Stephen F. Enloe

Canada Thistle (Cirsium arvense) Control with Aminopyralid in Range, Pasture, and Noncrop Areas

Canada thistle is a serious weed of many crop, rangeland, pasture, and natural areas throughout North America. Aminopyralid is a new pyridine carboxylic acid herbicide that has activity on Canada thistle at lower use rates than current standard treatments. The objectives of this study were to compare aminopyralid efficacy, rates, and application timing with several commercial standards for Canada thistle control. Studies were conducted across the Great Plains at ten locations, which encompassed a wide range of environments. Aminopyralid provided Canada thistle control comparable to picloram, picloram + 2,4-D amine, and clopyralid and better control than clopyralid + 2,4-D amine, dicamba, dicamba + 2,4-D amine and dicamba + diflufenzopyr. Canada thistle control was similar when aminopyralid was applied between 0.08 and 0.11 kg ai/ha and application timing (spring bolting vs. fall rosette/regrowth) did not strongly influence control 1 yr after treatment (YAT). Aminopyralid provided effective Canada thistle control at lower use rates than current commercial standards and might be useful in areas where herbicides such as picloram and clopyralid are not recommended for use. Nomenclature: Aminopyralid; clopyralid; dicamba; diflufenzopyr; picloram; 2,4-D amine; Canada thistle, Cirsium arvense L. Scop. CIRAR.

Comparison of Aminocyclopyrachlor to Common Herbicides for Kudzu (Pueraria montana) Management

Abstract Kudzu is an invasive perennial climbing vine characterized by fast growth rates and tolerance to control measures. Repeated applications with high rates of 2,4-D plus picloram provide effective kudzu control, but picloram use is not permitted in certain states due to groundwater pollution concerns. Studies were conducted in Alabama and Florida to compare kudzu control with aminocyclopyrachlor, a new herbicide, to control provided by aminopyralid, clopyralid, metsulfuron methyl, and picloram plus 2,4-D, which are common treatments for kudzu management. Two annual applications of the same herbicide treatment were evaluated for effects on kudzu cover, kudzu volume index, and cover of other vegetation. Aminocyclopyrachlor at 140 to 280 g ae ha−1 (2 to 4 oz ae ac−1) was as effective as the standard 4.48 kg ae ha−1 (4 lb ae ac−1) 2,4-D amine plus 1.2 kg ae ha−1 picloram for kudzu control. There were no differences in kudzu control among the three rates of aminocyclopyrachlor tested. Colonization by graminoids, forbs, and Rubus spp. at 2 yr was greatest for herbicides providing the best kudzu control: aminocyclopyrachlor, and 2,4-D plus picloram. Herbicide treatments were more effective in controlling kudzu at the Alabama location, but repeated annual applications for 2 yr did not completely eliminate kudzu with any treatment at either site. Nomenclature: 2,4-D; aminocyclopyrachlor; aminopyralid; clopyralid; metsulfuron methyl; picloram; kudzu, Pueraria montana (Lour.) Merr., USDA code PUMOL, EPPO code PUELO. Interpretive Summary: The new herbicide, aminocyclopyrachlor, applied at 140 to 280 g ae ha−1 controlled kudzu as well as the standard treatment for kudzu management, 4.48 kg ae 2,4-D amine plus 1.21 kg ae picloram, and generally better than the other common kudzu herbicides metsulfuron methyl, aminopyralid, and clopyralid. Graminoids, Rubus spp., and forbs recolonized plots receiving the most effective treatments for kudzu control. Graminoid (grasses, sedges) cover increased following aminocyclopyrachlor treatments, an important consideration for restoration of infestations where kudzu was planted for erosion control in the region. Annual herbicide applications for 2 yr did not eradicate kudzu, confirming the need for multiple years of treatment or more frequent applications.

