Atrazine May Overcome the Time-of-Day Effect on Liberty Efficacy

Liberty is a common postemergence herbicide used for weed control in Liberty Link corn. However, Liberty applied within two hours of sundown fails to control many weeds like velvetleaf. The impact of this time-of-day (TOD) effect on corn grain yield is unknown. Therefore, field experiments were conducted near Columbia and Novelty, MO in 2000, 2001, and 2002 to determine if the TOD effect impacts corn grain yield and whether atrazine added to Liberty could overcome the TOD effect. All plots were treated preemergence with Dual II Magnum followed by a postemergence application of Liberty (0.31 or 0.44 lb a.i./acre) or Liberty plus atrazine (0.25 + 0.83, 0.31 + 0.83, 0.31 + 1.03, or 0.44 + 1.03 lb/acre) at 6, 2, and 0 hours before sundown (HBS). Liberty or Liberty plus atrazine applied to velvetleaf at sundown resulted in reduced weed control when compared to 6 HBS in four of the six site-years. Other weed species exhibited a similar visual response to postemergence applications of Liberty or Liberty plus atrazine. In addition, when a visual TOD effect was observed, total weed biomass following postemergence applications at sundown versus those at 6 and 2 HBS was up to 11-fold greater 3 weeks after treatment. The addition of atrazine increased velvetleaf control when a TOD effect was observed in only one instance (Columbia 2002). When a TOD effect was observed, grain yield was up to 56% lower when Liberty at 0.31 lb/acre was applied at sundown compared to 6 and 2 HBS. In the three site-years that a TOD effect was observed, the addition of atrazine to at least 0.31 lb/acre of Liberty resulted in little yield loss compared to the weed-free control. This indicates that those injured weeds that remained after treatment did not influence crop yield. However, it remains inadvisable for producers to apply Liberty with or without atrazine within two hours of sundown as described by the Liberty label.