Veritas Opti Herbicide Antagonism

Tank mixing fungicides with early post-emergent herbicides for single-pass disease and weed control is becoming increasingly necessary with stubble retention resulting in higher early disease inoculum levels and larger cropping programs.  However, some fungicide active ingredients and formulation types can compromise weed control, particularly with GRPA herbicides on grass weeds with developing levels of resistance.

In 2017 – ADAMA contracted independent researchers Plant Science Consulting to conduct screening trials to evaluate the efficacy of the GRPA herbicide - Platinum (clethodim) at a range of rates (125ml, 250, 500mL/Ha) in mixtures with a range of fungicides on populations of wild oats and ryegrass with varying degrees of resistance development.  Fungicides evaluated in tank mix with Platinum included Veritas Opti, Dithane Rainshield, Cavalry Weatherguard & Echo 900.

On susceptible populations of both ryegrass and wild oats – no antagonism was observed between the Platinum and any of the fungicides evaluated.

Resistant Populations

Ryegrass
At the 500mL/Ha rate of Platinum – both Veritas Opti and Dithane Rainshield were not antagonistic.  In contrast both chlorothalonil products – Cavalry Weatherguard and Echo 900 were significantly antagonistic reducing the efficacy of 500ml/Ha of Platinum by over 50%, (FIG 1).

Wild Oats
On a strongly resistant population of Wild oats significant antagonism was observed at the 250mL/Ha rate.   All fungicides, apart from Veritas Opti reduced the efficacy of Platinum by at least 50%.  Similar to the ryegrass results chlorothalonil products were antagonistic reducing the efficacy of Platinum by over 40% (FIG 2).

These results indicate that tank mixing herbicides with fungicides, particularly protectant types can compromise weed control – particularly where resistance to GRPA herbicides may be present.  Veritas Opti due to its unique suspension concentrate formulation was shown to be one of the least antagonistic products evaluated.

For more information on tank mixing compatibility with ADAMA fungicides – please contact your local ADAMA representative.