Optimal Timing for Sequential Mesotrione Application to Control Annual Bluegrass (Poa annua) In Newly Seeded Kentucky Bluegrass (Poa pratensis) Turf.

Field studies were initiated in the fall 2009 and fall 2010 to evaluate the control of annual bluegrass in newly seeded Kentucky bluegrass to mesotrione applied at planting (PRE), and PRE followed by (fb) sequential treatments 2, 4, 6, 8, and 10 weeks after turfgrass emergence (WAE) at rates ranging from 0.21 to 0.42 kg ai/ha.  POST applications included NIS at 0.25% v/v.   Applications were made with a CO₂ backpack sprayer equipped with a single 9504EVS nozzle tip delivering 375 L/ha at 222 kPa.  The experimental design was a randomized complete block with 4 replications, with a plot size of 0.9 x 1.8 meters.  Due to significant year by treatment interactions, data was separated by year in the spring.  There was no reduction in Kentucky bluegrass cover across all application regimes.  Rate by timing interaction was non-significant, and therefore rate was combined across all timings and timings were combined across mesotrione rates.  Increasing the rate of mesotrione increased the percent control of Kentucky bluegrass from 50 to 62.35 % in 2009 and 36.75 to 64 % in 2010.  No definitive trend was observed across years to determine the optimum timing for the POST sequential mesotrione application.  However, in both years, mesotrione applied 2 WAE tended to provide the highest and most consistent levels of annual bluegrass control.