Vanelle F. Peterson

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.

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

Preemergent Control of Medusahead on California Annual Rangelands With Aminopyralid

Abstract Medusahead (Taeniatherum caput-medusae [L.] Nevski), the most problematic invasive grass on many California rangelands, is difficult to control selectively in grasslands. Prescribed burning, grazing, and herbicides have been tested with some success but are not practical in all situations. The selective herbicide aminopyralid, normally used for control of certain broadleaf species such as thistles, suppresses some annual grasses when applied pre- or early postemergence. In 2009–2010, we tested the efficacy of aminopyralid for medusahead control in preemergence applications at three foothill rangeland sites in northern California. We compared a rate series of aminopyralid (53, 88, 123, and 245 g · ha−1 acid equivalent [ae]) with rimsulfuron (18 and 35 g · ha−1 active ingredient) and imazapic (140 g · ha−1 ae). Plots were 3 × 9 m with four replications at each site. Treatments were applied in early fall 2009. In May 2010, we took visual cover estimates and biomass/seedhead samples in three quadrats per plot. In regression analysis, medusahead cover was found to decrease consistently with increasing rates of aminopyralid. Medusahead control at the highest rates of aminopyralid was consistent across the three sites, averaging 89% ± 3 standard deviation (SD) with 245 g · ha−1 ae and 59% ± 10 SD with 123 g · ha−1 ae. Aminopyralid at lower rates, rimsulfuron, and imazapic were less consistent. Cover of other annual grasses increased in plots treated with aminopyralid at all sites. Aminopyralid has potential utility for suppressing medusahead, particularly in sites also infested with invasive members of the Asteraceae. However, the most effective rate (245 g · ha−1 ae) is registered for use only as a spot application. In situations where this rate can be justifiably used, it would be expected to give season-long control of medusahead, as well as longer-term control of thistles and other susceptible species. Resumen Medusahead (Taeniatherum caput-medusae [L.] Nevski), la especie de pasto invasiva más problemática en muchas áreas de California, es difícil de controlar selectivamente en los pastizales. Se ha probado el fuego prescrito, el pastoreo, y los herbicidas con algo de éxito pero no son prácticos en todas las situaciones. El herbicida selectivo aminopyralid, normalmente usado para el control de ciertas especies de hoja ancha como cardo, suprime algunos pastos anuales cuando se aplica durante la etapa de pre o al inicio de pos-emergencia. In 2009–2010 se evaluó la eficiencia de aminopyralid para controlar medusahead en aplicaciones pre-emergentes en tres sitios localizados en pastizales del norte de California. Se compararon una serie de rangos de aplicación de aminopyralid (53, 88, 123, y 245 g · ha−1 acido equivalente [ae]) con rimsulfuron (18 y 35 g · ha−1 de ingrediente activo) e imazapic (140 g · ha−1 ae). Las parcelas fueron de 3 m por 9 m con cuatro repeticiones en cada sitio. Los tratamientos se aplicaron durante el inicio del verano de 2009. En mayo de 2010 se tomo una estimación visual de la cobertura y muestras de biomasa/inflorescencia en tres cuadrantes por parcela. En los estudios de regresión encontramos que la obertura de medussahead disminuyó consistentemente con el incremento en el rango de aplicación de aminopyralid. El control de medusahead bajo los niveles más elevados de aminopyralid fue consistente en los tres sitios, promediando 89% ± 3 SD con 245 g · ha−1 ae y 59% ± 10 SD con 123 g · ha−1 ae. La aplicación de aminopyralid a bajos niveles, rimsulfuron, e imazapic fueron menos consistentes. La cobertura de otras especies de pastos anuales se incrementó en las parcelas tratadas con aminopyralid en todos los sitios. Aminopyralid tiene un potencial para suprimir medusahead, particularmente en sitios infestados con los miembros invasivos de asterácea. Sin embargo, la tasa más efectiva (245 g · ha−1 ae) está documentada para uso solo de aplicación a un sitio específico. En situaciones donde esta tasa puede ser usada justificadamente, se esperaría que ofreciera control durante toda la temporada sobre medusahead así como control a largo plazo de cardos y algunas especies perenes.

Control of yellow starthistle (Centaurea solstitialis) and coast fiddleneck (Amsinckia menziesii) with aminopyralid.

