Canada thistle can cause greater than 50% yield loss in small grain crops, but little is known about production losses when the weed invades pasture and wildlands. Change in grass, forb, and woody species production from Canada thistle infestations was evaluated in two separate studies in North Dakota. The first measured change in production following aminopyralid applied at 120 g ha−1 to control Canada thistle at two prairie sites. In general, grass, broadleaf, woody, and total plant yields were similar between treated and untreated prairie, regardless of the near-complete control of Canada thistle following aminopyralid application. Grass yield increased by 365 kg ha−1 the year after treatment at one location, with no change in forb or woody species production. Plant production was also estimated at 20 ungrazed wildland preserves located within two Major Land Resource Areas (MLRAs). Similar to the prairie sites, minimal differences in production between Canada thistle–infested and noninfested sites were observed. The only exception was an increase in grass production of 425 kg ha−1 at one of the MLRAs, with no change in broadleaf or woody species production between the Canada thistle–infested and noninfested sites. In contrast to cropland, pasture and wildland production of other species was not consistently reduced by Canada thistle.

Cuphea is a new crop of temperate regions that produces seed oil that can substitute for imported coconut and palm kernel oils. Only four herbicides are known to be tolerated by cuphea to date. More herbicides, especially POST products, are needed for continued commercialization. In Minnesota and North Dakota, where cuphea currently is grown, greater control of Canada thistle and biennial wormwood is needed in cuphea. Because clopyralid is effective on both of these species, it was tested at rates ranging from about 25 to 850 g ae ha−1 in greenhouse and field trials. Visual assessment of injury, height, growth, and seed yield of cuphea were not reduced significantly in field-grown plants when clopyralid was applied at rates up to 400 g ae ha−1. Thus, at the rate commonly used in other crops, 200 g ae ha−1, clopyralid can be applied safely to cuphea.

Crops transformed to provide resistance to herbicides with two different mechanisms of action provide new opportunities for control of herbicide-resistant weeds. However, unexpected interactions may develop, especially for herbicides not generally used in tank-mixtures. The objectives of this study were to evaluate weed control and determine herbicide interactions and fluorescence responses with combinations of glyphosate and glufosinate on selected weeds prevalent in Michigan cropping systems. Field studies to determine herbicide interactions resulted in synergism only at 0.84 kg ae ha−1 of glyphosate and 0.47 kg ai ha−1 glufosinate in 2008. Early synergism (7 d after treatment [DAT]) was observed in the field at several combined rates for common lambsquarters and velvetleaf in 2009, and in the greenhouse for giant foxtail. Differences between years were perhaps due to the effect of environmental conditions on herbicide absorption and translocation. Antagonism was observed in the field in 2009 for velvetleaf, common lambsquarters, and giant foxtail especially at 840 g ae ha−1 glyphosate and 118 g ai ha−1 glufosinate, 28 DAT. Antagonism was also observed in the greenhouse for giant foxtail and Canada thistle, 28 DAT. Fluorescence measurements on Canada thistle in the greenhouse showed that glufosinate and glufosinate plus glyphosate acted rapidly to quench electron transport of photosystem II (PS II) system of photosynthesis, and the fluorescence characteristics of the glyphosate and glufosinate combinations were indistinguishable from glufosinate alone.

Riparian habitats are important components of an ecosystem; however, their hydrology combined with anthropogenic effects facilitates the establishment and spread of invasive plant species. We used a maximum-entropy predictive habitat model, MAXENT, to predict the distributions of five invasive plant species (Canada thistle, musk thistle, Russian olive, phragmites, and saltcedar) along the North Platte River in Nebraska. Projections for each species were highly accurate. Elevation and distance from river were most important variables for each species. Saltcedar and phragmites appear to have restricted distributions in the study area, whereas Russian olive and thistle species were broadly distributed. Results from this study hold promise for the development of proactive management approaches to identify and control areas of high abundance and prevent further spread of invasive plants along the North Platte River.

