Summer fallow weeds should be sprayed before grazing with sheep.
Weeds growing over summer use moisture and nutrients that would otherwise be available to the next crop. Weeds use available nutrients such as Nitrogen directly, but they also dry the soil profile and reduce Nitrogen mineralisation. Research by the NSW DPI shows that for every 1mm of soil moisture consumed by summer weed growth, 0.64 kg/ha of Nitrogen is lost. This equates to a $7.70 /ha Nitrogen loss for every 10 mm of summer rainfall consumed by weeds.
Storing moisture in the soil from summer rain:
- Increases soil moisture stored deeper in the profile,
- Increases Nitrogen mineralisation
- Increases the likelihood of being able to sow the crop on time,
- May lead to better early crop growth which in turn increases crop competition and ability to withstand insect and weed pressure,
- Improves the efficacy of pre-emergent herbicides,
- Reduces risk by improving the reliability of the next crop
Grazing weeds after spraying can improve weed control by helping to finish off hard to kill species like fleabane, targeting surviving plants or reducing the development of herbicide resistance. Grazing weeds before spraying, makes them harder to kill due to plants having larger root systems and limited leaf area. Many summer weed species are sensitive to stresses such as heat, moisture or grazing. A combination of these stresses plus increasing age and size of weeds, can make them significantly harder and more expensive to kill. Summer fallow spraying should target small, young plants to improve efficacy and reduce the cost of control.
The benefit to the subsequent crop from spraying summer weeds far outweighs the grazing value. Sheep can be grazed on stubbles once weeds have been sprayed and the Grazing Withholding Period (WHP) has elapsed, with little effect on soil structure or moisture storage, provided that 70% groundcover is maintained. There is a marginal benefit for livestock in grazing living rather than dead plants, but this is of less significance than the benefit of controlling summer weeds to the subsequent crop.
Around 60 to 80% of summer rainfall can be lost to evaporation, depending on the size of each rainfall event and subsequent weather conditions. When soils are dry, rainfall less than 15mm will generally be lost to evaporation, unless follow up rainfall occurs within two weeks. By contrast, when soils already contain moisture, more is stored from each subsequent fall. Mild, overcast weather after rain also increases the amount stored in the soil.
GRDC’s Summer Fallow Weed Management Manual points to research conducted over many years, showing sites which had an average return on investment of $6.07/ha for every $1.00 spent on summer weed control. In low rainfall areas with variable growing season rainfall, this figure is likely to be higher.
Rainfall totals of 50 to 100 mm and more have been received throughout southern NSW recently, with a range of summer weed species having been observed germinating since October. Given the current soil moisture, these weeds will require spraying soon after the crop is harvested. Interestingly, much lower summer weed numbers have appeared in more competitive crops sown on narrow rows.
Priority should be given to controlling weeds such as fleabane, mallow and wireweed, which become significantly harder to kill once established. By contrast, volunteer wheat and canola along with weeds such as milk thistles or hairy panic, can be more easily controlled, allowing them to be placed lower on the priority list. Chemical interactions and residual effects must also be given consideration.
Click here for GRDC Summer Fallow Weed Management Manual