Some weeds one endeavours to eliminate and others one tries to reduce their impact, but accept it is almost impossible and impractical to eliminate them.
St Barnaby’s thistle is an example. While some properties have it scattered across most of the farm with high soil seed banks, for others like our situation, one aims to eliminate it before it gets a hold on.
We had kept the farm free of St Barnaby’s, but in 2017, via spreading contaminated legume seed (as part of a program introducing new species), we accidentally introduced it at a low level to 200 hectares (500 acres). We should have been more careful.
Early identification and a strategy to prevent weed seed set, especially in the first two years before it gets away (one plant can set thousands of seeds) then became our plan.
St Barnaby’s, like many weeds, has some “hard” seed, which means they can remain dormant for a few seasons and then germinate and become a problem.
In our situation we were fortunate to note scattered randomly spaced St Barnaby’s at the start of flowering. Because it is a widespread problem, we were aware of what it looks like (an important aspect of eliminating weeds early). While there are many yellow flowering plants (commonly nuisance weeds) around in spring, St Barnaby’s is distinct.
Four-wheel bike surveillance and plant removal was practical in our situation, with several hundred plants removed prior to seed set in 2017. Repeat surveillance in 2018 has only found less than 200 plants, again removed prior to seed drop.
Further surveillance will be necessary for probably the next few springs, and probably forever to capture the odd invading plants before a problem develops. Clearly a case of a “stich in time saves nine” or much more.
Most effective weed control or eradication is commonly by using a combination of strategies (integrated weed control) including herbicides, individual weed elimination, and competitive pastures. 2018, being an especially dry year illustrated the major role of competitive pastures that have largely out-competed St Barnaby’s for spring early summer moisture, and has largely prevented seed set of this mainly late spring early summer maturing weed.
In our case much of the infested pasture was tropical grass with species Premier digit, Consol lovegrass and Bambatsi panic. With density of one or more plants a square metre, these are capable of almost out-competing weeds like St Barnaby’s in years with dry springs early summers. In wetter years, although competitive, there can be enough soil water for weeds to set seed.
Other infested areas on our farm included improved native grass pastures, with many of the species summer perennials. These also compete strongly against St Barnaby’s in a dry spring early summer. In both introduced tropical and native grasses, winter legumes can also be helpful in competing against the weeds but because of the dry did not help much in 2018.
St Barnaby’s, like many weeds, has some “hard” seed, which means they can remain dormant for a few seasons and then germinate and become a problem.
- Bob Freebairn
Where weed levels are high, a three-year winter cropping program combined with fallow weed control prior to sowing down aggressive persistent perennials, is a sound strategy on arable land. A number of effective in-crop herbicides can be used to kill weeds like St Barnaby’s, to drastically reduce the soil weed seed bank.
Knowledge of likely weed threats in given areas is critical for early elimination. Constant surveillance is important, especially on boundaries (including public roadways) and entry point to a property. Surveys have shown visiting vehicles often have weed seeds in them.
Newer legumes like serradella have few herbicide options for broadleaf weed removal. Even older legumes (sub clover, medics) can be damaged by many herbicides. Some herbicides relatively safe on legumes are not always reliable at killing some broadleaf weeds. Hence the need to largely eliminate the weed base before sowing down to competitive pastures.
Some herbicides, for example Lontrel (clopyralid) can be residual for lengthy periods against some legumes and can also impact on legume Rhizobia nodules. This is especially important for herbicide use in-crop the year (in some cases the previous two years) leading up to legume sowing. Fallow weed control products also need to be checked for residual issues.
Next week: Research tests biological inputs in cropping systems.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
