Planting clean seed avoids the risk of spreading herbicide resistance and ensures the crop gets a head start on the weeds. If weeds are planted at the same time as the crop they have access to all the same resources as the crop and can often out-compete the establishing crop.
This can potentially reduce crop yield and almost certainly means that the weeds will set abundant seed and most likely shed that seed before the crop is harvested, increasing the weed pressure in future years.
The best way to ensure clean crop seed is to buy certified weed-free seed each year. But many growers prefer to retain some grain on-farm for planting the next year. For best results growers usually harvest seed from their cleanest paddocks and conduct some form of seed cleaning either on or off-farm.
However, research shows that there is a tendency to underestimate weed seed contamination in seed retained for planting.
An AHRI study on 74 farms across the Western Australian grainbelt showed 73% of cleaned crop seed samples had some level of weed seed contamination. The up-side is that 25% of cleaned samples were weed-free, so it can be done!
This means that many unknowingly introduce significant levels of weed and volunteer crop seeds into the farming system at seeding time, even when crop seed has been cleaned. More alarmingly, many of these weed seed populations are resistant to a range of commonly used post-emergent herbicides.
Uncleaned crop seed samples can have almost 25 times more contamination than cleaned crop seed. It is important to remember that resistance will evolve faster from introducing resistant weed seeds into a paddock, compared to resistance evolving independently in that paddock.
The cleaning method used strongly influences contamination levels – a ‘gravity table’ is the most effective, followed by other methods such as rotary screens and sieves.
Contamination levels of each cleaning method for all contaminants.
Crop type also has a significant effect on the amount of contamination, with wheat containing much higher annual ryegrass seed numbers than barley, possibly because barley was more likely to out-compete weeds during the growing season.
Another advantage of having seed professionally cleaned and graded is that larger crop seeds can be retained, promoting stronger seedling vigour and higher germination rates.
Systems promoting farm hygiene such as meticulous seed cleaning and sanitising tillage, sowing and harvesting equipment between paddocks will help prevent the introduction of new weed species, noxious weeds and herbicide resistance.