The Mallee region has been dry sowing for a month now with careful considerations going into weed control methods such as pre-emergent herbicide selection. With the recent rainfall in the region, it’s worth considering just how effective your pre-emergents are performing so you can adjust your in-crop herbicide strategies if required.
When assessing pre-emergent performance, consider moisture (from the sky and already in the soil), the weed burden, paddock condition (stubble or bare earth) and, of course, the type of herbicide.
In a GroundCover article from August 2017, Chris Preston says “the key factors to breaking down most herbicides in soil are moisture, temperature and soil organic matter” and points to the herbicide half-life as a good start to calculate herbicide activity after a period of time.
“These values are averages … however, the herbicides with longer field half-lives are likely to have broken down less than herbicides with shorter field half-lives.”
Table 1. Pre-emergent herbicide half lives
Source: “Impacts of dry conditions on herbicide effectiveness and weed management decisions”
Pre-emergents applied into dry soil with little to no rain since sowing should still be present with little breakdown. Herbicides with a higher half-life such as Sakura® are likely to have more of the original amount than something like Butisan®.
Research conducted in Western Australia by the WA No Tillage Farmers Association in 2016 supports this and found pre-emergents applied early all decayed slowly in dry conditions. Soil type had little effect.
However under wet conditions, herbicide decay was increased greatly.
Therefore, crops sown into dry conditions in early April, will be effective if applied in the right conditions specified on the label. If the soil is wetting and drying during the period of seeding to germination, there could be issues with getting the expected job. This will cause the pre-emergent to break down before it has acted on the weed.
BCG’s Senior Manager Business Development and Innovation, Cameron Taylor sums it up: “timely germination of the crop will generally give it the best chance if higher yields and securing a profitable return” he said.
“In general, if you are delaying sowing as a method to control weeds you are probably better off looking to another control method such as hay, sheep or crop/variety type selection to avoid sowing outside your optimal flowering windows”.