Russian Knapweed (Acroptilon repens) Control with Low Rates of Aminopyralid on Range and Pasture

Abstract Russian knapweed is an invasive weed of rangeland, pasture, and natural areas throughout western North America. Aminopyralid is a new pyridine carboxylic acid herbicide that has activity on Russian knapweed at lower use rates than current standard treatments. The objectives of this study were to compare aminopyralid efficacy at the bud to early flower timing and the fall timing with commercial standards for Russian knapweed control. Studies were conducted at five locations in California, Utah, and Wyoming in heavily infested pastures or rangeland. When applied in summer at the bud to early flower timing, aminopyralid at 0.07 kg ae/ha controlled Russian knapweed effectively and was comparable to picloram + 2,4-D amine (0.56 + 1.12 kg ae/ha) at 12 and 24 months after treatment (MAT). The addition of 2,4-D with lower rates of aminopyralid did not improve control. When applied after seed set at the fall timing, control from aminopyralid at 0.05 kg ae/ha and higher was also comparable to picloram (0.56 kg ae/ha) and better than clopyralid (0.42 kg ae/ha) and imazapic (0.18 and 0.21 kg ae/ha) 12 and 21 MAT. Aminopyralid controlled Russian knapweed effectively at lower use rates than current commercial standards and good control lasted for at least 21 to 24 MAT. Nomenclature: Aminopyralid; clopyralid; imazapic; picloram; 2,4-D amine; Russian knapweed, Acroptilon repens (L.) DC. ACRRE

Propoxycarbazone-Sodium and Imazapic Effects on Downy Brome (Bromus tectorum) and Newly Seeded Perennial Grasses

Abstract Vigorous stands of perennial grasses can effectively provide long-term control of many invasive plants on rangelands. However, in degraded conditions, successful reestablishment of perennial grasses can be compromised by invasive annual grasses, such as downy brome. Propoxycarbazone-sodium is a selective herbicide currently labeled for downy brome control in small grains, but its potential use on rangelands is unknown. Studies were conducted from 2004 through 2008 at three rangeland sites in Colorado and Nebraska to evaluate downy brome control and perennial grass injury with propoxycarbazone-sodium and imazapic. Propoxycarbazone-sodium provided satisfactory downy brome control with grass injury equal to or less than imazapic when rainfall followed the fall application. A second set of studies was conducted from 2007 to 2008 at Lingle, WY, and Scottsbluff, NE, to determine the plant-back interval and postemergence application response of seven perennial grass species to propoxycarbazone-sodium and imazapic. Grass tolerance to both herbicides was good when applied 90 and 120 d before planting (DBP). However, grass injury increased as plant-back interval decreased. The greatest impact on plant biomass was observed from herbicide applied at planting or after planting. Crested and intermediate wheatgrass (Agropyron cristatum and Thinopyrum intermedium) biomass production was not affected when herbicides were applied 90 or 120 DBP. Western wheatgrass (Pascopyrum smithii) and Russian wildrye (Psathyrostachys juncea) showed tolerance to imazapic applied before planting. Smooth brome (Bromus inermis), sheep fescue (Festuca ovina), and orchardgrass (Dactylis glomerata) showed the least amount of tolerance to propoxycarbazone-sodium and imazapic. Nomenclature: Propoxycarbazone-sodium; imazapic; downy brome, Bromus tectorum L.; crested wheatgrass, Agropyron cristatum (L.) Gaertn.; intermediate wheatgrass, Thinopyrum intermedium (Host) Barkworth & D.R. Dewey; orchardgrass, Dactylis glomerata L.; Russian wildrye, Psathyrostachys juncea (Fisch.) Nevski; sheep fescue, Festuca ovina L.; smooth brome, Bromus inermis Leyss.; western wheatgrass, Pascopyrum smithii (Rydb.) Á. Löve Interpretative Summary: The establishment of perennial grasses that can provide long-term control of invasive plants can be jeopardized by infestations of downy brome. Propoxycarbazone-sodium is a selective herbicide for downy brome control in small grains, but its performance on established perennial grass stands in rangelands was unknown. A study was conducted in Colorado and Nebraska and indicated satisfactory downy brome control with propoxycarbazone-sodium at 45 g ha−1 (0.04 lb ac−1) when applied in fall. Downy brome control was consistent when fall applications were followed by seasonal rainfall. An additional finding of the study was that grass injury was equal to or less than that caused by imazapic. In a second study, the response of seven perennial grasses to propoxycarbazone-sodium at 60 g ha−1 and imazapic at 105 g ha−1 applied 120, 90, and 30 d before planting (DBP), at planting, and 30 d after planting (DAP) was examined. Grass tolerance to both herbicides was good for applications 90 and 120 DBP. Grass injury increased as herbicide applications were made closer to planting. Crested and intermediate wheatgrass biomass production was not affected when herbicides were applied 90 or 120 DBP. Western wheatgrass and Russian wildrye showed tolerance to imazapic applied before planting. When planting orchardgrass, smooth brome, or sheep fescue, neither propoxycarbazone-sodium nor imazapic should be included as part of the weed control program.