Abstract Yellow starthistle is the most widespread broadleaf invasive plant in the western United States, and it is particularly prevalent in California. Prior to the registration of aminopyralid in 2005, the standard for chemical control of yellow starthistle was the herbicide clopyralid. We report on a compilation of several independent trials comparing the efficacy of aminopyralid and clopyralid on yellow starthistle. Treatments were applied at several rates and timings at 11 locations in four states between 2001 and 2007. Treatments were made pre-emergence and postemergence at the seedling and rosette stages of yellow starthistle. Results showed that aminopyralid, even at the low rate of 18 g ae ha−1, provided nearly complete control of yellow starthistle when treatments were made at the seedling stage. However, less consistent control (80 to 100%) resulted with applications made at the pre-emergence and rosette stages. At the seedling stage, aminopyralid is about four times more effective on yellow starthistle compared to clopyralid, based on the rate of acid equivalent. In the Central Valley of California, complete control was obtained at the lowest registered rate (53 g ae ha−1) when applications were made from December through February. At two locations we also evaluated control of the poisonous native plant coast fiddleneck. Although clopyralid does not adequately control coast fiddleneck, aminopyralid provided almost complete control when applied in the winter growing season. Applications of aminopyralid at the rosette stage resulted in a two-fold increase in annual forage grass biomass the following year. These results indicate that aminopyralid is a valuable tool for land managers and can play an important role in integrated management strategies for yellow starthistle and coast fiddleneck. Nomenclature: Aminopyralid; clopyralid; yellow starthistle; Centaurea solstitialis L.; coast fiddleneck; Amsinckia menziesii (Lehm.) A. Nels. & J. F. Macbr. var. intermedia (Fisch. & C. A. Mey.) Ganders. Interpretive Summary: Yellow starthistle is one of the most invasive species of rangelands and natural areas in the western United States. Several control options have been developed including mowing, grazing, burning, biological control, and herbicides. Until 2005, the herbicide clopyralid was considered the most effective chemical option for yellow starthistle control. However, clopyralid does not provide effective control of coast fiddleneck, which often co-occurs with yellow starthistle in rangelands of California. Coast fiddleneck is toxic to livestock and often increases in cover following clopyralid treatment. Aminopyralid was registered for use in noncrop areas in 2005. In this study we compared the efficacy of aminopyralid and clopyralid for yellow starthistle and coast fiddleneck control at several rates and timings at 11 locations in four states during the time period ranging from 2001 to 2007. Our results indicate that aminopyralid requires about one-fourth the active ingredient compared to clopyralid to achieve similar control of yellow starthistle at the seedling stage. Unlike clopyralid, aminopyralid also provides excellent control of coast fiddleneck. Although pre-emergence applications and late-season applications (rosette stage) also control yellow starthistle, those timings were less consistent at lower rates and therefore required higher rates as per label instructions. Late-stage applications were more effective given higher rainfall. We did find that late-season application resulted in increased annual forage grass production the following year. In the Central Valley of California, the optimal timing of application was from December through February. In cooler climatic regions the optimal application timing window might be somewhat later. Our results indicate that aminopyralid is a very effective tool for the management of yellow starthistle and coast fiddleneck and could be incorporated into an integrated management program.

Effects of Aminopyralid on Ponderosa Pine (Pinus ponderosa)

Abstract Invasive weed control within cleared, forested sites in the inland Northwest is complicated by the susceptibility of ponderosa pine to synthetic auxin herbicide injury, used to control broadleaf weeds. Herbicide injury may lead to decreased canopy volume and variable growth patterns of ponderosa pine, which is a commercially important tree species. Herbicide injury to ponderosa pine can be decreased with dormant-season applications, a timing suited to control many weeds that may occur within ponderosa pine sites. However, spring-timed herbicide applications are needed to control other weeds, such as meadow hawkweed, and that application timing coincides with active ponderosa pine growth. In this study, we determined the level of injury to ponderosa pine resulting from spring-timed aminopyralid, clopyralid, and picloram applications beneath ponderosa pine canopies. Herbicide injury to leader and lateral candles and needle elongation was evaluated 1 and 12 mo after treatment (MAT). Low rates of aminopyralid alone (0.05 kg ae ha−1 [3 fl oz ac−1]) and aminopyralid + clopyralid (0.05 + 0.10 kg ae ha−1) resulted in herbicide injury ratings that did not differ from untreated trees. The high rate of aminopyralid (0.12 kg ae ha−1) resulted in leader candle injury on 75% of treated trees, 5% of which were necrotic at 12 MAT. Herbicide injury was observed on 30% of lateral candles. In comparison, picloram (0.28 kg ae ha−1) treatments resulted in necrosis or mortality of leader and lateral candles on 65% and 40% of trees, respectively, at 12 MAT. Results suggest that use of low rates of aminopyralid alone or in combination with low rates of clopyralid minimizes the risk of nontarget injury to ponderosa pine (> 5 yr old) while controlling hawkweed with a spring application. Nomenclature: Aminopyralid; clopyralid; picloram; meadow hawkweed, Hieracium caespitosum Dumort; ponderosa pine; Pinus ponderosa Lawson & C. Lawson. Interpretive Summary: Broadleaf weeds under ponderosa pine canopies are often controlled in the fall when trees are dormant but the weeds are susceptible to treatment. Meadow hawkweed, however, is not susceptible to fall herbicide treatments. Managing broadleaf weeds in the spring risks injury to ponderosa pine, thus herbicide application rates were tested to determine whether injury could be minimized. Spring applications of low rates of aminopyralid (0.05 kg ae ha−1) with or without low rates of clopyralid (0.10 kg ae ha−1) did minimize the risk of injury to ponderosa pine when applied below the canopy. Our results suggest that use of high aminopyralid label rates (0.12 kg ae ha−1) or picloram (0.28 kg ae ha−1) should be avoided when targeting invasive weeds in the spring because of likely negative effects to ponderosa pine. In particular, the results of this study should improve meadow hawkweed management decisions in the Pacific Northwest, which requires spring-timed applications but are effective at low use rates.

