Considerations for Terminating Giant Miscanthus on Maryland Farms
Introduction
Giant miscanthus (Miscanthus x giganteus). Photo credit: Kurt Vollmer, University of Maryland.
Giant miscanthus (Miscanthusx giganteus) is a perennial nonnative grass hybrid with several potential uses, including as a bioenergy crop, as a source for animal bedding, as a source for building material and insulation, as a source of eco-friendly disposable dinnerware, or as a food additive. Compared to other bioenergy crops, miscanthus produces 67% more biomass than switchgrass (Heaton et al., 2008), it requires lower management inputs (tillage, fertilizer, pesticides) than corn (Lewandowski et al., 2000), and ethanol derived from miscanthus produces a higher net energy balance compared to ethanol derived from corn (Yuan et al., 2008). In addition to these features, giant miscanthus has the potential to be grown on marginal lands. These include areas with unproductive soils, poor drainage, high water tables, and steep slopes. Giant miscanthus production is especially important when considering its tolerance to adverse environmental conditions such as wet and saline soils (Chen et. al., 2017), a currently expanding problem for many Maryland farmers because of saltwater intrusion. Furthermore, dense stands of miscanthus can reduce runoff and help prevent soil erosion (Mazur and Kowalczyk-Juśko, 2021).
Despite these advantages, there is little information about how to manage miscanthus should it become weedy, or should a farmer desire to plant another crop. It is estimated that giant miscanthus can have a productive lifespan of 15 to 20 years (Heaton et al., 2019). Since miscanthus propagates from underground rhizomes, it can be especially difficult to control, as simply removing the aboveground biomass will not provide permanent control. Therefore, research was needed to evaluate management options for controlling unwanted populations of giant miscanthus.
Miscanthus Management in Agronomic Fields
Researchers from the University of Maryland instituted two trials in Ridgley, MD to evaluate different strategies for terminating established (>1 year) giant miscanthus. The first study evaluated miscanthus eradication methods in no-till production systems. Treatments included herbicide applications made once (1x) or twice (2x), mowing once, or twice, or a combination of mowing followed by an herbicide application (1x or 2x) (Table 1). In the second study, the study area was disced once in May, 2022 to break up miscanthus rhizomes before implementing the same treatment protocols used in the first study. Each study was arranged in a randomized complete block design with four replications in the first study, and three replications in the second study. In the first study, plots were 10 ft. x 25 ft., and in the second study, plots were 8 ft x 10 ft. Plots were mowed with a rotary mower at the specified time according to each treatment. As a result, miscanthus height varied at the time of treatment. Herbicide treatments consisted of Roundup PowerMax® (glyphosate) applied at 32 fl. oz./A with 1.3 lb. ammonium sulfate/A. Applications were made using a compressed CO₂-pressurized backpack sprayer with a spray volume of 18 gal at 22 PSI and Teejet 8003 extended range flat-fan nozzles. Percent stand reduction (stunting) was rated on a visual scale as compared to the untreated check, with 0 being no stand reduction and 100 being complete stand reduction. The following spring, the number of new shoots was counted, and aboveground biomass harvested using a 2.7 ft² quadrat. Dry biomass was recorded for each plot following a period of at least 7 days of drying in the UMD Wye REC greenhouse.
Table 1. Approaches to managing giant miscanthus in agronomic fields.
Treatment
Frequency
Dates
Untreated
Herbicide
1xᵇ
18 May
2x
18 May, 1 July
Mow
1x
18 May
2x
18 May, 1 July
Mow fb Herbicide
1x
18 May, 6 Jun
2x
18 May, 6 Jun, 1 July, 14 July
ᵃ Herbicide applications consisted of Roundup PowerMax® applied at 32 fl. oz./A (1.13lb. Glyphosate acid/A) plus 1.3 lb. ammonium sulfate/A.
ᵇ Abbreviations: 1x, once; 2x, twice; fb: followed by.
In-season Treatment Effects on Giant Miscanthus
Mowing twice followed by subsequent glyphosate applications after each mowing consistently showed greater miscanthus stunting (Table 2).
In the disced trial, this treatment was comparable to a single mowing followed by glyphosate application and mowing alone, resulting in a 35% to 92% reduction in miscanthus vigor by late summer.
Similarly, results from the no-till trial showed mowing twice followed by two subsequent glyphosate applications were comparable to two glyphosate applications without mowing, resulting in a 79% reduction in miscanthus vigor by late summer.
