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Integrated Canada Thistle Control in Pasture | |
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Introduction
Canada thistle is a long-lived perennial weed common in pastures throughout central Alberta, particularly where moisture is abundant. Canada thistle is persistent, in large part due to an extensive creeping root system. This weed falls into the noxious category under the Weeds Act, indicating that where present, steps must be taken by landowners to prevent its spread. Although Canada thistle has been intensively researched in the past, most work has taken place within cultivated cropping systems. As a result, there is limited information on the impact of this weed in permanent pastures, nor on the most effective means of control. This review outlines results from a research program conducted between 1999 and 2002 addressing questions dealing with the management of this noxious weed in permanent pastures of central Alberta. This program combined resources from the Alberta Agricultural Research Institute, the University of Alberta, AAFRD, over a dozen counties, as well as industry (BASF Canada and Dow AgroSciences).
Yield Loss
Our first objective was to quantify forage losses associated with increasing levels of thistle infestation. Although yield loss data is readily available for annual crops, this information has been deficient for permanent pastures, resulting in uncertainty over the importance of thistle control. Between 1999 and 2001, we conducted 8 yield loss assessment trials throughout central Alberta. Our results frequently demonstrated a significant negative association between thistle abundance [either stem densities (stems/m2) or biomass (kg/ha-dry matter)] and forage production (e.g. see Fig. 1A). At several sites, initial yield loss relationships were subsequently validated when the removal of thistle from plots increased forage yields the following year, with greater thistle removal being associated with greater yield gains (Fig. 1B).
Results of this study supported the concern that Canada thistle reduces pasture forage yields, and this information should provide a strong incentive for producers to control this weed. However, our results also indicated that the presence of a yield loss and the magnitude of that effect was inconsistent among sites (6 of 8 demonstrated some significant negative effect), likely due to high variability in soil, moisture, and forage types throughout the region, as well as their level of pre-existing management (i.e. grazing history, fertilization, etc.).
Efficacy Trial
Our second major objective examined various control options. In particular, we evaluated various one-time control treatments and their longer-term effectiveness (up to 3 years). We compared 4 herbicides to mowing and a check treatment at each of 4 sites across central Alberta. We also examined these treatments with and without annual fertilization (100-45-10-15 kg/ha of N-P-K-S, respectively) as other research has shown that enhanced fertility can limit the impact of weeds. Fertilization was intended to eliminate all macronutrient deficiencies. The 4 herbicides we used represented a range of active ingredients and modes of action, and included 2,4-D ester 700 (2.5 L/ha), Lontrel (0.6 L/ha), Grazon P+D (3.7 L/ha), and Dyvel DS (3.25 L/ha). All treatments were applied in early July of 1999 when thistle root carbohydrates were deemed to be at a minimum, a time when thistle plants are susceptible to damage.
Initial levels of thistle control in 1999, the year of treatment, were similar between herbicides, ranging from 84 to 93% mortality. Subsequent reductions in both thistle biomass (Fig. 2) and thistle density (Fig. 3) reflected this control. Two other important effects were evident in the year of treatment. The first was an increase in thistle biomass with fertilization in the absence of thistle control (Fig. 3). Thus, the addition of fertilizer in the absence of thistle control is likely to waste fertilizer and money, and appears to make the thistle infestation worse. Second, one-time mowing increased thistle density (Fig. 2). The latter response is a reflection of the ability of thistle to regrow from basal buds, and highlights the fact that infrequent mowing is more cosmetic than effective in reducing thistle populations.
Residual effects of the treatments were also observed for 2 subsequent growing seasons (2000 and 2001). In particular, thistle density (Fig. 4) and biomass continued to be lower within herbicide treated plots. Moreover, the combined use of fertilizer and herbicide resulted in a greater reduction in thistle abundance through both years. This is likely a result of the increased vigor of the forage stand associated with fertilization, which in turn, suppressed recovery of damaged thistle. Additionally, there were minor differences among herbicide treatments with respect to their residual level of thistle control. Whether these differences are significant, however, particularly in terms of a cost-benefit analysis, remains to be determined.
In terms of forage response, thistle removal within the herbicide treatments resulted in increased grass biomass one year later (Fig. 5). These results reinforce the yield loss findings reported earlier, and suggest thistle removal is effective in enhancing forage availability.