Management of weeds is often a barrier to conversion from conventional to organic agriculture. Tef is a C4 annual cereal that is valued for its small seeds, rapid establishment, and wide adaptation. The objective of this study was to evaluate tef as a smother crop for management of weeds during transition to organic production. Greenhouse and field trials were conducted in 2008 and 2009 to evaluate the growth of eight tef varieties and their effect on Canada thistle and annual weeds. In greenhouse studies, tef decreased the biomass of Canada thistle shoots and roots 44 to 74%, depending on variety. Emergence of Canada thistle shoots was affected by the planting depth of their roots. Tef variety Corvalis suppressed Canada thistle biomass and accumulated more biomass than most other tef varieties. In field studies, tef varieties suppressed annual weed biomass by 35 to 54% with varieties Corvalis, Dessie, and VA-T1 being least suppressive in 2008, but there were no differences between varieties in 2009. Canada thistle growth was suppressed an average of 73% by tef in 2008 and 37% in 2009, a year of cooler temperatures and unseasonal rainfall. Differences between varieties in suppressing Canada thistle and annual weeds were mostly inconsistent between years. However, tef variety Tiffany did consistently suppress biomass, height, and percentage cover of Canada thistle and other weeds in the field study in 2008 and 2009.

Weeds can infest management-intensive grazed pastures and impact forage quantity, forage quality, and animal health. Common burdock, plumeless thistle, and Canada thistle are three common pasture weeds in the midwestern United States that are managed to avoid these impacts. Experiments were established at two sites to determine if increasing grazing heights from fall through summer would reduce emergence and survival of burdock, plumeless thistle, and Canada thistle seedlings. Five simulated grazing heights (5, 10, 15, and 20 cm and a not-clipped treatment) were implemented in October 2008 and repeated in May through August. Density of all species was reduced from May to September, with reductions ranging from 65 to 78%, regardless of treatment. Treatments that left at least 15 cm of residual grass had reduced densities of burdock and Canada thistle compared to the 10-cm treatment. Regression analysis demonstrated that reduction in burdock and summed planted weed density was related to increased intercepted photosynthetically active radiation from forage in April. However, total biomass yield was reduced up to 60% when grazing heights were increased from 5 to 20 cm, although differences were only observed at the fall and early spring grazing events. Relative forage quality (RFQ) was similar across treatments, except at the third grazing event for which the 15 and 20-cm treatments had reduced RFQ compared with other treatments. Results suggest that increasing grazing heights can reduce emergence and survival of burdock and Canada thistle but can also result in a reduction in forage quantity in the fall and early spring.

Canada thistle is a perennial spreading weed that is difficult to control in farming systems with reduced reliance upon herbicides for weed management. Experiments were conducted from 2006 to 2008 at Champaign, IL, to evaluate the combined effects of summer annual cover crops and mowing on Canada thistle growth and survival. Whole plot treatments were fallow, buckwheat, sudangrass–cowpea mixture (MIX), and sudangrass. The subplot treatments were mowing frequencies (0 to 2 times). Cover crop and mowing did not interact to suppress Canada thistle. MIX and sudangrass produced more standing biomass, greater regrowth, and more surface mulch following mowing than the buckwheat. A single season with sudangrass or MIX reduced Canada thistle shoot density and mass to less than 20% of the initial values through two growing seasons. Mowing alone only suppressed Canada thistle shoot density and mass on the site with greater initial density. A sudangrass or MIX cover crop alone or combined with mowing suppresses Canada thistle, but intensive management must continue for several years to eliminate patches.

Canada thistle is difficult to manage in organic farming systems and others with reduced reliance on herbicides. Previous field studies found that defoliation or sudangrass interference suppressed Canada thistle. Our objective was to understand the factors causing suppression of Canada thistle observed in the field. Three greenhouse studies were conducted utilizing frequency of defoliation, sudangrass interference and defoliation, and interspecific phytotoxicity to discern mechanisms of Canada thistle suppression. Increased defoliation frequency (up to four defoliations) decreased Canada thistle shoot height, shoot and root mass, and root-to-shoot ratio. Plants with larger root mass had greater shoot mass and number (r = 0.87 and 0.73, respectively), indicating a probable interdependence of root size (carbohydrate reserves), bud density, and subsequent shoot growth. In the sudangrass interference and defoliation study, Canada thistle shoot dry mass was 38.7, 2.76, and 0.39 g pot−1 in the defoliation only, sudangrass interference only, and defoliation + interference + surface mulch treatments, respectively. Sudangrass interference by itself was effective in suppressing thistle growth; combining interference with defoliation did not further reduce growth (2.76 and 2.83 g pot−1, respectively). In the experiment minimizing interspecific competition, we found no evidence of sudangrass having a phytotoxic effect on Canada thistle. Overall results indicate that sudangrass competition or frequent shoot removal suppresses growth of Canada thistle.