Signs of recovery were already evident by late fall.
In the disced trial, only the herbicide treatments (without mowing) showed a greater reduction in overall stand quality from August to December.
There was no reduction in overall stand quality in the no-till trial.
Table 2. In-season growth reduction of giant miscanthus following herbicide and mowing treatments in 2022.ᵃ
Treatment
Frequency
Disced
No Till
Aug. 5
Dec. 13
Aug. 5
Dec. 13
____ stunting (%) ____
Herbicideᵇ
1xᶜ
20 b
27 b
15 d
15 b
2x
17 b
47 ab
79 ab
57 a
Mow
1x
35 ab
17 b
0 d
0 b
2x
38 ab
25 b
73 b
0 b
Mow fb Herbicide
1x
82 ab
70 ab
44 c
46 a
2x
92 a
88 a
92 a
69 a
ᵃ Means in the same column followed by the same letter are not significantly different according to Tukey’s HSD (α=0.05).
ᵇ Herbicide applications consisted of Roundup PowerMax® applied at 32 fl. oz./A (1.13 lb. glyphosate acid/A) plus 1.3 lb. ammonium sulfate/A.
ᶜ Abbreviations: 1x, once; 2x, twice; fb: followed by.
Treatment Effects on Miscanthus the Following Spring
In the disced trial, shoot density was lowest when mowed twice followed by subsequent glyphosate applications after each mowing, but was comparable to mowing once followed by a glyphosate application or two glyphosate applications (Figure 1, Table 3).
Similarly, a field experiment in Illinois showed that spring tillage with one or two spring glyphosate applications reduced miscanthus shoot number 38% to 67%, and aboveground biomass 94 and 95%, respectively (Anderson et. al., 2011a).
Figure 1. Spring miscanthus regrowth in (a) non-treated vs (b) two mowing followed by subsequent herbicide applications. The study area was disced the previous year before implementing treatments. Photo Credit: Ellie Rogers, University of Maryland.
Table 3. Miscanthus density and biomass in May 2023 following applications in 2022.ᵃ
Treatment
Frequency
Density
Biomass
Disced
No-Till
Disced
No-Till
_ shoots ft⁻² _
__ oz. ft⁻²__
Untreated
1216 a
1087 abc
0.07
0.42 ab
Herbicideᵇ
1xᶜ
1496 a
1561 a
0.11
0.39 ab
2x
344 bc
581 c
0.03
0.13 b
Mow
1x
1216 a
1152 abc
0.10
0.37 ab
2 x
958 ab
1249 abc
0.10
0.27 ab
Mow fb Herbicide
1 x
226 bc
797 bc
0.02
0.21 b
2 x
11 c
1485 ab
0.02
0.55 a
ᵃ Means in the same column followed by the same letter are not significantly different according to Tukey’s HSD (α=0.05).
ᵇ Herbicide applications consisted of Roundup PowerMax® applied at 32 fl. oz./A (1.13 lb. glyphosate acid/A) plus 1.3 lb. ammonium sulfate/A.
ᶜ Abbreviations: 1x, once; 2x, twice; fb: followed by.
In the no-till trial, no treatments showed significantly lower shoot density compared to the untreated (Figure 2).
No significant difference in spring miscanthus biomass was observed in the disced study, and no treatments were different from the untreated in the no-till study.
Figure 2. Spring miscanthus regrowth in (a) non-treated vs (b) two mowing followed by subsequent herbicide applications. No additional tillage occurred prior to implementing treatments the previous year. Photo credit: Ellie Rogers, University of Maryland.
Miscanthus Management in Riparian Areas
In Maryland, giant miscanthus plantings have been proposed for salt-affected and actively flooded fields. This can be especially problematic since miscanthus grown in wetland habitats can have greater belowground biomass and associated starch reserves compared to miscanthus grown in upland habitats (Turnage et al., 2022). Additionally, only certain herbicides are approved for use on and around water bodies. With this in mind, a separate study evaluated two different aquatic herbicide formulations, Rodeo® (glyphosate) and Polaris® (imazapyr) for controlling miscanthus in riparian areas (Table 4). The study was arranged in a randomized complete block design with four replications. Plants were established from rhizomes in 3-gal pots and herbicides applied when plants reached 3 feet in height. Plants were visually rated for overall injury on a 0 to 100% scale 95 days after application. The number of shoots were counted, and the shoot heights recorded. Percent shoot reduction and percent height reduction were calculated based on the shoot number and height of untreated plants.