Does Superior Competitive Ability Explain Yellow Starthistle's (Centaurea solstitialis) Successful Invasion of Annual Grasslands in California?

Abstract Yellow starthistle represents one of the most spectacular examples of biological invasion in the western United States. However, the mechanisms leading to its success have not been clearly elucidated. Although its success has been attributed to superior competitive ability, few competition studies have been performed with yellow starthistle to test this assertion. Yellow starthistle and wild oat (a dominant component of California annual grasslands) were grown in monocultures and mixtures to assess the strength of competitive interactions between them. For either species, intraspecific competition exerted a greater influence over mean plant weight than did interspecific competition. A companion study revealed temporal separation in the phenology of these plants, explaining the weak role of interspecific competition. Additional measurements of growth and soil moisture dynamics in large 270-cm-tall by 50-cm-diam polyvinyl chloride columns also showed a lack of interspecific competition and confirmed that water use patterns differed between these species, indicating niche partitioning. Wild oat reduced soil moisture to 5% but only to a depth of approximately 150 cm. Yellow starthistle depleted soil moisture to less than 5% throughout the column to a depth of at least 270 cm. These patterns were present when wild oat and yellow starthistle were grown individually or together in the columns, indicating that yellow starthistle had a greater impact on soil moisture and to greater depths. Yellow starthistles invasion of grasslands in California does not appear to be due to superior competitive ability, but may be due to its ability to access deeper soil moisture. These results support the empty niche hypothesis that implies that invasive species are successful in new habitats because they access resources not available to resident species. Nomenclature: Yellow starthistle; Centaurea solstitialis L. CENSO; wild oat; Avena fatua L. Interpretive Summary: Yellow starthistle has invaded California grasslands within the last 100 yr. The mechanism for this successful invasion is not known for certain, even though it would be useful in managing this troublesome weed. We grew yellow starthistle in competition experiments with wild oat. The results indicate that yellow starthistle is not a strong competitor in relation to wild oat. Soil water measurements indicate that yellow starthistle used water from deeper in the soil profile than wild oat, suggesting that these species avoid competition in this way. Results from this study support the “empty niche hypothesis,” which implies that yellow starthistle has successfully invaded California grasslands because there are unused resources, i.e., deep soil water, available to it in these systems. These results support the idea that controlling yellow starthistle without establishing a plant community that also utilizes this deep soil water will not provide a long-term solution to its invasion.

Herbicide Treatment and Application Method Influence Root Sprouting in Chinese Tallowtree (Triadica sebifera)

