Simple computer simulations determined the long-term impact on the weed seed-bank of the currently promoted strategies to prevent development of herbicide resistance. These simulations, using locally derived data, showed that the most effective strategy was the combination of crop rotation (summer and winter crops), rotation of herbicide groups and growing more competitive crops. Rotation of herbicide groups alone was not sufficient to prevent rapid development of resistance.
How effective are management options currently being promoted to minimise the risk for development of herbicide resistance?
- Maximise crop competition (to reduce in-crop seed production of resistant (R) and susceptible (S) weeds) using more competitive crops and cultivars, higher sowing rates, optimum agronomy and disease control
- Crop rotation (to increase options for controlling R weeds)
- Crop choice with better range of available herbicides
- Weed control in fallow with alternative methods such as cultivation, grazing, non-selective herbicides
- Herbicide rotation (to minimise seed production of R weeds)
- Rotation between different modes of action
- Tank mix or sequential applications such as selective spray topping
- Other options such as delayed sowing, stubble burn, autumn tickle.
Simple computer simulations were used to determine the impact of different strategies and combination of strategies on the weed seed-bank. In recent years, local research has measured the effectiveness of various techniques on weed seed production (the source of replenishment of the seed-bank) and on weed seed persistence in the soil. The simulations used wild oats with a starting seed-bank of 1000 seeds/m2 of which 1% were resistant to Group A herbicides. The simulations were done for a 5 year period. The effectiveness of the different strategies is shown in the following table.
Table 1. Simulated changes in wild oat seed-bank following various strategies for 5 years, starting with 1000 seeds/m2 of which 1% was resistant to Group A herbicides.
Weed management strategy for 5 years
Total seed bank
Resistant seeds in seed-bank
Continuous winter cropping with poorly competitive wheat, and continuous and exclusive use of Group A herbicides
Continuous winter cropping with more competitive wheat, and continuous and exclusive use of Group A herbicides
Continuous winter cropping with more competitive wheat, and continuous and exclusive use of Group A herbicides, plus autumn tickle with delayed sowing and knockdown herbicide pre-sowing
Crop rotation of competitive wheat with barley, and continuous and exclusive use of Group A herbicides
Continuous winter cropping with poorly competitive wheat, but rotation of herbicide groups every 2 years
Continuous winter cropping with more competitive wheat, but rotation of herbicide groups every 2 years
Continuous winter cropping with more competitive wheat, and continuous use of Group A herbicides, plus use of SST* with Group K every 2 years
Crop rotation of competitive wheat for 2 years, long fallow followed by 2 years of sorghum, with full control in winter fallows using cultivation, Group M and or Group C herbicides
|Crop rotation of competitive wheat for 2 years, long fallow followed by 2 years of sorghum, with full control in winter fallows using cultivation, Group M and or Group C herbicides, but no Group A herbicides in wheat.||12||<1||3|
*SST = selective spray topping, i.e. the late application of herbicide to stop seed set.
The absolute weed seed numbers will differ with weed species, herbicide effectiveness and seasons, but the simulation results demonstrate the relative efficiency of different strategies. The best approach is to include several weed management options, especially crop rotation, herbicide group rotation, inclusion of timely cultivation, and better crop competitiveness.
Dr Steve Walker
Ph: (07) 4639 8838