Introduction
Aminocyclopyrachlor (AMCP) controls many noxious and invasive broadleaf weeds such as Canada thistle [Cirsium arvense (L.) Scop.], field bindweed (Convolvulus arvensis L.) (Lindenmayer et al. Reference Lindenmayer, Nissen, Westra, Shaner and Brunk2012), Russian knapweed [Rhaponticum repens (L.) Hidalgo] (Lindenmayer et al. Reference Lindenmayer, Westra, Brunk, Nissen and Shaner2010), leafy spurge (Euphorbia esula L.), perennial sowthistle (Sonchus arvensis L.) (Lym Reference Lym2010), prickly lettuce (Lactuca serriola L.) (Bell et al. Reference Bell, Burke and Prather2011), houndstongue (Cynoglossum officinale L.), absinth wormwood (Artemisia absinthium L.), and yellow toadflax (Linaria vulgaris Mill.) (Conklin and Lym Reference Conklin and Lym2012). AMCP has moderate to long soil residual, which reduces reinfestation from targeted weeds. AMCP time to 50% dissipation (DT50) averaged 20 d in four different soil types from the Northern Great Plains, with the longest DT50 of >112 d found in cool, dry, low organic matter soil (Conklin and Lym Reference Conklin and Lym2013).
Management Implications
Native forbs are an essential component of prairie communities. Floristically diverse plant communities, including more than 200 species of plants, with a majority being forbs, were once commonly found in the tallgrass prairie ecosystems of the Northern Great Plains. Invasion by noxious weeds has become a major threat to the biodiversity and ecosystem sustainability in remnant and restored prairies. Aminocyclopyrachlor (AMCP) will control many noxious and invasive broadleaf weeds and has moderate to long soil residual. The wide spectrum of weed control and soil residual from aminocyclopyrachlor can have negative effects on non-target species. Data from field evaluations are important to determine which forbs may be damaged by AMCP. However, locating adequate populations of native forbs to evaluate tolerance to herbicide applications is often difficult. A greenhouse study evaluated tolerance of 10 native prairie forbs to spring-applied AMCP. Blueflag iris (Iris versicolor L.) and harebell (Campanula rotundifolia L.) were relatively tolerant and would likely be unharmed following an application of AMCP in the field. Wild licorice (Glycyrrhiza lepidota Pursh), prairie wild rose (Rosa arkansana Porter), purple prairie clover (Dalea purpurea Vent.), and wild bergamot (Monarda fistulosa L.) were moderately susceptible to AMCP; however, plants might regrow in the field, since some survived at the highest AMCP application rate (105 g ha−1) evaluated. Skyblue aster [Symphyotrichum oolentangiense (Riddell) G. L. Nesom], Canada goldenrod (Solidago canadensis L.), blue cardinal-flower (Lobelia siphilitica L.), and blacksamson echinacea (Echinacea angustifolia DC.) were susceptible to AMCP even when applied at 35 g ha−1. The susceptibility of greenhouse-grown forbs to AMCP was the same or similar to species evaluated in the field with no instance of contradictory findings. Thus, these data could be used to estimate tolerance to AMCP of forb species that have not been tested in the field. Land managers need to consider both the susceptibility of the target weed and the potential of injury to desirable plant species when choosing which herbicide to apply and at what rate and timing of application to maximize weed control and minimize non-target plant injury.
The wide spectrum of weed control and soil residual from AMCP can have negative effects on non-target species (Hergert et al. Reference Hergert, Mealor and Kniss2015). AMCP reduced most of the 16 forbs observed in Colorado by Sebastian et al. (Reference Sebastian, Beck, Sebastian and Rodgers2012) in both density and richness. Smooth white aster [Symphyotrichum porter (A. Gray) G. L. Nesom] and heath aster [Symphyotrichum ericoides (L.) G. L. Nesom] were completely eliminated. Sagebrush (Artemisia spp.) and flax (Linum spp.) species density was reduced by 74% following AMCP application in Wyoming (Hergert et al. Reference Hergert, Mealor and Kniss2015). In a separate Wyoming study, forb cover was reduced but grass cover increased from 56% to 91% (Greet et al. Reference Greet, Mealor and Kniss2016). In Colorado, shrub density was decreased by 45% to 76% and richness by 20% to 50%, depending on application rate (Sebastian et al. Reference Sebastian, Beck, Sebastian and Rodgers2012).