Wiping for Thistle Control
The efficacy trial was accompanied by an evaluation of weed wipers for controlling thistle with herbicides. Weed wipers apply a concentrated herbicide through a saturated sponge, wick, or carpet, directly onto weeds. A key prerequisite to the successful use of wipers is that the target weed (e.g. Canada thistle) must be taller than the forage. This is normally obtained through livestock grazing as cattle reduce the height of forage plants and leave taller weeds behind. Wipers have 2 potential advantages compared to broadcast spraying, including a reduction in the amount of herbicide used (per unit area) and the retention of desirable plants (i.e. legumes) in the understory that may be herbicide susceptible.
To date, the only herbicide with recommended label specifications for applying herbicide with a wiper is glyphosate (i.e. Roundup). An initial trial was conducted at 4 locations in central Alberta during 1999 examining thistle and forage responses to wiping. Glyphosate at 33% concentration was applied bi-directionally using a 3 m Smucker wiper to 4 replicates at each location, and contrasted to untreated check plots. All pastures were grazed prior to treatment.
Wiping with glyphosate resulted in a marked reduction in thistle density and biomass at all locations in 1999, a pattern still apparent in 2000 (Fig. 6). However, reduced thistle coincided with a decline in grass biomass and increased forbs (Fig. 7). Data from 2001 indicated the forbs were largely annual weeds. The loss of grass biomass appeared to result from the application of glyphosate to grasses that escaped grazing and were located within the taller thistle canopy, and were therefore exposed to herbicide. Dead patches of grass were evident after treatment, likely a result of glyphosate translocation into the crowns and roots of grasses. In addition, the patchy removal of grasses allowed short-lived, weedy annual and biennial species (e.g. flixweed, stinkweed, pigweed, lamb’s quarters, etc.) to germinate and increase. Thus, although wiper applied glyphosate was effective in reducing thistle, it also reduced forage production and increased annual weeds.
Based on the results obtained in the first wiping trial, a second trial was established in 2000 at 3 locations comparing the use of Roundup, Lontrel, Grazon, and Dyvel DS. All locations were mixed stands of perennial grass, clover, and Canada thistle. Differences in the uptake and translocation of these herbicides, as well as their selectivity and soil residual properties, were hypothesized to allow the latter 3 treatments to retain more grasses and legumes in the forage stand. Herbicide concentrations were relativized on a cost equivalency basis (using Roundup as the benchmark), resulting in concentrations of 33, 2, 20, and 24%, respectively, for the 4 herbicides listed above.
Results of the second trial showed that although the Roundup treatment resulted in the greatest individual control of thistle in the year of treatment, all herbicides effectively reduced thistle. Similar results were evident one year later in 2001, with Grazon and Lontrel resulting in the lowest levels of thistle (Fig. 8). Most importantly, one year after application the Roundup treatment had the least biomass of grass, and again underwent an increase in annual forbs, a trend not evident with other herbicides (Fig. 9). Thus, the use of selective herbicides for the control of thistle in permanent pastures appears to be an effective alternative to glyphosate for not only achieving thistle control, but also maintaining forage production following treatment.
Grazing Systems for Thistle Management
The management of permanent pastures to prevent the invasion and spread of weeds such as Canada thistle is partially dependent on maintaining a vigorous, competitive forage stand capable of preventing weed establishment and spread. Maintaining a competitive forage stand, in turn, is largely determined by the type of defoliation regime (i.e. grazing system) used. Continuous or season-long grazing, where cattle remain in a single pasture for the duration of the grazing season, is likely to maximize the patchiness of forage use, with some areas over-utilized and others under-utilized. Weakened forage plants in over-utilized areas are poor competitors against weeds, and the increased bare soil associated with overgrazing creates microsites for weed establishment.
Livestock producers have 2 general strategies to implement rotational grazing, both of which are intended to promote more uniform use of pastures and/or maintain a vigorous forage stand. These strategies involve altering defoliation intensity and frequency, and are reflected in 2 grazing systems. In high intensity-low frequency (HILF) grazing, infrequent but relatively intensive (e.g. 70-80% use) defoliation is accompanied by a long rest period before regrazing to allow rapidly growing grasses sufficient time to recover. Brief but intensive use is thought to increase the uniformity of use among all vegetation, including weeds. The opposite approach, known as short duration (SD) grazing, uses more frequent, but relatively light defoliation (e.g. 30-40% use) to maximize forage plant vigor and ensure their competitiveness against weeds.