Table 4. Miscanthus density and height reduction in response to aquatic herbicide formulations 95 days after application.ᵃ
Treatmentᵇ
Application Rate
Density
Height
___ fl. oz. a-1 ___
__ % reduction __
Polaris
48
28 c
0 b
Polaris
96
35 bc
0 b
Rodeo
112
93 abc
37 ab
Rodeo
224
92 ab
21 ab
Rodeo + Polaris
48 + 112
91 ab
42 ab
Rodeo + Polaris
96 + 224
100 a
100 a
ᵃ Means in the same column followed by the same letter are not significantly different according to Tukey’s HSD (α = 0.05).
ᵇ Polaris® applications consisted of 0.75 or 1.5 lb. imazapyr/A. Rodeo® applications consisted of 3.5 or 7 lb. glyphosate acid/A. All herbicide treatments included a nonionic surfactant applied at 0.25 %V/V.
Results
Rodeo and Rodeo plus Polaris treatments resulted in 78% to 100% injury 95 days after application (data not presented).
Rodeo and combinations of Rodeo + Polaris resulted in at least a 91% reduction in miscanthus density, with no plants reported with the higher rates of Rodeo + Polaris (Table 4).
Rodeo and combinations of Rodeo + Polaris reduced miscanthus height at least 21%, while Polaris only treatments resulted in no reduction in miscanthus height.
However, other studies reported applications of glyphosate or imazapyr reduced miscanthus height 66% to 100%, and biomass 84% to 100% compared to nontreated plants 12 weeks after treatment (Turnage el. al., 2022)
Implications for Management
Figure 3. Miscanthus plots (center) showing signs of recovery within a year of herbicide and or mowing treatments. Photo credit: Kurt Vollmer, University of Maryland.
Our results showed that multiple treatments might be required over successive years as plants begin to recover within a year of treatment (Figure 3).
One time discing in conjunction with sequential herbicide applications or mowing followed by herbicide applications appears to have the greatest effect of reducing miscanthus biomass and subsequent recovery the following season.
Although mowing and tillage are often not options in riparian areas, herbicides with an aquatic label such as Rodeo or Rodeo + Polaris should be sufficient to manage populations of giant miscanthus.
However, this study did not access the efficacy of these herbicides on an established (>1 year old) populations of giant miscanthus.
Given these results, farmers wanting to plant giant miscanthus should consider the long-term commitments of planting this species, as conversion back to crop land (especially no-till or organic) could be challenging.
If planting a broadleaf crop, such as an herbicide-tolerant soybean, in-season miscanthus control may be achieved with glyphosate.
One study reported that, while complete miscanthus control was not achieved, corn and soybean yields improved with two sequential glyphosate applications compared to one application (Anderson et al., 2011b).
However, other research has shown that in order to reduce miscanthus biomass 85% to 100%, glyphosate had to be applied at 4 and 6 lb. ae/A (Barksdale et al., 2020).
This would result in an $18 to $32/A increase compared to the 1.13 lb. ae/A rate used in our field study.
Acknowledgments
The authors acknowledge funding support for this research by the USDA Agricultural Research Service through award #58-8042-0-079. We would also like to thank Dave Tribbett, Taylor Tribbett and Twin Maples Farm for providing the field site, Angelina Duschel, Jadon Cook, Joe Crank, Louis Thorne, CJ Chansler, and the horticultural crew at University of Maryland Wye Research and Education Center.
References
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Anderson, E.K., Voigt, T.B., Bollero, G.A., & Hager, A.G. (2011), Rotating a Field of Mature Miscanthus × giganteus to Glyphosate-Resistant Crops. Agronomy Journal, 103: 1383-1388. https://doi.org/10.2134/agronj2011.0091
Barksdale, N., Byrd, J. D., Zaccaro, M. L. M., & Russell, D. P. (2020). Evaluation of herbicide efficacy and application timing for giant miscanthus (Miscanthus x giganteus) biomass reduction. Weed Technology, 34(3), 371– 376. https://doi.org/10.1017/wet.2020.10
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Turnage, G., Byrd, J.D., & Madsen, J.D. (2022). Miscanthus × giganteus growth and control in simulated upland and wetland habitats. Invasive Plant Sci Manage. 15:25-32. doi:10.1017/inp.2022.1
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KURT VOLLMER
kvollmer@umd.edu
ELLIE ROGERS
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