Are Combinations of Amicarbazone and Mesotrione Synergistic?.

Herbicide synergism is rare, but can provide powerful weed control. Mesotrione is a relatively new herbicide that controls a broad spectrum of weeds in cool-season turfgrasses. Amicarbazone was recently registered for use in turf primarily to control annual bluegrass (Poa annua L.). Research in other cropping systems has shown that mesotrione is synergized by herbicides whose mode of action is the inhibition of photosystem II. Amicarbazone is a PS II inhibitor and this research was initiated to see if the combination of mesotrione and amicarbazone would result in significantly more effective control than can be achieved with either herbicide alone. Mesotrione can effectively control annual bluegrass when 5-10 sequential applications are applied on 2-5 day application spacing. While effective, this approach is extremely labor intensive and difficult to implement. Similarly, amicarbazone can control annual bluegrass but usually a series of up to 4 applications are required before control is achieved. We tested various rates of mesotrione plus amicarbazone for their efficacy on annual bluegrass. We found that two applications of mesotrione and amicarbazone at 0.14 + 0.05 kg ai ha-1 spaced 10 days apart provided very high levels of annual bluegrass control.