Dry Seeding Considerations

When starting with a dry soil, the first rainfall is likely to move rapidly down the profile, especially in sandy soils that have much larger air pores. Herbicide moving with this initial rainfall will move further than applications where the same herbicide was applied to a ‘full profile’ at application, where subsequent rainfall will not penetrate as far. This occurs with all herbicides; however individual herbicide differences also need to be understood.

The ability of a herbicide to wash off stubble and how far it will move with and after the initial rainfall, depends on both the herbicide solubility and its propensity to bind to soil and organic matter. Both are important. The table below groups herbicides of roughly similar mobility. However, as many biological processes are involved, this is a guide only, with individual situations sometimes making a herbicide either more or less mobile than suggested.

‘Mobile’ herbicides will easily move into the soil following rainfall. Once in the soil, most of the herbicide will want to primarily remain in the soil moisture phase, with only a small amount of herbicide being bound to soil or organic matter. Highly mobile herbicides will move with water both horizontally and vertically in the soil moisture. Even if sowing with a knife point and press wheel system and the herbicide is moved into the interrow at application, these herbicides are likely to move back into the planting furrow following significant rainfall. When used in dry sowing, and especially in sandy soils, these mobile herbicides can move along distance following rainfall (i.e. to the depth of the wetting front), and can often lead to crop damage unless the crop has an adequate level of tolerance to that herbicide.

Less mobile herbicides either have significantly lower solubility and/or much tighter binding to soil and organic matter, and hence do not move as extensively following the initial rainfall. Generally, if these lower mobility herbicides are applied by a correctly setup knife point system, the majority of the herbicide will be moved into the interrow, and will be less likely to move back into the planting furrow. Note – herbicide binding is a chemical reaction that takes some time (days) to occur and typically requires some level of soil moisture for most herbicides. This means that if the herbicide was not incorporated or was incorporated during application but there was not enough soil moisture for binding to reach an equilibrium, then there may be a significant proportion of herbicide unbound at the time of the break. If the opening rainfall event is large, then even relatively immobile herbicides may sometimes move further than expected.

No pre-emergent herbicide binds completely to soil – if it did then it would be unavailable for the weeds to take up and hence it would not provide weed control. Herbicides with higher binding are generally considered less mobile as there is a lower amount of herbicide in the soil moisture phase at any given time – although this lower concentration of herbicide in the soil moisture may still move with soil moisture.

*information sourced from GRDC