Agronomy Library > Forages & Forage Seed

Controlling Perennial Weed Problems in Reduced Tillage Systems
Author: K. Neil Harker, Agriculture & Agri-Food Canada, Lacombe, Alberta
Date Created: June 04, 1997
Last Reviewed: February 14, 2007
Summary · Left uncontrolled, perennial weeds can severely reduce crop yields in any tillage system. · Early emerging perennial weeds such as quackgrass are often adequately controlled by pre-seeding "burn-off" treatments, which include glyphosate (Roundup or Touchdown). However, early seeding often precludes the effectiveness of pre-seeding treatments on weeds such as toadflax, Canada thistle, or perennial sowthistle, which may emerge after crop seeding. In this case, in-crop herbicides such as clopyralid (Lontrel) may be important for weeds such as Canada thistle or perennial sowthistle. · A few in-crop herbicides may suppress toadflax (Estaprop, Refine Extra, Ally), but there is no early post-emergence herbicide treatment that provides a high level of toadflax control. · Post-harvest perennial weed control can be effective on many perennial weeds, but the success of post-harvest glyphosate has been limited by weather conditions, lack of time, and fall frosts. · The most effective treatment of perennial weeds is the application of preharvest glyphosate. Preharvest glyphosate is the most consistent and effective treatment for the majority of perennial weed problems. Whether it is time-of-year, weed growth stage, canopy microclimate, or other more mysterious factors, the preharvest timing for glyphosate application is a critical factor for perennial weed kill. Since it has not been fully determined why glyphosate is so effective at this stage, I will concentrate on the data from numerous experiments which documents the effectiveness of the preharvest timing. · Weed control data in the year after preharvest glyphosate are as follows for several perennial weed species: · quackgrass (91 %) · Canada thistle (85 %) · perennial sowthistle (84 %) · toadflax (91 %) · dandelion (87 %) · field horsetail (suppression). · Unfortunately, seed growers are restricted from using preharvest glyphosate on seed crops. Preharvest glyphosate applications can be particularly damaging on seed peas.