Galvanised burr control is one of the biggest issues I receive enquiry about. It is an issue on millions of hectares of western/north western NSW pastures and in many cases it is outcompeting pastures for soil water, space and nutrients. Adult plants are largely unpalatable. Competitive pasture is the best long term control approach.
Drought between 2017 early 2020 saw many pastures heavily grazed over an extended period with native grass species commonly badly thinning out. In contrast galvanised burr, largely because its burrs make it unpalatable, was un-grazed and consequently increased its dominance in many paddocks. Once dominant it makes it hard for native grasses to re-establish against strong weed establishment.
On arable land a logical and sound strategy is to crop weed infested paddocks for at least three years and replant to a strong and long term persistent tropical grass pasture. While weed seed reserves are unlikely to be totally eliminated via a three year cropping phase, with appropriate in-crop and fallow herbicide use, it can go a long way to dramatically reducing soil weed seed reserves.
After the cropping phase (either for grain or grazing or both) paddocks can be sown down to tropical grass. Basically this is fight " fire with fire". Species choice will depend on rainfall environment and soil type. For example in western warmer districts choices can include buffel grass, Premier digit grass and Consol love gras. While more a medium lighter textures soil weed, in heavy soil infested areas Mitchell grass and bambatsi panic also suit. In more moderate temperature areas Premier digit and Consol are commonly the best choice.
There are many examples of established perennial pastures sown after a cropping clean up stage relatively free of galvanised burr decades after establishment. Sound perennial grass management is part of competitive perennial pastures. Periodic pasture resting to ensure good root reserves, especially important for persistence over droughts, and not grazing into the ground are important.
While soil fertility issues are commonly not assessed as feasible in drier areas, good competitive, as well as productive and high quality pastures only happen if soil deficiencies are addressed. Quite a lot of NSW DPI research has shown that commonly drier areas require less frequent fertiliser use, and generally moderate rates, therefore more affordable programs, than for higher rainfall areas. Including legumes, if not present, in the tropical grass pasture helps ensure better soil fertility as well as quality production.
Non-arable areas of galvanised burr are more difficult to control but progress can be made to reduce the weeds impact. Two of galvanised burrs features help in long term control. Firstly plants generally only live for around two to four years, but they are prolific seeders, hence reestablishment is common and can be continuous.
Secondly young galvanised burr plants die when basal buds are removed via grazing, generally best done via sheep. Basal buds are the growth points from where branches emerge. They appear about six weeks after germination, with often a big germination after excellent rain events. Although spines appear at the same time they are relatively soft and grazable. Timed heavy seedling grazing can result in large seedling kills.
In burr infested non-croppable areas grazing management that encourages revegetation and strengthening of native grasses helps with control. Good native grass grazing management is similar to that for introduced grasses. Flexible rotational grazing that allows for germination, establishment, as well as replenished of root reserves of existing plants, as well as periodic seeding are all important. More subdivision and more watering points is an important consideration for better pasture management. Selective herbicides when conditions are suitable (generally mild and after rain) can kill or retard burrs while not harming grasses. If herbicides are used it is important to treat outside the winter legume phase. It is important not to prioritise herbicides above other strategies, such as correcting soil deficiencies.
Next week: Farming successfully with climate change and climate variability.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
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