AMCP decreased total foliar cover, species richness, evenness, and diversity in treated communities compared with nontreated in native prairie sites found in Minnesota and North Dakota (Thilmony and Lym Reference Thilmony and Lym2017). High seral forb cover was reduced from 19.8% to 2.9% in North Dakota and 18.5% to 2% in Minnesota 14 mo after treatment (MAT) with AMCP at 140 g ha−1 (2 oz acre−1).
Native forbs are an essential component of prairie communities. Floristically diverse plant communities, including more than 200 species of plants, with a majority being forbs, were once commonly found in the tallgrass prairie ecosystems of the Northern Great Plains (Beran et al. Reference Beran, Gaussoin and Masters1999; Jordan et al. Reference Jordan, Peters and Allen1988; Weaver Reference Weaver1954). Native forbs increase diversity (Hooper et al. Reference Hooper, Chapin, Ewel, Hector, Inchausti and Lavorel2005), provide cover and seed for wildlife, require low maintenance, and increase aesthetic attributes.
Data from field evaluations are important to determine which forbs may be damaged by AMCP application to control weeds. However, locating adequate populations of native forbs in close proximity to evaluate herbicide tolerance is often difficult. The purpose of this research was to determine the susceptibility of native prairie forb species to AMCP under greenhouse conditions. Forb species selected in this study were from families that are difficult to evaluate in the field because of their rarity or tendency to grow singularly in the wild. Representatives of commonly found native prairie plants were also included so results could be compared with field trials. Greenhouse trials are often a more rapid and cost-effective alternative to field trials, especially for evaluation of relatively rare or low-abundance forbs.
Materials and Methods
Ten native forb species were evaluated for susceptibility to AMCP in greenhouse trials. Species included were wild licorice (Glycyrrhiza lepidota Pursh), skyblue aster [Symphyotrichum oolentangiense (Riddell) G.L. Nesom], blueflag iris (Iris versicolor L.), Canada goldenrod (Solidago canadensis L.), blue cardinal-flower (Lobelia siphilitica L.), harebell (Campanula rotundifolia L.), prairie rose (Rosa arkansana Porter), blacksamson echinacea (Echinacea angustifolia DC.), purple prairie clover (Dalea purpurea Vent.), and wild bergamot (Monarda fistulosa L.). These species were chosen either because populations were too sparse to adequately evaluate in the field or so that results could be compared with a field study of AMCP applied to forbs (Thilmony and Lym Reference Thilmony and Lym2017).
Prairie forbs were either purchased from a nursery (Prairie Restorations, Princeton, MN) in August 2014 or 2015 or grown from seed or root from local collections. The forbs were then transplanted into cone-tainers (6.3-cm diameter by 25-cm deep [2.5 by 9.8 in]; DeepotsTM, Stuewe & Sons, Tangent, OR) with a blend of commercial mix (Sunshine Mix No. 1, patented formulation with wetting agents; Sun Gro Horticulture, Agawam, MA) and sandy loam soil (4:1 by volume). Plants were grown for 4 to 8 wk in a greenhouse at a maintained temperature of ~24 C (75 F) with a 16-h photoperiod of natural and supplemental light using metal-halide lamps with an intensity of 450 µE m−2 s−1. Plants were watered as needed and fertilized with a diluted 20-20-20 nutrient solution (Jack’s All Purpose Water Soluble Plant Food, JR Peters, Allentown, PA) one to two times before treatment. Imidacloprid was applied at 5 mg ai (0.00017 oz) per cone-tainer to G. lepidota, S. oolentangiense, L. siphilitica, C. rotundifolia, and R. arkansana to control mealybugs [Pseudococcus longispinus (Targioni Tozzetti)], aphids [Myzus persicae (Sulzer)], and spider mites (Tetranychus urticae Koch).
