Farmers in parched South Australia and Victoria have been warned to consider plantback risks when formulating their rotations for the upcoming year.

There has been extremely below average rainfall across spring and summer, meaning some residual herbicides have not received enough rain to dissipate the risk for the upcoming crop.

Popular chemistry groups using Clearfield technology require 250mm to minimise risk, while Overwatch, also a popular choice in chemical rotations, also needs 250mm to avoid plantback risk.

With this in mind, Trengove Consulting agronomist Ben Smith, based in SA's mid north, said growers of many commodities, in particular sensitive crops such as hay oats, needed to be aware of the risk of plantback damage.

"It's a real watch-out for oat hay growers this season, particularly in South Australia and the western half of Victoria where there will be areas that haven't had effective rainfall or enough time pass to deal with herbicide residues."

He cautioned that even if there had technically been enough rain, farmers needed to consider whether it was 'effective rain' in terms of incorporating the herbicide.

"For rainfall to be effective it must penetrate into the soil, keeping it moist to a depth of around 5-10cm for a reasonable period."

"It needs to persist long enough for the soil microbes to have the time to break down any herbicide residues," Mr Smith said.

"While you may have actually received the rainfall volume on the label, if it all came down in heavy falls that ran off or there were long gaps between events, you may not have had enough moisture in the soil long enough to degrade all your residues."

Overwatch has a replanting interval of nine months and 250mml interim rainfall for oats, while Sakura and Mateno Complete have a 21-month and 550ml rainfall plantback to oats.

Any in-season application of imidazolinone (Group 2) products, like Intervix and Intercept, needs to be considered as in many areas there hasn't been enough rain to satisfy plantback requirements.

He said one option could be imi tolerant varieties, such as the new oat releases Kingbale and Archer which will tolerate the chemical residues.

Other products to watch out for include Reflex (fomesafen, Group 14), Callisto (mesotrione, Group 27) and Talinor (bicyclopyrone, Group 27 and bromoxynil, Group 6) all with nine month, 250 mm plantback restrictions which may not have been met.

Mr Smith said with some chemicals soil type also needed to be consider, saying soil pH was an important consideration with group 2s.