Soil active pre emergent herbicides are widely used in horticultural and broadacre cropping systems throughout Australia. Such herbicides provide a number of benefits in these cropping systems; however, variations in crop safety and weed efficacy, and herbicide carry over to susceptible crops are issues associated with the use of pre emergent herbicides. This 3-year Horticulture Australia Ltd project examined some of the key factors influencing herbicide activity and persistence, and also looked at developing more rapid and cost effective analysis methods for quantifying herbicide levels in soils and relating these to crop growth responses.

The attraction of herbicides to soil particles (sorption) is important in controlling the activity and also the breakdown of pre emergent herbicides. The sorption of pendimethalon, clomazone, and metribuzin varied according to soil type, in each case increasing in the order of kurosol, sodosol, vertosol, ferrosol.

Herbicide breakdown studies showed half-life values of colmazone ranging from 79 to 124 days, while values for pendimethalin ranged from 64 to 117 days. Metribuzin dissipated more quickly with half-life values of between 30 and 64 days. Soil moisture was shown to be a critical factor for herbicide breakdown on all soils. Cultivation of the soil did not increase the rate of herbicide breakdown when compared to herbicide applied to the soil surface and not cultivated.

A series of replicated trails and sampling from commercial crops as conducted to determine critical herbicide concentrations affecting crop growth. Poppy crops were found to be particularly sensitive to pendimethalin, with concentrations above 300-400 ppb significantly reducing yield. Critical concentration for clomazone in onions was 400-500 ppb, oxyfluorfen in zucchini was 100-150 ppb, and metribuzin in swedes was 140 ppb. No negative effects on yield were observed with oxyfluorfen at 254 ppb in squash and sulfentrazone at 24 ppb in poppies and onions.

Herbicide concentrations have shown to provide a reliable indication of crop yield responses in the study and could potentially be used to plan crop rotations and diagnose herbicide carry over effects and susceptible crops. The use of soil herbicide analysis along with adjusting herbicide rates for different soil types and the strategic use of agronomic practices such as irrigation to increase herbicide breakdown can be effectively used to minimise the impact of residual herbicides on following crops.

An effective way of communicating information on herbicide soil type interactions to end users is via herbicide labels. Information generated as part of this project has been used in the development of rate recommendations based on soil type for both Command and Frontier Optima. This information with be documented on the product labels. Regular field days, conference presentations, and seminars were also conducted throughout the project.