This study determined the potential of the tortoise beetle (Coleoptera: Chrysomelidae) to control Canada thistle (Asteraceae) in Iran. Genetic analysis of the tortoise beetle, based on mitochondrial DNA, confirmed the presence of the species in Iran. A field experiment using five insect densities (0 to 20 larvae plant−1) showed a positive correlation between the number of larvae transferred and impact. Feeding by 20 larvae reduced total biomass of Canada thistle by 78% and the number of capitula by 94%. More important, when grown in competition with wheat, four and eight egg batches (corresponding to approximately 12 and 24 larvae) per Canada thistle plant increased wheat ear weight by 46 and 82%, respectively. Host range studies with 22 crop and 21 weed species using no-choice and multiple-choice tests under laboratory and field conditions and parallel data from a field survey showed that joint feeding and oviposition were restricted to Canada thistle and a few other weed species. Limited feeding, without oviposition, was recorded on an additional seven weed species but also on safflower (10 to 15% reduction in biomass), and common sunflower (< 10%); the latter only under no-choice conditions. The growing period of either crop species, however, does not coincide with the feeding period of the tortoise beetle in the field. Findings indicate that the tortoise beetle is a promising biological control agent for Canada thistle in arable crops and grasslands in Iran. Other complementary methods will likely be needed to prevent substantial yield losses.

Canada thistle poses a particular threat to organic producers in temperate agriculture due to its ability to reproduce through an extensive system of underground roots. The Canada thistle life cycle, growth, and development are seasonally affected, and exploiting this biology may be useful for weed management. The objective of this study was to evaluate smother crop mixtures, seeded at different times, for Canada thistle control. Field trials were established in 2009 and 2010 to evaluate the ability of smother crop mixtures to suppress Canada thistle growth and development. Canada thistle aboveground biomass was suppressed 50% in 2009 and 87% in 2010 by the sorghum–sudangrass mixture, averaged over planting times. The oat mixture suppressed annual weed biomass more than 58% in 2009 and 67% in 2010 in all planting dates. Percent cover of Canada thistle was affected by crop mixture in 2009 and 2010, with sorghum–sudangrass being the most suppressive. The sorghum–sudangrass mixture was more suppressive of Canada thistle, probably because it included soybean and sunflower, all high-biomass, competitive crops. Planting date affected smother crop suppression of Canada thistle growth, but the effect was not consistent between 2009 and 2010 due to differences in weather conditions.

In the present field study, the capability of Canada thistle to develop shoots from intact roots and root fragments at different soil depths was studied. The experiments were performed on four sites with high-density Canada thistle, with three or four replications per treatment. At each site, the soil in the plots was removed layer by layer (to 30 or 40 cm, depending on the site), within a 1 by 1-m quadrat, and spread out on a plastic sheet. All roots and other plant parts were removed, and the soil was either replaced without any root material (two sites), or the roots of the thistles were cut into 10-cm-long fragments and replaced into the source holes (two sites). The measured variables were shoot number and biomass. The number of shoots of Canada thistle decreased with increasing depth (P < 0.001) and increased with time. Additionally, the two factors interacted (P < 0.001) such that shoot development was slower from greater depths. Roots from ≤ 20 cm depth produced higher biomasses than did roots from below 20 cm depth. Replacement of root fragments did not affect the amount of biomass produced. It was concluded that the intact root system contributed considerably more to the total biomass produced by Canada thistle than did the root fragments in the upper soil layers.