To evaluate the effect of these defoliation regimes on thistle abundance, a clipping study was conducted at 4 locations from 1999 to 2001 evaluating accumulated forage yields and thistle responses. Defoliation treatments attempted to mimic the selective grazing behaviour of livestock (i.e. an animal’s tendency to remove non-thistle herbage). Defoliation regimes simulated continuous (clipping every 2 weeks to 2 cm stubble height), HILF (every 4 weeks to 2 cm), and SD grazing (every 2 weeks to 10 cm), along with a deferred treatment (clipping at maximum biomass in August). Each defoliation regime was investigated with and without annual fertilization.
Marked differences in both accumulated forage yield (Fig. 10) as well as thistle abundance (Fig. 11) were evident among defoliation treatments. Furthermore, treatment differences continued to develop through time, with the greatest effects evident in the last year (2001). Regardless of fertilization, forage yields were greatest within the deferred treatment, followed by the HILF, SD, and continuous treatment (Fig. 10). This trend suggests defoliation intensity is not as important as frequency for maximizing forage production in central Alberta. Grasslands found in pastures of this region appear well-adapted to relatively intense defoliation, provided it occurs quickly and they receive a long rest period (4 weeks) with adequate moisture to facilitate recovery and maintain plant vigor and growth.
Notably, the thistle data displayed the opposite trend, with the greatest thistle density and biomass within continuously defoliated plots (Fig. 11). In contrast, the HILF and deferred treatment had the least thistle, with fertilization resulting in a further decline in thistle abundance. Overall, these results reinforce the notion that HILF grazing is effective in maximizing accumulated forage yield and reducing thistle abundance within weedy pastures. HILF grazing appears to convey a competitive advantage away from thistle and in favor of more rapidly growing, defoliation tolerant forage grasses.
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Thistle Biocontrol With Grazing
There has been a growing interest among producers for utilizing livestock grazing as a means for controlling weeds such as thistle. Although low in palatability, Canada thistle plants are non-toxic and relatively high in forage quality (e.g. crude protein). Continuous grazing tends to maximize sward patchiness, leading to a loss of forage vigor and an increase in weeds. As shown previously, HILF grazing can increase forage production, as well as limit thistle abundance through increased competition from the forage stand. An additional effect of HILF grazing, however, is the increased grazing pressure within the sward during each grazing period, and its direct effect on vegetation. High stocking densities increase the uniformity of defoliation (or trampling) of all plants, including weeds. Defoliation of weeds, in turn, may increase the ultimate level of weed control.
To test the effect of HILF grazing for direct thistle control, 4 trials were conducted comparing HILF, SD, and continuous grazing for their affect on thistle abundance. Grazing regimes were similar to those examined in the clipping study. Results indicate HILF grazing once again generated the greatest decline in above-ground thistle stem densities (e.g. Fig. 13). It is also apparent that each grazing period within the HILF rotation caused an incremental or additive decline in thistle abundance through time. Collectively, these findings suggest the close manipulation of livestock grazing, as obtained with HILF grazing, can be effective in reducing above-ground thistle abundance.
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Conclusion
Overall, these Results indicate it is important for livestock producers to control Canada thistle, which is an effective competitor and uses nutrients (including fertilizer), resulting in a loss of forage production. It is also apparent that there are many tools to control thistle. Where thistle is an existing problem, the use of herbicides in conjunction with fertilization can be effective in controlling thistle and increasing forage yields. Similarly, changing from a continuous to a HILF rotational grazing system can reduce thistle, as well as increase forage. Although weeds such as Canada thistle will likely never be eliminated, the integrated use of many tools, including herbicides, fertilization, and rotational grazing, can work together to minimize their impact on pasture and livestock production.
Edward Bork, Range Management Specialist
410E, AgFor Center, University of Alberta
Edmonton, AB T6G 2P5
The contents of this page are no longer available.
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For more information about the content of this document, contact Valerie Sowiak.
This information published to the web on March 10, 2004.
Last Reviewed/Revised on April 6, 2006.
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