AMCP was applied when plants reached the late-spring growth stage to simulate a spring treatment for weed control (Table 1). Plants were spaced apart for treatment to maximize spray coverage. AMCP at 0, 35, 70, and 105 g ha−1 (0, 0.5, 1, and 1.5 oz acre−1) was applied with an air-pressurized cabinet-type sprayer equipped with an 80015 nozzle, delivering 160 L ha−1 (17 gal acre−1) at 240 kPa (35 psi). All herbicide treatments were applied with an MSO plus silicone-based NIS blend (Dyne-Amic®, Helena Chemical, Collierville, TN) at 0.25% v/v to maximize potential injury.
Table 1 Species, family, growth stage, and plant height of forbs when treated with aminocyclopyrachlor in a greenhouse to simulate exposure received during a spring weed control application.
a Abbreviations: FLW, flowering; VEG, vegetative.
Visual evaluations for plant response, such as epinasty, leaf cupping, and reduced height compared with untreated plants, were recorded 2 wk after treatment (WAT) on a scale of 0% to 100% (0 = no effect; 100 = all visible material dead). The top growth was then removed 3 cm above the soil surface and discarded. Plants were then allowed to regrow for 8 to 12 wk, dependent on species, in the same greenhouse conditions as previously described. After the regrowth period, injury was visually estimated, and plant material was harvested, dried at 50 C for 96 h, and weighed to estimate the effect of AMCP on plant production.
The experiment was a randomized complete-block design with four replicates and was repeated. Each species was analyzed as an individual experiment. Plant injury ratings and regrowth weights were assessed using the PROC GLM procedure of SAS to determine differences in injury among application rates. Mean separations were tested using F-protected LSD (P=0.05), and homogeneity of variance was assessed using error mean squares from each run. A combined analysis was performed when error mean squares differed by less than a factor of 10. Plant susceptibility to AMCP was categorized as tolerant (<20% injury), moderately tolerant (21% to 50% injury), moderately susceptible (51% to 80% injury), and susceptible (>80% injury) based on plant regrowth response and injury evaluations.
Results and Discussion
Prairie forb susceptibility to AMCP varied among species (Tables 2 and 3). Iris versicolor and C. rotundifolia were the most tolerant to AMCP of the 10 species evaluated, whereas S. oolentangiense, S. canadensis, L. siphilitica, and E. angustifolia were the most susceptible.
Table 2 Effect of aminocyclopyrachlor (AMCP) on various greenhouse-grown forbs 2 and 10 to 14 wk after treatment (WAT).
a AMCP applied with an MSO plus silicone-based NIS blend at 0.25% v/v (Dyne-Amic®, Helena Chemical, 225 Schilling Boulevard, Suite 300, Collierville, TN 38017) at 0.25% v/v.
b Injury ratings were based on a scale of 0% to 100% (0 = no effect; 100 = all visible material dead).
Iris versicolor was tolerant to AMCP at all application rates (Table 2). Plant injury at 2 and 10 WAT was minimal and difficult to visually discern from control plants. Regrowth weight was similar among treated and control I. versicolor plants and averaged 1.4 g (Table 3). All plants quickly regrew shortly after top growth was removed at 2 WAT. Based on these results, I. versicolor likely will not be affected by AMCP in the field, as the herbicide was applied with an MSO plus silicone-based NIS blend surfactant to maximize potential injury in the greenhouse.
Table 3 Effect of aminocyclopyrachlor (AMCP) on regrowth of various greenhouse-grown forbs 10 to 14 wks after treatment.
a AMCP applied with an MSO plus silicone-based NIS blend at 0.25% v/v (Dyne-Amic®, Helena Chemical, 225 Schilling Boulevard, Suite 300, Collierville, TN 38017).
b Abbreviations: MS, moderately susceptible; MT, moderately tolerant; S, susceptible; T, tolerant.
c Field trial results are combined from reports from Hergert et al. (Reference Hergert, Mealor and Kniss2015), Sebastian et al. (Reference Sebastian, Beck, Sebastian and Rodgers2012), Thilmony and Lym (Reference Thilmony and Lym2017), and/or Wallace and Prather (Reference Wallace and Prather2012).
d Susceptibility rating from field trial followed by “/number” indicates number of plants from the same species, genus, or family evaluated in the field that received that rating.
