Cultivation - fallow and pre-sowing
Cultivation is useful as a 'one-off' tactic in reduced tillage or no-tillage operations, to achieve effective weed control. Over-reliance on cultivation can reduce surface stubble cover, increase erosion risk, degrade soil structure and reduce soil moisture retention. Poorly timed cultivation will result in a poor seedbed, leading to reduced crop emergence, herbicide damage and reduced herbicide performance.
- Well timed cultivation effectively kills weeds through:
- plant burial
- seed burial, thus reducing the ability to germinate
- root severing
- plant desiccation, where plants are left on the soil surface to die
- breaking seed dormancy
- seed placed in a more favourable environment to encourage germination for subsequent control
- The impact of cultivation will depend on the weed species. Seed of annual ryegrass will more rapidly lose viability at deeper soil depths than when shallowly buried. By contrast, seed viability of other species (for example, wild radish and common sowthistle) increase with depth of burial.
- In preparing a seedbed, cultivation provides a weed-free environment for the emerging crop.
- Cultivation can control weeds in situations where herbicides are ineffective or not an option, that is, when weeds are too stressed to respond to herbicides (especially over the summer fallow), weeds are resistant to herbicides or herbicide sensitive crops are nearby.
- Pre-sowing cultivation or full disturbance cultivation at sowing reduces the reliance on knockdown herbicides and therefore the likelihood of weed populations developing herbicide resistance.
- Cultivation for weed management can be an additional benefit obtained when incorporating soil ameliorants (such as lime or gypsum) or breaking up a plough pan.
Issues to consider
- Strategic cultivation must take into account whole-farm practicalities. Avoid repeated cultivation – use it strategically in situations where no suitable alternatives are available. Cultivation can increase weed control costs through increased labour and machinery inputs.
- Maintain soil structure by cultivating at suitable soil moisture levels and appropriate implement ground speed. Cultivating when the soil is too wet can cause ‘smearing’ and compaction. On the other hand, cultivation when the soil is too dry can destroy soil structure. Both will lead to reduced water infiltration and storage, and soil aeration. Travelling faster than the recommended ground speed for a particular implement type will greatly increase the damage to soil structure.
- The tillage implement used will influence the level of soil disturbance and thereby the effect on the weeds present. A disc plough or mouldboard invert soil and bury weed seed. However, for those weeds species where burial prolongs the life of the seed bank, scarifiers and cultivators that cause little soil disturbance (operation at less than 10 centimetres (cm) depth) can be used to kill weeds and may simulate weed seed germination (for effective control once the weeds have emerged).
- Choice of cultivation practice can influence weed density and spectrum. For example, species like skeleton weed that reproduce vegetatively will be encouraged by cultivation, whereas weeds like fleabane or silver grass will be effectively controlled by cultivation and are encouraged by the reduced tillage system.
- Cultivation is most successful when used against small weeds (before flowering commences). Root systems of large weeds may be extensive, making removal difficult. Weeds that are not fully dislodged by the cultivation may re-root if the surface soil remains moist.