Weeding out bad Johnson
.A SUBJECT that has been giving us trouble for many years is the weed Johnson grass. I have provided many control recommendations through the years for this perennial grass problem.
This particular grass weed is notorious for its hosting capability in spreading sorghum and maize leaf mosaic diseases by aphids and for sorghum midge escalations.
Having said that, it has provided a lot of feed for livestock through the years, despite its potential to poison livestock from a build-up of cyanide-type glycosides in the animal’s system, especially after some adverse plant environmental growing conditions.
So from railway lines to roadsides and on to creek flats, and of course on to our valuable cropping lands, Johnson grass is numbered in the top 10 of the world’s worst weeds.
With seed dormancy and seed viability being recorded in other parts of Australia to last several years, along with its rhizome root structure, this plant can be difficult to control and it spreads very easily.
So with up to 20,000 seeds produced on some large, mature plants, it can certainly build to a big problem very quickly – a big problem that in the very short time of weeks can eventuate in our grain sorghum or maize crops.
If not controlled, then the amount of Johnson grass seed ending up in the grain sample can make it unsaleable and probably not even acceptable for export.
You may have seedling-type plants and old mature plants in the very next row, and this is important to know.
With inter-row cultivation – or scuffling as it was called –
that is a very slow process and not 100 per cent effective for control.
Also it may spread pieces of root rhizome down the paddock. Still, it can at least reduce the competition angle against your coarse grain row crop. Obviously a robust rate of glyphosate pre-plant can reduce the Johnson grass numbers, but it still won’t eliminate the old mature plants with a 5m root system underground.
With an underground root system this large, even robust rates of glyphosate are not going to do a complete job.
My best advice has been to
tine cultivate only these large patches of Johnson grass to break up the root structure and promote a new green shoot from those fractured rhizome root structures.
In other words, the end result we need is for sections of broken up roots to generate a green shoot above ground so glyphosate when applied has a better chance of reaching the root ends.
Far from perfect, but many years of agronomy work has given me confidence this is the best method for old established patches of JG.
We do have maize varieties with an IT attached to their
name and that stands for imidazolinone tolerant and allows us to use a herbicide called Lightning to be applied over the top of certain maize varieties to control many annual weeds, including Johnson grass seedlings.
Don’t spray this on any other variety of crop as heavy damage will occur.
If the weeds are much older than seedling stage, the control rate drops off as the size of the JG, or should I correctly say the size of the root system, is much larger than you envisage from the above-ground portion.
Small weeds with a small root system are going to be better controlled by this handy group B herbicide.
It is difficult to explain in writing about controlling a weed like Johnson grass in cultivation.
Suffice to say, it needs attention to at least protect our export markets, so a herbicide and cultural program is required.
We have enough challenges in the weed world with increasing levels of herbicide resistance, so we certainly do not need any others, like the perennial Johnson grass, to alter our herbicide resistance plan we all have in place.