Campanula rotundifolia was moderately tolerant to AMCP (Tables 2 and 3). Injury averaged approximately 32% at 2 and 10 wk after AMCP was applied at 35 to 105 g ha−1. Regrowth of treated plants was similar to the control except when AMCP was applied at the highest rate of 105 g ha−1. Short-term cupping of C. rotundifolia basal leaves was observed following an AMCP application in the greenhouse, which would likely be short-lived in the field as well. Conversely, C. rotundifolia was extremely susceptible to aminopyralid, as all treated plants died, even when the herbicide was applied at 30 g ha−1 in a similar greenhouse study (Mikkelson and Lym Reference Mikkelson and Lym2013).
Glycyrrhiza lepidota, R. arkansana, D. purpurea, and M. fistulosa were all considered moderately susceptible to AMCP, which was similar to the results from field trials conducted by Thilmony and Lym (Reference Thilmony and Lym2017) (Table 3). Glycyrrhiza lepidota regrowth averaged 0.4 g when AMCP was applied at 70 g ha−1, compared with 1.6 g for the control, a 75% reduction in regrowth, even though plants were grown an additional 4 wk (14 wk total) to allow maximum regrowth. These findings were consistent with a field study using AMCP at 170 g ha−1, in which G. lepidota was reduced from 1.8% to 0.1% foliar cover at 10 MAT (Thilmony and Lym Reference Thilmony and Lym2017). However, by 14 MAT, G. lepidota cover increased to 1.5%, and plants exhibited minimal signs of injury.
Rosa arkansana regrowth declined as AMCP application rate increased, and injury ranged from 9% to 37% at 10 WAT (Tables 2 and 3). Rosa arkansana foliar cover was reduced but not eliminated at 14 MAT when AMCP was applied at 170 g ha−1 in the field (Thilmony and Lym Reference Thilmony and Lym2017). Other plants in the Rosaceae family such as Woods’ rose (Rosa woodsii Lindl.) and wild strawberry (Fragaria virginiana Duchesne) were also moderately susceptible to AMCP (Thilmony and Lym Reference Thilmony and Lym2017; Wallace and Prather Reference Wallace and Prather2012). Rosa woodsii cover decreased slightly at 13 MAT but recovered by 25 MAT in Idaho, while F. virginiana was reduced 55% at 14 MAT in Minnesota.
Dalea purpurea dry weight at 10 WAT was 1.2 g when AMCP was applied at 35 g ha−1 compared with the control at 1.9 g, but weight decreased to 0.4 g or less when the herbicide was applied at 70 and 105 g ha−1 (Table 3). The reduced regrowth observed in the greenhouse following AMCP application was consistent with field trials that included D. purpurea. Dalea purpurea was not present at 10 MAT, but plants recovered slightly by 14 MAT in MN (Thilmony and Lym Reference Thilmony and Lym2017). In Idaho, legume biomass was reduced by more than 71% for 3 yr following a single AMCP application (Miller et al. Reference Miller, Bork, Hall and Summers2015).
Monarda fistulosa regrowth decreased as AMCP application rate increased and ranged from 0.1 to 0.4 g compared with 0.7 g for the control (Table 3). Although M. fistulosa top growth was reduced in the greenhouse, especially at high application rates, most plants survived at 10 WAT (Table 2). AMCP effect on other plants in the genus Monarda has not been reported, but healall (Prunella vulgaris L.), another member of the Lamiaceae family, was completely eliminated when AMCP was applied at 170 g ha−1 (Thilmony and Lym Reference Thilmony and Lym2017).