Chinese tallowtree is an invasive tree found throughout the southeastern United States and in California. Its negative effects can be seen in numerous natural and managed ecosystems, including bottomland hardwood forests, pastures, pine plantations, and along lakes, ponds, streams, and rivers. Despite its troublesome presence for many decades, relatively few effective control strategies are available. Root sprouting following management efforts is a major impediment to successful control. Studies were conducted in Alabama and Louisiana at three locations to test several herbicides for cut stump, basal bark, and foliar individual plant treatment (IPT) methods. Herbicide treatments included triclopyr amine and ester formulations, imazamox, aminopyralid, aminocyclopyrachlor, and fluroxypyr. Data were collected just before leaf senescence at one and two growing seasons after treatment and included Chinese tallowtree foliar cover, number of stump or root collar sprouts, and number of sprouts originating from lateral roots within a 1-m radius of each tree. For the cut stump and basal bark studies, most herbicide treatments prevented sprouting from the stump or root collar region better than they did from the lateral roots. Aminopyralid reduced total sprouting better than all other treatments in the cut stump study. The high rates of aminocyclopyrachlor and fluroxypyr resulted in the highest mortality in the basal bark study. Aminocyclopyrachlor reduced total sprouting better than all other herbicides in the foliar treatment study. Triclopyr amine and ester formulations, which are commercial standards, did not consistently control Chinese tallowtree across these IPT studies. These studies provide some promising treatments to increase the number of effective tools that can be used to manage Chinese tallowtree. Additional research is needed to address the prolific nature of lateral root sprouting following any of these treatment methods. Nomenclature: Aminocyclopyrachlor; aminopyralid; fluroxypyr; imazamox; triclopyr; Chinese tallowtree, Triadica sebifera (L.) Small. Management Implications: Chinese tallowtree is an increasing problem across the southeastern United States. However, there is little published research examining effective control methods. We compared several newer herbicides with the commercial standards triclopyr ester and triclopyr amine for foliar, cut stump, and basal bark individual plant treatment (IPT) methods in natural areas. We examined both root collar and lateral root sprouting response to herbicide treatment, which is rarely done in invasive plant studies. We found that aminocyclopyrachlor, aminopyralid, fluroxypyr, and imazamox all controlled Chinese tallowtree similar to, or better than, triclopyr in specific IPT studies. Aminocyclopyrachlor resulted in greater mortality of Chinese tallowtree than triclopyr did in foliar and basal bark studies, whereas aminopyralid was more effective than triclopyr in cut stump studies. Fluroxypyr resulted in greater mortality than triclopyr in cut stump and basal bark studies but not in foliar studies. This research highlights the strong need to continue to evaluate new tools for invasive plant control and the need to continuously collect better data on invasive plant responses to treatment.

Impacts of Prescribed Fire, Glyphosate, and Seeding on Cogongrass, Species Richness, and Species Diversity in Longleaf Pine

Abstract Cogongrass [Imperata cylindrica (L.) Beauv.] is a warm-season, rhizomatous grass native to southeast Asia that has invaded thousands of hectares in the southeastern United States. Its negative impacts on pine forests have been well documented, and aggressive control is widely recommended. Although repeated herbicide treatments are effective for suppression, integrated strategies of prescribed burning coupled with herbicide treatment and revegetation are lacking in pine systems. In particular, longleaf pine forests, which are typically open, fire-dependent, communities, are highly susceptible to cogongrass, which is a pyrogenic species. To address management goals for cogongrass control and herbaceous restoration in longleaf pine forests better, field studies were conducted in southwestern Alabama from 2010 to 2012. Two longleaf pine forests with near-monotypic stands of cogongrass in the understory were selected for study. Treatments included combinations of winter prescribed fire, spring and fall glyphosate herbicide treatments, and seeding a mix of native, herbaceous species. Data were collected for three growing seasons following study initiation, and included seasonal herbaceous species cover and final cogongrass shoot and rhizome biomass. Species richness and diversity were calculated and analyzed to ascertain treatment effects over the duration of the study. Burning slightly improved cogongrass control with glyphosate, but had no effect on total cover, species richness, or species diversity. Three glyphosate treatments reduced total vegetative cover and nearly eliminated cogongrass cover, shoot, and rhizome biomass. Glyphosate and glyphosate + seeding also increased herbaceous species richness and diversity. However, aboveground productivity in treated plots was significantly lower than productivity in the untreated control, which was almost exclusively cogongrass. These studies indicate that glyphosate and integrated strategies utilizing glyphosate and seeding are very useful for cogongrass management and increasing herbaceous species richness and diversity in longleaf pine. Nomenclature: Glyphosate, cogongrass, Imperata cylindrica (L.) Beauv., longleaf pine, Pinus palustris Mill. Management Implications: Cogongrass is a serious threat to southeastern pine ecosystems. Although numerous control studies have been conducted, information is still lacking for cogongrass management in longleaf pine. With longleaf pine, many land managers want to recreate mixed pine/open savannah conditions with a species-rich herbaceous understory. However, cogongrass is a significant obstacle to that goal, as it forms near-monotypic stands beneath longleaf pine that are of little value. To address this problem, we tested multiple integrated methods, including prescribed burning, glyphosate treatment, and seeding native herbaceous species. We found that glyphosate treatment following prescribed burning provided a minor but significant increase in cogongrass control compared to glyphosate-treated, unburned plots. However, the effect was short-lived and not useful when follow-up glyphosate treatments were applied. Glyphosate treatment combined with seeding native species generally increased cover, species richness, and diversity compared to glyphosate treatment alone. Burning and seeding without glyphosate treatment was ineffective, as cogongrass quickly recovered. In summary, we found that glyphosate treatment was critical for cogongrass control and improvements in native species richness and diversity.

