alternative herbicides with no known prior history of evaluation for control of OWB.

Experimental procedures
Herbicide test plots were evaluated in nearly solid stands of yellow bluestem and Caucasian bluestem. Plots were evaluated for OWB frequency, or the presence of OWB rooted within 100 small squares, each 4 × 4 inches in size within a square 40 × 40 inch frame, prior to herbicide application, and were also evaluated for OWB control 60 days following herbicide treatment. Control ratings were based on the amount of injury and the reduced production compared to untreated OWB plants.
One year after herbicide treatment, OWB control and frequency were evaluated again to see if control observed during the year of treatment carried over into the next year. Herbicides were applied to plots arranged in a randomized complete block design with four replications. New locations of separate yellow and Caucasian bluestem stands were used in 2019 and 2020, for a total of four distinct and complete experiment locations. Herbicide treatments included the following rates of active ingredient: 1) fluazifop-P 6.0 oz/acre, 2) glyphosate 2.0 lb/acre, 3) halosulfuron 0.9975 oz/acre, 4) imazapyr 0.5 lb/acre, 5) mesotrione 3.0 oz/acre, 6) rimisulfuron 0.75 oz/acre, 7) sulfosulfuron 0.9975 oz/acre, and 8) untreated control. All herbicides were sprayed at 30 psi with an equivalent of 17 gallons/acre water carrier, and included a non-ionic surfactant at 0.5% v/v.

Results and Discussion
The frequency of OWB plants prior to herbicide application was not different between any of the herbicides. Combined across years, yellow bluestem frequency within the large frame was 93% and Caucasian bluestem frequency was 80% prior to herbicides being applied. Sixty days following application, glyphosate tended to display the greatest control of both yellow and Caucasian bluestem, followed by fluazifop-P and rimisulfuron ( Table 1). Control of OWB 60 days after treatment with imazapyr was highly variable, with poor control in 2019 but much improved control, more closely resembling glyphosate control in 2020. Imazapyr variability was unexpected as it is one of the two primary herbicide recommendations for OWB spot control in pasture. During the year of treatment, fluazifop-P and rimisulfuron showed as much potential for OWB control as imazapyr. However, that control was short-lived and did not extend to the next growing season. One year after herbicide application, glyphosate provided much better control of both yellow and Caucasian bluestem than all other herbicides applied in the trial (Table 2). Glyphosate control 1 year after treatment was significantly greater in plots treated in 2019 than 2020; however, in both years of treatment, control with glyphosate was much greater than with any other herbicide. Greater control of both OWB species 1 year after treatment with glyphosate also translated into lower frequency of both OWB species 1 year after treatment. Frequency of both OWB species was below 26% when treating with glyphosate (Table 3). All other herbicides had an OWB frequency of near 90% or greater 1 year after treatment. Imazapyr has provided adequate control of OWB during the year after treatment in multiple prior experiments and in actual production pastures. The lack of control and the high OWB frequency with imazapyr during the year after treatment was unexpected in the current experiment.

Implications
Glyphosate continues to be the most reliable herbicide for control of OWB. However, glyphosate is non-selective and will also kill all other desirable pasture vegetation if treated. Imazapyr was variable in this trial, but has also provided adequate OWB control in prior experiments and in general production pastures. Most native tall and mid-grasses show tolerance and survival to low rates of imazapyr that are able to provide some control of OWB. Fluazifop-P and rimisulfuron showed initial OWB injury and control that was as good as imazapyr, although control did not extend into the next season in this trial. These two herbicides may have potential for further experimentation to control OWB.
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