The wet months of July and August are also the coldest, so when the soil was wet enough for degradation to happen, it happened at the slowest rate. To simulate a failed lupin crop, simazine was sprayed and incorporated, then cereals sown into the area at weekly intervals to estimate the time delay required before recropping. When the simazine was applied in late June, enough residue remained even after 12 weeks to kill all cereals planted then. When the simazine was applied in early May however, 4-5 weeks degradation was enough to allow cereals to grow with no effects. All other things being equal, imidazolinones will be more persistent on acid soils and sulphonyl ureas on alkaline soils.
Triazines are very slightly more persistent on alkaline soils.
Any paddock where any of these products was applied in June or later should be regarded as having potentially damaging levels of residues, and so should areas of earlier applications if there were significant periods when the soil surface was dry. If the 2001 season breaks in April the paddock will probably be safe if seeding is delayed for a month, provided the soil surface remains moist. Any summer rain will help with breakdown but the effect of such rains is hard to predict. They do occur when soil temperatures are high and therefore breakdown is rapid - as long as the soil remains moist. However, drying is usually quick, at least in the top few centimetres, and so the degradation period is short.