Perennial grass weeds are perennial problem

How to prevent them from taking hold

Tablelands Telegraph – November 2021

Phil Cranney, Senior Land Services Officer, Pastures

The last 18 months of above average rainfall is hiding the sleeping giant of lost farm production in annual grass dominant pastures. Stocking rates that do not match the phenomenal grass growth have seen some extremely low utilisation rates of paddocks. It could be over six months before farmers open a gate and drive around checking how many days grazing are in these paddocks.

It is a tight rope we walk when trying to spell paddocks to ensure ground cover increases to acceptable low erosion risk levels. Unfortunately, the same long rest periods that benefit some of our native grasses, also mean that perennial weeds can thrive under this management regime without other beneficial perennial grasses to compete with the invasive seedlings.

Invasive perennial grass weeds vary from district to district, but here are the top 4 culprits:

  1. Serrated tussock (Nassella trichotoma)
  2. African love grass (Eragrostis curvula)
  3. Chilean needle grass (Nassella neesiana)
  4. Coolatai grass (Hyparrhenia hirta)

Often these grasses as seedlings are vulnerable to competition from improved pastures such as Phalaris, Cocksfoot, Fescue and sub-clovers. Sub-clover has also been shown to be an excellent competitor for soil water and nutrients against serrated tussock seedlings. However, if you do not have any of perennial pastures in your paddocks, there is little competition to fight the invasive perennials grass weeds that are busy establishing a robust root system to produce the next generation of seedlings.

So, what can we do to prevent these grass weeds from taking hold in a once productive native pasture? First, prioritise your monitoring time. Prioritise your paddocks using the following five criteria:

  1. Steep un-arable paddocks that are dominated by annuals
  2. Paddocks adjoining neighbours that have significant grass weed uncontrolled burden
  3. Paddocks with a history of perennial grass weeds
  4. Paddocks that were lower than 70% ground cover in February 2020
  5. Low fertility paddocks

Prioritise monitoring and control efforts this spring and summer to those paddocks that score high on the above criteria.

There has never been a greater incentive to control perennial weeds such as serrated tussock. Using the NSW DPI and NSW Weeds Action research modelling the impact of different control methods on serrated tussock, we can quickly do some impressive “back of a beer coaster” numbers.

Using a 40ha paddock of 50% annual grasses and 50% perennial grasses stocked at 6DSE/Ha, it took less than five years for serrated tussock to infest 25ha of the paddock under a drying climate scenario with no control of the weed.

In comparison, using a combination of boom and spot spraying, the model showed that the infestation was limited to well below 3ha, or less than 10% of the total area.

Therefore, using 1000ha of “at risk” paddocks (50% annual and 50% perennial pasture), 625ha becomes fully infested with serrated tussock in less than five years. Only 325ha can be run at 6 DSE/Ha and the rest would only run pprox.. 2 DSE/Ha.

Using $50/DSE gross margin return from a traditional merino breeding operation, your income will take a massive hit, from $300 000 to $160 000 ($97 500 + $62 500). I reckon $140 000/year can buy a lot of contract weed spraying and pasture improvement.

The paper ‘Determining the value of pasture to limit serrated tussock invasion’ (Geoff Millar, Aaron Simmons, Karl Behrendt, Marja Simpson, Warwick B. Badgery) outlines the following key messages:

  • Competitive perennial pastures limit serrated tussock seedlings
    • >2t DM/ha prevents seedlings germinating
    • Seedling survival over the first summer is lower in perennial (0.3%) than annual pastures (18%)
  • There are niches that are susceptible to invasion:
    • Vulnerable areas of the paddock/landscape/farm
    • Seasons that have higher levels of invasion
  • Annual pastures have a higher invasion rate of serrated tussock than perennial native pastures
    • 40% higher invasion rate and 44% lower profit for a pasture that started with half annual and half perennial than a perennial pasture
  • Higher stocking rates have higher invasion rates of serrated tussock than low stocking rates
    • On average the 6 DSE treatments had a 32% higher invasion rate than the 4 DSE treatments
  • Changes in rainfall patterns predicted with climate change will influence future serrated tussock invasion rates
    • On average there was a 37% higher invasion rate in the dry compared to the wet scenario
    • There was a 35% lower profitability with a dry than the wet climate, with less of a reduction in the absence of serrated tussock (14%)
  • Spot spraying was the most cost-effective control options in perennial pastures
    • A 14% reduction in profit compared to No ST
  • Spot spraying was the least cost effective in half annual and half perennial pastures
    • Invasion rates of serrated tussock were too high to make spot spraying viable
    • Spot + boom spraying was most successful and reduced profit by 30% compared to no serrated tussock
  • Are conservative stocking rates worth it for serrated tussock management?
    • In a perennial pasture without serrated tussock, increasing stocking rate from 4 to 6 DSE increased profitability by 26%
    • With best bet serrated tussock management (spot spraying) higher stocking rates were 15% better off
    • In half annual and half perennial pastures without serrated tussock increasing stocking rate from 4 to 6 DSE increased profitability by 14%
    • When best bet management was used (spot + boom spraying) there was an 18% reduction in profit at the higher stocking rate

In pastures vulnerable to serrated tussock invasion, conservative stocking rates are more successful.

If you are not sure what grass weeds you have, download the NSW DPI WeedWise app. It has great photos, descriptions and control methods for all invasive perennial grass weeds that impact our central tablelands productivity and natural resources. Your local council are your first point of contact for weeds.

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