UNDERSTANDING the interaction of pre-emergent herbicides and the level of soil moisture at seeding goes a long way when it comes to deciding which herbicides to apply in dry seeding conditions.
Rainfall after pre-emergent application assists with the incorporation of most pre-emergent herbicides, however the soil moisture at the time of sowing also affects the mobility of some products into soils of different textures and the volatilisation of some products.
Independent Consultants Australia Network (ICAN) senior consultant Mark Congreve said when seeding needed to start prior to significant breaking rains and without the forecast of imminent rainfall, it was even more important that the maximum amount of product reached the soil surface.
"To achieve this, use large coarse droplet size, higher pressure and higher water rates - around 100 to 120 litres per hectare, and preferably solid over air-inducted droplets," Mr Congreve said.
"The aim is to minimise the amount of product left on stubble as you can't rely on rain to wash it off the stubble and into the soil.
"Narrow fan nozzles assist in reducing stubble capture and if the rig is travelling at 20 kilometres per hour or more consider rear facing nozzles to reduce the forward momentum of these large heavy droplets."
Mr Congreve said pre-emergent herbicides were an important component of many herbicide programs but should never be left to do the heavy lifting alone, as to do so exerted immense selection pressure on these herbicides.
When it came to dry seeding, getting the product onto the soil surface, incorporating by sowing and potential environmental losses are all critically important.
Mr Congreve said stubble could intercept herbicides so if there was no rain forecast it was necessary to use other strategies to get the product onto the soil surface.
"The seeder will be doing most of the work to incorporate the product, so growers need to ensure excellent soil coverage of herbicide in the interrow," he said.
"Especially for those herbicides that are more prone to breakdown or loss through UV exposure, volatility and stubble binding.
"Many of the ryegrass pre-emergent products can be damaging to the emerging crop so it is important to also ensure that the seeder adequately moves treated soil out of the planting line."
When it does rain, pre-emergent products will behave differently, according to the soil texture and soil moisture.
For example, in coarse sands rainfall will readily wash the herbicide down the profile, potentially below where the weed seeds are sitting in the profile.
Whereas if the soil has some moisture at planting, or is of finer texture, the movement down the profile will usually be less pronounced.
Mr Congreve said if the first rainfall event was quite heavy, there could be both vertical and sideways movement of the product into the crop row where it can cause damage.
"Consider the mobility of the product you plan to apply," he said.
"Highly mobile herbicides will move a considerable distance after the first rainfall event and have a greater risk of causing crop damage, however less mobile herbicides can still move further than predicted if the soil is porous and the first rainfall is substantial.
"Where rainfall continues to be low and the herbicide treated soil remains relatively dry during weed germination, the efficacy of all pre-emergent herbicides will be reduced."
In general, herbicides that have either higher solubility, or some vapour activity, may perform slightly better under those conditions.
Whereas low solubility herbicides that rely on the roots taking up herbicide dissolved in the soil water are likely to be the most compromised under these ongoing dry conditions.
Mixtures are key to managing herbicide resistance in weeds as they can broaden the activity on the weed spectrum present and in some situations the herbicides in the mix can have an additive effect in terms of weed control.
Pairing products with different mobility may increase the chance of effective control in uncertain conditions, however there may be a higher risk of crop damage if a more mobile herbicide is included in the mix.
If using two different modes of action herbicides they are likely to break down independently and therefore it is unlikely that 'mixing' will result in increased length of residual control.
Mr Congreve said a third consideration was the frequency of use of a certain product over time, even in mixes.
"Resistance can still increase due to selection pressure if there are survivors allowed to set seed," he said.
"There can also be a build-up of the microbes that break down certain products and this can speed up the degradation process if the same product is used frequently in mixes.
"To avoid some of these pitfalls, try to plan a five year herbicide program that includes as much diversity as possible in both pre-emergent and post-emergent herbicides and knockdowns."
