St John's wort has been favoured by the run of wet years between 2020 and 2022 and has spread across NSW and adjoining states onto many farms that were previously free of the weed.
Properties previously with low weed levels commonly have increased densities.
Now is a good time to plan feasible control programs for mid-spring onwards when the weed is clearly visible.
Some infestations are so bad it is difficult to determine where to start control strategies.
A sensible program may be to map the weeds presence and develop a plan to endeavour to eradicate from boundaries and work inwards.
Where infestations are minimal, a plan to spot spray via total area surveillance with repeat treatments is commonly necessary to capture late maturing or earlier missed plants.
Check on roadsides and adjoining land as infestations easily spread via various means.
St John's wort if left uncontrolled can quickly increase in density, competing adversely with pasture and, for some livestock types, can cause significant poisoning.
In non-arable and timbered areas, the weed can be especially difficult and expensive to control if it gets out of hand.
As the saying goes, 'a stitch in time saves nine', is especially true for weeds like wort.
Yet, St John's wort is one weed that gives us an advantages in control.
Its flowers are a clearly visible bright yellow and remain in bloom for some time.
Arable paddocks have most control options including cropping.
A three-year cropping phase that includes fallow and in-crop herbicide control can be effective in killing existing plants and driving down the soil weed seed bank.
Then follow with a perennial grass-based pasture sown to species likely to last for many years and capable of outcompeting invading weed seeds that could develop into weed plants.
If the weed is already out of control, as hundreds of thousands of hectares are, the best approach can be to encourage a combination of management strategies.
These include competitive pastures, using appropriate grazing management, encourage - unpredictable - biological predators, as well as tactical and strategic use of herbicides.
Toxicity of St John's wort is least between the months of June and September.
Research shows heavy grazing during this time is relatively low risk to animal health and also reduces weed vigour and density.
Strong perennial pasture, either introduced or native, and high soil fertility helps encourage the more competitive pasture likely to best compete against seedlings and parent wort.
Herbicides that are non-pasture destructive and effective are an important control strategy.
Scone Consulting Pasture Agronomist Ross Watson - formerly a NSW Department of Primary Industries district agronomist, with a wort research background of over 30 years - notes Grazon Extra (Triclopyr, Picloram and Aminopyralid) is very good for spot treatment.
Its residual activity is harmful to legume growth, but grasses are not affected.
Starane advanced (Fluroxypyr) is cheaper, safe on grasses as well as legumes, and well suited to broad area spraying.
Mr Watson says the best time to apply both products is at full flower.
Glyphosate, often in combination with other products, is widely used when treating fallows prior to cropping phases to eliminate the weed.
Mr Watson considers St John's wort, for many areas, the number one pasture weed threat in non-arable country.
He advises a multi-pronged "integrated control program" that commonly includes changes to grazing management, especially for infested hills.
He feels for many situations further subdivision, combined with strategic and controlled grazing and resting of paddocks, utilising goats or sheep where appropriate, encouraging competitive pastures with appropriate fertiliser use, and strategic use of selective herbicides, should all be considered as part of an integrated control program.
A range of biological control insect agents can help but often are disappointing.
Next week: Managing to get through the next, or currently developing, drought.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email firstname.lastname@example.org or contact 0428 752 149.