Factors Affecting Poa Annua L. Control with Mesotrione.

Mesotrione can control annual bluegrass (Poa annua L.) in Kentucky bluegrass, but only when 5-10 low rate applications are applied with an application interval of 2-5 days. This labor-intensive approach is often difficult for turf managers to implement. The purpose of this research was to determine if spray parameters or cultural practices could be varied in order to increase mesotrione activity on annual bluegrass. Experiments were conducted to determine the effects of adjuvants, spray volume, and nitrogen fertilization on the efficacy of mesotrione on annual bluegrass. Spray volumes of 190 to 935 L ha-1 were tested using mesotrione applications of 186 g ha-1 applied weekly for 3 weeks. Control decreased with increasing spray volume. Greatest control, 94%, was observed when mesotrione was applied at 190 L ha-1. Fertilization was also important in improving annual bluegrass control. Urea was applied either at mesotrione treatment initiation or 7 days after treatments were initiated. Urea was applied at rates of 0, 24, 48, or 72 Kg N ha-1. Annual bluegrass control increased linearly with increasing nitrogen fertilization. When no fertilizer was applied before mesotrione applications, annual bluegrass control was only 12% from a standard set of mesotrione applications. However, control reached 74% when urea was applied at 72 kg ha-1. A series of adjuvants were tested and there were no differences between the various adjuvant types; however, adding urea-ammonium nitrate solution at 2.5% v/v significantly increased control regardless of adjuvant used. Manipulating these various parameters can result in more consistent and higher levels of annual bluegrass control.