The effects of soil moisture and temperature on trifluralin degradation were investigated in 3 Australian soils under field and laboratory conditions. For the field trials, the amount of trifluralin persisting over 1 year into the next cropping season varied from 9 to 24%. From laboratory experiments, separate relationships were derived for the effects of temperature and moisture 1 on trifluralin degradation. Trifluralin degradation I increased with both increasing moisture and increasing temperature. The results indicate potential for unusually long persistence of trifluralin in years of low rainfall when the soil becomes very dry. It is therefore recommended that analysis for trifluralin residues or activity be undertaken if a sensitive crop such as oat, sorghum, or millet, or a sensitive pasture species such as phalaris or ryegrass, is sown after very dry climatic conditions following trifluralin application. The wheatbelt regions of Australia do not experience cold conditions for long enough periods for low temperatures to be a major factor contributing to trifluralin persistence. Degradation was compared for trifluralin applied as a formulation of the herbicide and as the active ingredient alone, to determine the validity of results obtained using the pure herbicide. In half of the 6 field and laboratory studies, there was a statistically significant difference between degradation of trifluralin as the active ingredient and as the formulation. This indicates a need to use the formulated compound in laboratory experiments if results are to reflect normal agricultural practices.

Summary. The persistence of trifluralin was investigated at 3 sites in southern Australia over 8 years. Persistence varied from year-to-year, with the highest persistence being recorded at all sites in the dry year of 1984. Trifluralin persistence (expressed as a percentage of the initial measured concentration) 1 year after application was lowest at Rutherglen (7–22%), followed by Dooen (7–26%) and Walpeup (13–120%). Trifluralin degradation at Walpeup in 1984 was negligible. The resultant high residue concentrations would have posed a threat to subsequent crops. However, 1 year later after the subsequent fallow, the residue concentration had dropped to more typical levels. Trifluralin did not accumulate after repeated application (either 1 or 2 years apart) at all 3 sites. It was found important to measure the amount of trifluralin in the soil immediately after application because the trifluralin concentration could not be assumed from the rate. Soil concentrations immediately after application ranged from 32 to 138% of that expected from the nominal application rate. Therefore, measuring initial concentrations when comparisons were to be made between treatments, years and sites, was essential. In addition to studies of 1 year persistence, trifluralin concentration was also measured 2 years after application at Walpeup and Dooen. There was evidence to suggest that the rate of degradation over the same 1 year period, was different for a recent application when compared with weathered trifluralin residues from a previous application. Correlations between persistence and various weather factors indicated that the most influential factors were those that occurred in the first 60 days after application. These included total rainfall, effective rainfall (rainfall minus evaporation) and average maximum temperature.