Tropical grasses are kryptonite for summer perennial weeds

AG ADVICE - November 2021

Phil Cranney - Senior Land Services Officer, Pastures

The top three summer perennial weed supervillains to look out for this spring are:

  1. Blue heliotrope Heliotropium amplexicaule
  2. Silverleaf nightshade Solanum elaeagnifoliu)
  3. Sticky nightshade Solanum sisymbriifolium

Herbicides often get all the attention for the control of these supervillains, but the true heroes of this story are the beneficial perennial grasses that stop these perennial weeds from invading your productive paddocks.

However, there is good evidence to suggest that there are alternatives to herbicide control of these top four invasive weeds.

Of course, the big advantage that some farmers have is the ability to cultivate and sow crops in these paddocks. In this scenario, we can use the Batman and Robin, double punch to help defeat the enemy. “Robin” is the short-term crop and “Batman” is the preparation and establishment of a long-term perennial tropical grass.

You can send in Robin to soften-up the enemy with a competitive millet crop sown in September/October on the slopes, or October/November on the Tablelands. There are many selective herbicide options for the clean-up of these broadleaf weeds in crop. The Robin approach, combining in-crop herbicides with a knock-down herbicide fallow application pre-sowing to store moisture, will stop these perennial weeds from setting seed and deplete their root reserves.

The unassuming crime/weed fighter Robin does not stop there. The millet can be terminated, after grazing/hay/harvest, in late January. This now allows the farmer to use affordable knockdown herbicides to again stop the broadleaf perennial weed from seeding and weaken its root reserves during a short fallow period.

Depending on soil moisture and sowing conditions, a short season hard seeded legume can be planted in mid-late autumn. While the supervillains (summer perennial weeds) are dormant, Robin plants the legume seeds of success to ensure a productive pasture in the long term. Selective herbicides can then be used to reduce the annual winter grass burden.

Depending on the season, this legume could be grazed, before being locked up to maximise the seeding event from September to November. Using a short season legume is critical, to ensure there is still an opportunity to stop the perennial broadleaf weed from seeding early in summer. After a year of fighting the enemy, Robin has a well-earned break and tags Batman into the battle.

Depending on soil moisture and the abundance of the weed regrowth, Batman may decide to lead with heavier translocated herbicides may be needed to deliver the knock-out blow in early summer. If there is enough moisture by the end of December, you could use another millet crop to compete with the summer broadleaf weeds. Allow the millet crop to regrow after grazing to a height >30cm, before spraying out for the winter fallow.

Another option, especially if there is still a significant seed bank of summer annual grass weeds like Black grass and Hairy Panic, would be to fallow until early autumn. Batman plants an erect, strong straw strength, short-season cereal in autumn for grazing only. This gives Batman another opportunity to use selective herbicides to control broadleaf weeds in the cereal.

The cereal is grazed, then let regrow to a decent height (>30cm), before being sprayed out no later than September. The standing stubble provides an important structure to reduce evaporation while the tropical grass is germinating in late spring or early summer.

The six big weapons deployed by Batman critical to ensure tropical grass establishment success are:

  1. Soil test and fix any nutrient deficiencies or toxicities
  2. Reduce the weed burden for two summers prior to planting
  3. Maximise the soil moisture profile
  4. Quality seed (check purity and germination)
  5. Shallow seed placement, aim for 10mm
  6. Stubble cover reduces evaporation

In the early 2000’s Bob Thompson, former NSW DPI Agronomist, showed that tropical grasses could beat silverleaf nightshade on the Southwest Slopes with an integrated weed management approach.

We know that our farmers are all superheroes that survived the last drought. So, it is time to put your cape back on and fight those perennial supervillain weeds.

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