Evaluating the Effectiveness of Non-chemical Management Methods for Reducing Losses in Pasture Forage Quality, Quantity, and Utilization from Canada Thistle

Project Overview

Project Type: Graduate Student
Funds awarded in 2012: $9,984.00
Projected End Date: 12/31/2014
Grant Recipient: University of Wisconsin
Region: North Central
State: Wisconsin
Graduate Student:
Faculty Advisor:
Dr. Mark Renz
University of Wisconsin

Annual Reports


  • Agronomic: grass (misc. perennial), hay
  • Animals: bovine
  • Animal Products: dairy


  • Animal Production: feed/forage, grazing management, grazing - rotational, pasture fertility, stocking rate, stockpiled forages
  • Crop Production: application rate management, nutrient cycling
  • Education and Training: extension, focus group, networking, on-farm/ranch research
  • Farm Business Management: budgets/cost and returns
  • Pest Management: biological control, chemical control, competition, mulching - vegetative, weed ecology
  • Production Systems: organic agriculture, permaculture, transitioning to organic
  • Soil Management: organic matter


    We compared the efficacy of a fall herbicide application followed by rotational grazing; two mob grazing treatments (one year followed by rotational grazing and two consecutive years); and a rotationally grazed control on Canada thistle density and forage production and utilization. Herbicide application was the most effective treatment, while Canada thistle density increased at two sites and non-significantly decreased at a third when mob grazed for two years. However, forage production was increased by 24-76% in mob grazed plots and herbicide application reduced clover and other broadleaf biomass, causing a 25-38% reduction in forage production.


    Management intensive rotational grazing has become an established practice throughout Wisconsin and the Upper Midwest due to its economic, environmental, and production benefits (Dartt et al. 1999; Paine and Gildersleeve, 2011; Taylor and Foltz, 2006; Lyons et al. 2000). One such benefit is the prevention of weed emergence and suppression of infestations by competitive, desirable forage species in pastures that are managed using rotational grazing (Wardle et al. 1995; Trumble and Kok 1982). While weed species are less common in rotationally grazed pastures, weed infestations can still occur. Canada thistle (Cirsium arvense L.) (hereafter CT) has been identified as a weed of particular concern in temperate regions (Moore 1975). CT can reduce the production and utilization of desirable forage which can result in losses in animal performance (Undersander et al. 2002). For example, desirable yield losses caused by Canada thistle presence have been estimated at 2 kg ha-1/ha for each kilogram of thistle biomass (Grekul and Bork 2004). In addition, Canada thistle’s spiny leaves can reduce forage utilization by up to 60% (De Bruijn and Bork 2006).


    While weed control is desired by producers, Canada thistle is notably hard to suppress due to its aggressive perennial roots (Moore 1975). Most research conducted on controlling Canada thistle in pastures andor grasslands has focused on the use of herbicides, many of which, though effective (Enloe et al. 2007), can cause injury to clover populations (Bork et al. 2007) and are not registered for use in organic systems (National Organic Program). One alternative to chemically-intensive control is altering grazing to improve suppression. It has been documented that control of weed species can be enhanced by increasing weed utilization by animals (Rinella 2009; Peterson 2013), physically injuring the weed with hoof action (Popay and Field 1996), and encouraging rapid forage regrowth that facilitates interspecific competition (De Bruijn et al. 2010).


    Rotational grazing systems can be modified to maximize the aforementioned impacts on Canada thistle. Mob grazing is one such practice, described by Allen et al. (2011) as Mob stocking and defined as “A method of stocking at a high grazing pressure for a short time to remove forage rapidly as a management strategy.” Among producers and popular press, Mob grazing (sometimes referred to as ultra high stocking density) has been implicated as a grazing management system that suppresses Canada thistle and increases forage production and utilization (Kidwell 2010; Lemus 2011; Johnson 2013). However, there has been little research conducted focusing specifically on Mob grazing or its reported benefits. One study researching big bluestem (Andropogon gerardii Vitman) establishment found Mob grazing to be ineffective at controlling weeds when compared with atrazine (Lawrence, 1995), while three experiments effectively used Mob grazing as a technique to decrease selectivity in pasture germplasm and persistence trials (Bittman and McCartney 1993; Gildersleeve 1987; McCartney and Bittman 1994). High intensity low frequency grazing (HILF) has also been documented to nearly eliminate Canada thistle after three years of grazing (De Bruijn and Bork 2005). Other studies have explored using sheep (Olson and Wallander 2001) and goats (Hejcman et al. 2014) as biological control agents, but few have focused on the use of cattle (Popay and Field, 1996). The limited and conflicting nature of the existing literature on Mob and other high intensity grazing regimes relating to weed control justifies further effort.


    Definitions among rotational grazing systems are seldom universally applicable, often making distinctions unclear and confusing. While HILF grazing has been shown to effectively suppress Canada thistle, this terminology appears to be used solely in research communities (Taylor 1993; De Bruijn, 2005). Mob grazing in its most recent incarnation is seen as a producer-generated term used by grazing communities that differs from HILF grazing by using higher stocking densities, shorter grazing events, longer rest periods, and utilizing more mature forage (Chapter 2, Thomas 2012; Kidwell 2010; Holin 2013). The lack of research exploring grazing and Canada thistle control in the Midwest and the increase in organic grazing operations both support the need for more research. Further, a focus on Mob grazing, an increasingly-adopted but little-studied grazing strategy, provides important information about the utility and productivity of this form of management intensive rotational grazing. The objective of our study was to evaluate the effect of Mob grazing on forage productivity, forage utilization, and Canada thistle suppression compared to other standard practices.

    Project objectives:

    The project’s objective is to develop an understanding of the efficacy of mob grazing as a biological weed abatement strategy as compared to herbicide application. Further, valuable agronomic information pertaining to mob grazing will be collected including yield, species composition, and forage utilization. The measurements that we take and the statistical and narrative interpretation that results will help to increase awareness of alternatives to chemical control methods as well as provide information about implementing such strategies. These outcomes will be directed towards members of the focus group, all of which are capable of distributing information to their considerable networks of producers and educators. Intermediate outcomes will include the extension of this information to grazing networks and sustainable agriculture organizations as well as publication in peer reviewed journals, all with the end goal of successful adoption of best practices recommended by our research. Long-term outcomes include an important contribution to a knowledge base of non-chemical weed control strategies and their implementation specific to Wisconsin and the upper Midwest. Additionally, as mob grazing continues to attract attention and interest within grazing communities, this study will pave the way for future research on the technique.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.