Forage Production and Canada Thistle Control in Mob Grazed Pastures: Comparing Herbicide Application to Livestock-Based Biological Control Strategies.

Canada thistle infestations can negatively affect pasture-based livestock systems by reducing forage production and utilization. Herbicides effectively suppress Canada thistle but also injure forage legumes, an important component in Midwestern pastures. Further, producers working in organic production systems don’t have access to chemical control and therefore need alternative suppression strategies. This study compared the efficacy of a fall herbicide application (Aminopyralid and 2,4-D at 979 +120 g ae/ha) 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 the resulting forage production and utilization. Rotationally grazed treatments were grazed 3-4 times and Mob grazed plots were grazed twice, once in the spring and once in the fall, in 2012 and 2013. Herbicide application followed by two years of rotational grazing was the most effective treatment across both years and all sites with substantial control lasting two years. By spring 2014, Canada thistle density had increased two to four fold in Mob grazed treatments at two of three sites compared to the rotationally grazed plots. At a third site non-significant reductions in stem density were observed. Mob grazing for two years increased forage production by 24-76% compared to the rotational control and those treated with a herbicide across sites in 2013. At the most productive site, herbicide application reduced clover and other broadleaf biomass, causing a 25-38% reduction in forage production when compared to rotationally grazed treatments. Mob grazing increased Canada thistle utilization at one of three sites compared to rotational grazing. While mob grazing did not provide improved thistle suppression after two years, the potential for increases in forage availability and utilization suggest that mob grazing may provide benefits beyond thistle control for producers. Additionally, reductions in forage production resulting from herbicide application in legume-rich pastures recommend further research into viable alternative control methods.