Symphyotrichum oolentangiense, S. canadensis, L. siphilitica, and E. angustifolia were all susceptible to AMCP at 70 g ha−1 or more with 0 to 0.1 g of regrowth at 10 WAT (Table 3). Symphyotrichum oolentangiense, S. canadensis, and E. angustifolia are in the Asteraceae family. All three species had >80% injury at 10 WAT following AMCP applied at 70 g ha−1, and no plants regrew when the herbicide was applied at 105 g ha−1 (Table 2). In field studies, S. canadensis cover was reduced an average of 97% at two sites (Thilmony and Lym Reference Thilmony and Lym2017), which closely compares to the results of this study. Many members of the Asteraceae family were sensitive to AMCP in field trials, including S. ericoides, S. porter (Sebastian et al. Reference Sebastian, Beck, Sebastian and Rodgers2012), Maximilian sunflower (Helianthus maximiliani Schrad.), and stiff goldenrod [Oligoneuron rigidum (L.) Small var. rigidum] (Thilmony and Lym Reference Thilmony and Lym2017). However, some Asteraceae members were much less sensitive to AMCP, including three Artemisia spp. (Hergert et al. Reference Hergert, Mealor and Kniss2015) and common sunflower (Helianthus annuus L.). The variation in results was expected, since Asteraceae is a large family, and similar work with aminopyralid found plant sensitivity varied from tolerant to moderately susceptible to susceptible even within the same genus of Asteraceae (Halstvedt Reference Halstvedt2015).
Lobelia siphilitica was susceptible to AMCP at all application rates evaluated, and regrowth was minimal (<0.1 g) at 10 WAT when treated with AMCP at 70 or 105 g ha−1 (Tables 2 and 3). Epinasty and leaf cupping was observed in the greenhouse and may occur shortly after a field application. Similarly, palespike lobelia (Lobelia spicata Lam.) was reduced but not eliminated by AMCP in the field (Thilmony and Lym Reference Thilmony and Lym2017). Lobelia siphilitica susceptibility to AMCP contrasts with C. rotundifolia, which was moderately tolerant and further illustrates the variance in sensitivity to this herbicide from plants in the same family (Campanulaceae).
Forbs in this and previous studies were generally more sensitive to AMCP than aminopyralid at commonly used application rates (Almquist and Lym Reference Almquist and Lym2010; Halstvedt Reference Halstvedt2015; Samuel and Lym Reference Samuel and Lym2008; Thilmony and Lym Reference Thilmony and Lym2017). However, land managers need to consider the weed to be controlled, application timing, herbicide rate, and forbs present when choosing the most effective treatment program. For instance, Rosa spp. were considered susceptible to aminopyralid but only moderately susceptible to AMCP (Tables 2 and 3) and regrew the year following application in the field (Thilmony and Lym Reference Thilmony and Lym2017). In contrast, D. purpurea and M. fistulosa were susceptible to AMCP in this study, but tolerant of aminopyralid (Halstvedt Reference Halstvedt2015). Both herbicides will control C. arvense, so a land manger could choose between AMCP and aminopyralid based on the forbs present and/or desired at the site following treatment. Timing of application should also be considered. For instance, slender cinquefoil (Potentilla gracilis Douglas ex Hook.) was susceptible to aminopyralid when fall applied but tolerant of spring treatments, while subalpine buckwheat (Eriogonum umbellatum Torrey var. majus Hooker) tolerated fall but not summer application (Halstvedt Reference Hooper, Chapin, Ewel, Hector, Inchausti and Lavorel2015).
The susceptibility of greenhouse-grown forbs to AMCP was the same or similar to species evaluated in the field with no instance of contradictory findings. Thus, these data can be used to estimate tolerance to AMCP of forb species that have not been tested in the field. Many forb species could be evaluated in greenhouse trials fairly rapidly, and this would likely be more cost-effective than similar trials in the field, especially if the forb species is rare or difficult to find in numbers high enough to conduct susceptibility trials. This information will be beneficial to land managers, who must balance the benefits of a herbicide application to control invasive weeds with the potential of unintended injury to desirable plant species.