Timing Aminopyralid to Prevent Seed Production Controls Medusahead (Taeniatherum caput-medusae) and Increases Forage Grasses

Abstract Exotic annual grasses such as medusahead [Taeniatherum caput-medusae (L.) Nevski] and downy brome (Bromus tectorum L.) dominate millions of hectares of grasslands in the western United States. Applying picloram, aminopyralid, and other growth regulator herbicides at late growth stages reduces seed production of most exotic annual grasses. In this study, we applied aminopyralid to T. caput-medusae to determine how reducing seed production in the current growing season influenced cover in the subsequent growing season. At eight annual grassland sites, we applied aminopyralid at 55, 123, and 245 g ae ha-1 in spring just before T. caput-medusae heading. The two higher rates were also applied pre-emergence (PRE) in fall to allow comparisons with this previously tested timing. When applied in spring during the roughly 10-d period between the flag leaf and inflorescence first becoming visible, just 55 g ae ha-1 of aminopyralid greatly limited seed production and subsequently reduced T. caput-medusae cover to nearly zero. Fall aminopyralid applications were less effective against T. caput-medusae, even at a rate of 245 g ae ha-1. The growing season of application, fall treatments, but not spring treatments, sometimes reduced cover of desirable winter annual forage grasses. The growing season after application, both spring and fall treatments tended to increase forage grasses, though spring treatments generally caused larger increases. Compared with other herbicide treatment options, preheading aminopyralid treatments are a relatively inexpensive, effective approach for controlling T. caput-medusae and increasing forage production.

Native Grass Establishment following Application of Pyridine Herbicides

Invasive species management is often more successful if desirable species are seeded after the target weed is controlled. However, control of invasive plants must be maintained following reseeding or the seeded species may fail to establish. A regional study conducted in Minnesota, North Dakota, and South Dakota evaluated the effect of aminopyralid, clopyralid, or picloram applied in the fall prior to fall-dormant seeding or seeding the following spring on cool- and warm-season native grass species establishment. Herbicides were applied at standard rates used to control invasive broadleaf weeds in the upper midwestern tallgrass prairie region of the United States. Cool-season species included Canada wildrye, green needlegrass, and intermediate wheatgrass. Warm-season species included big bluestem, little bluestem, sideoats grama, switchgrass, and Indiangrass. Aminopyralid did not reduce seedling establishment in either fall or spring seeding. Grasses generally were not affected by a pretreatment of the pyridine standards clopyralid or picloram either, with the exception of a slight reduction in fall-seeded establishment of intermediate wheatgrass. Picloram also slightly reduced fall-seeded establishment of Canada wildrye. Application of aminopyralid can safely be used to control susceptible invasive species preceding grass species establishment, with a safety margin similar to or slightly better than that with the pyridine standards clopyralid or picloram. Nomenclature: Aminopyralid; clopyralid; picloram; big bluestem (Andropogon gerardii Vitman); Canada wildrye (Elymus canadensis L.); green needlegrass [Nassella viridula (Trin.) Barkworth]; indiangrass [Sorghastrum nutans (L.) Nash]; intermediate wheatgrass [Thinopyrum intermedium (Host) Barkworth & D. R. Dewey]; little bluestem [Schizachyrium scoparium (Michx.) Nash]; sideoats grama [Bouteloua curtipendula (Michx.) Torr.]; switchgrass (Panicum virgatum L.).