Degradation of trifluralin in three Victorian soils —long-term field trials

Abstract

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.

Tillage systems affect trifluralin bioavailability in soil

“…However, there was consistency in the standard curves, with 50% inhibition of oat root growth occurring between 0.13 and 0.23 g trifluralin g Ϫ1 soil. This is similar to a previous study from Australia, where 50% inhibition of root growth was reported around 0.1 g trifluralin g Ϫ1 soil (Johnstone et al 1998).…”

“…In any case, weed control by trifluralin is not based on the total amount present but on its bioavailability. Therefore, a bioassay will provide information about the bioavailability of trifluralin (Johnstone et al 1998).…”

Target-Site Point Mutation Conferring Resistance to Trifluralin in Rigid Ryegrass (Lolium rigidum)

“…While the resistance trait is recessive, it may still be possible to select for resistance to trifluralin, so long as there is a rate of herbicide where there is greater survival of the heterozygote compared with the homozygous susceptible individuals. Trifluralin activity in the soil decays over a long period of time in Australian farming systems (Johnstone et al 1998). While heterozygous resistant individuals may be killed by the full dose of the herbicide, later-germinating weeds will be exposed to a lower dose, and differential survival may occur.…”

Accelerated Dissipation of Two Herbicides after Repeated Application in Field Experiments with Organically-Amended Soil

“…Rouchaud et al [27,28] have reported that the field dissipation of flufenacet accelerated with the higher temperatures in spring and summer due to increased soil microbial activities. Similarly, Johnstone et al [38] have found a relationship between the persistence of the herbicide trifluralin and weather conditions (total precipitation and maximum average temperatures) at 60 days after its repeated application in experimental plots.…”