Pushing toward cogongrass (Imperata cylindrica) patch eradication: the influence of herbicide treatment and application timing on cogongrass rhizome elimination.

Abstract Cogongrass, an invasive grass native to Asia, has infested thousands of hectares in the southeastern United States. Although numerous studies have examined cogongrass control, no published studies, to our knowledge, have tested strategies for cogongrass eradication. Cogongrass has a persistent, thick rhizome mat but an ephemeral seedbank; therefore, successful eradication methods must largely focus on the rhizomes. A field study to evaluate specific herbicide treatments and application timings for cogongrass patch eradication was conducted at two locations in southwestern Alabama. Herbicide treatments included glyphosate at 4.48 kg ai ha−1, imazapyr at 0.84 kg ai ha−1, and a tank mix of glyphosate and imazapyr at the same rates. Treatments were applied in May, August, or October for 3 consecutive yr, and the May glyphosate treatment included a second annual application each October. Cogongrass visual control, shoot biomass, rhizome biomass, rhizome depth, and total nonstructural carbohydrate (TNC) content were sampled during the course of the study. Cogongrass response to treatments varied by location but by 36 mo after initial treatment (MAIT), complete elimination of cogongrass shoot and rhizome biomass and 100% visual control was achieved in several herbicide treatment–timing combinations at both locations. These included glyphosate plus imazapyr at any application timing, imazapyr in August or October, and glyphosate applied in May and October each year. TNC levels of surviving healthy rhizomes were not affected by herbicide treatments, but a seasonal pattern was observed. The maximum live-rhizome depth was not influenced by any treatment, indicating that herbicides were not preferentially leaving deeper, surviving rhizomes. These results demonstrate, for the first time, that the entire rhizome layer of cogongrass can be eliminated within 3 yr with multiple treatment options and that cogongrass patch eradication is possible for many land managers. Nomenclature: Glyphosate, glyphosate plus imazapyr, imazapyr, cogongrass, Imperata cylindrica (L.) Beauv. IMPCY. Management Implications: Cogongrass is one of the most difficult weeds to manage because of its aggressive growth and persistent rhizomes, which often survive initial herbicide treatments. Historically, research efforts have been focused on cogongrass control, but no published studies have tested strategies for eradication. The present study is the first documented research to demonstrate complete elimination of cogongrass in 18 to 36 mo using repeated, annual herbicide applications. Treatments included glyphosate, imazapyr, and a tank-mix of both applied in the spring, summer, or fall for 3 consecutive yr. Verification of eradication was based on a highly rigid criterion involving measurements of cogongrass visual control, shoot biomass, rhizome biomass, rhizome depth, and total nonstructural carbohydrate (TNC) content over 3 yr. Cogongrass response to treatments varied by location. By 36 mo after initial treatment, the glyphosate plus imazapyr treatment applied at any timing, the imazapyr treatment applied in August or October, and the glyphosate treatment applied in May and October each year resulted in complete elimination of cogongrass shoot and rhizome biomass. The maximum live-rhizome depth (16 cm ± 2 SE) was not influenced by any treatment. During the 3-yr period, herbicides did not affect TNC levels of surviving rhizomes, indicating that repeated treatments directly killed rhizomes, rather than slowly exhausting energy reserves. We are not suggesting that cogongrass can be eradicated from the southeastern United States; however, with repeated glyphosate or imazapyr herbicide treatments, land managers do have a feasible means of eradicating cogongrass patches.

Macartney Rose (Rosa bracteata) Response to Herbicide and Mowing Treatments

Abstract Macartney rose is an aggressive thorny shrub that displaces forage species and hinders cattle grazing in rangelands and pastures of the southern United States. Historically, Macartney rose has proven to be extremely difficult to control even with high rates of soil residual herbicides such as picloram. Recent advances in herbicide chemistry warrant testing on this troublesome species. We compared mowing and late summer broadcast applications of thirteen herbicide treatments that included combinations of aminopyralid, fluroxypyr, metsulfuron, picloram, triclopyr, and 2,4-D. Treatments were applied to the same rose clumps for 2 consecutive yr. An additional mowing was done to one half of the rose clumps in each treatment 6 mo after the second herbicide treatment. At 11 mo after initial treatment (MAIT), mowing and all herbicide treatments performed very poorly and provided 35% control or less. At 12 mo after retreatment (24 MAIT), picloram + 2,4-D and aminopyralid + metsulfuron, both followed by mowing, were the most effective treatments, providing 72 to 91% control. All other treatments provided less than 70% control. However, complete clump mortality was very low across all treatments, ranging from 3 to 32%. These results indicate that Macartney rose suppression is possible with certain new herbicides, but complete clump kill is still lacking. Nomenclature: Aminopyralid, fluroxypyr, metsulfuron, picloram, triclopyr, 2,4-D, Macartney rose, Rosa bracteata J. C. Wendl. ROSBC Management Implications: Macartney rose is one of the most difficult-to-control thorny shrubs in pastures across the Southeast. We evaluated a suite of newer pasture herbicides that included several combinations of 2,4-D, aminopyralid, fluroxypyr, metsulfuron, picloram, and triclopyr. Broadcast treatments were applied for 2 consecutive yr in August over the same individual rose clumps and mowing was done 6 mo after the second application. We found that none of the herbicides and rates tested effectively killed entire clumps but that Macartney rose was suppressed best with picloram + 2,4-D treatments. Triclopyr was ineffective when applied alone or when tank-mixed with aminopyralid + 2,4-D or aminopyralid + metsulfuron. Metsulfuron was also ineffective and did not improve control when applied with aminopyralid alone or in combination with 2,4-D. These results indicate that typical annual broadcast applications currently utilized for broadleaf pasture weed control will not effectively eliminate the invasive shrub Macartney rose and more integrated methods need to be evaluated.

Competitive Ability of Black Henbane (Hyoscyamus niger) When Grown with Three Native Grasses

Abstract Black henbane is a poisonous, invasive plant in the family Solanaceae, and is typically associated with highly disturbed environments, such as pipelines, roadsides, and mammalian burrows. Often, such disturbances require reseeding for successful restoration; thus, the potential exists for competition between henbane and perennial grasses commonly used in restoration projects. These competitive interactions have not, to our knowledge, been evaluated. We conducted a greenhouse study to compare the response of henbane when grown alone and in combination with three common, cool season, perennial, northern mixed prairie grass species. We examined both seedling and mature grass response to the presence or absence of henbane and the response of henbane to the grasses. Using the relative neighbor-effect index, black henbane was found to be a very poor competitor with mature grasses and two out of three seedling grasses tested. All measures of henbane growth were significantly lower among plants grown with a mature grass pot companion. Total biomass of henbane was up to 99% lower when grown with mature grasses. Mature grasses were not negatively affected when grown in combination with henbane. Western wheatgrass (Pascopyrum smithii) was the only seedling grass that was competitive with henbane but was also the only seedling grass negatively affected by henbane in both biomass and tiller production. These experiments suggest that henbane is not well suited for invasion of mature grass stands but may negatively influence some perennial grass seedlings in restoration situations. Nomenclature: Black henbane, Hyoscyamus niger L.; western wheatgrass, Pascopyrum smithii (Rydb.) Á. Löve Interpretive Summary: Black henbane is a poisonous, invasive annual or biennial in the family Solanaceae that appears to be an opportunistic invader, proliferating when competition is minimal. Black henbane can be successful in situations with limited competition and high nutrient and moisture availability. As such, henbane tends to be associated with disturbed conditions in which reseeding of native vegetation may be required. Revegetation with perennial grasses is an excellent method for long-term control of invasive plants but is often species and site specific. The results from these experiments indicate that black henbane is a poor competitor with Sandberg bluegrass and Idaho fescue seedlings, but it does negatively affect western wheatgrass seedlings. However, once established, all three grasses strongly suppressed henbane. For reclamation efforts along disturbance corridors, such as gas lines where black henbane often appears, multiple grass species may be effective in suppressing henbane.