Why Giant Rats Tail Grass Keeps Coming Back After Spraying
Giant Rats Tail Grass (GRT) is one of the most frustrating pasture weeds for farmers and lifestyle property owners. You spray an infested area, see the plants die off, and then find new plants coming through a few months later.
In most cases, the spray has worked on the plants it reached. The problem is that GRT spreads by seed, and spraying doesn't deal with the seed already in the paddock. Understanding where the new plants come from makes it easier to plan your follow-up and keep the weed under control.
Reasons GRT Returns After Spraying
Seed Already in the Soil
GRT produces a large amount of seed, and that seed can stay in the soil for years. Spraying kills the plants you treat, but the seed on the ground is left behind and can germinate when conditions suit.
Glyphosate won't stop these new plants, as it only works on the plants it touches at the time of spraying and doesn't stay active in the soil.
Plants Producing Seed While They Die
GRT doesn't die straight away after spraying. Flupropanate can take three to four months to kill the plants, and plants treated with glyphosate or flupropanate can still produce seed during that time. This means a sprayed paddock can still gain new seed, especially if the plants were close to seeding when they were treated.
Plants That Were Missed
It's easy to miss plants when spraying, particularly young GRT, which looks a lot like other pasture grasses before it forms seed heads. Plants along fence lines and in hard-to-reach areas are also easy to skip, and any plant that's missed can grow on and produce seed.
Thin or Bare Pasture
GRT grows best where pasture is thin, bare or overgrazed. Stock don't like eating it, so they graze the better grasses around it and leave the GRT behind. After spraying, bare patches give new seedlings space to grow, and without strong pasture to compete with, GRT can quickly fill those gaps.
Seed Brought In From Other Areas
GRT seed can be carried on vehicles, machinery and stock. Moving any of these from an infested paddock into a clean one can spread the weed to new parts of the property.
When to Spray GRT
Since GRT spreads by seed, the aim is to kill plants before they produce it.
Glyphosate works best on plants that are actively growing, while flupropanate needs rainfall to become active. In Queensland, applying flupropanate in October or November, when conditions are dry, can kill plants before they set seed. Plants sprayed in February or March may still produce some seed while they die.
Always follow the product label for rates, timing and grazing withholding periods.
Follow-Up Checks After Spraying
One spray is rarely enough, so it's worth checking treated paddocks regularly. Look for:
- Plants that are still green once the spray has had time to work
- New seedlings coming up through the pasture
- GRT along fence lines, gateways and hard-to-reach areas
- Bare or thin patches where new plants could grow
Treat these plants early, while there are only a few and before they set seed. Keeping a record of where GRT keeps appearing also shows you where to check first next time.
Using a Weedwiper on Scattered Plants
After the main treatment, you may be left with scattered GRT plants through the paddock. Spraying the whole area again uses more chemical and can set back the surrounding pasture.
Because GRT usually grows taller than the pasture around it, a weedwiper can treat it without touching the grass below. Its herbicide-soaked wicks are set just above the pasture and wipe chemical onto the taller weeds as you drive through. A few features help when wiping GRT:
- Two rows of thick wicks that stay wet without dripping onto the pasture
- A bruise bar at the front that damages the weed so it takes in more chemical
- Wicks that can be raised to around 500mm, with a hand winder for small height adjustments
- Wing wheels that follow uneven ground so contact stays even
Graziers often wipe GRT at around 10 km/h and go over the paddock in both directions to reach both sides of each plant. Wiping also doesn't cause spray drift, so it can be used when it's too windy to spray.
Keeping Pasture Strong
Spraying and wiping kill GRT plants, but they don't stop new seedlings from coming up. Thick, healthy pasture makes it much harder for those seedlings to establish.
Avoid overgrazing paddocks where GRT has been a problem, and give treated areas time to recover. Where pasture is thin, fertiliser or sowing improved pasture grasses can help fill the gaps.
Keep Checking After the First Spray
GRT comes back because of seed in the soil, plants that produce seed while dying, missed plants and thin pasture. There's no quick fix, and control usually takes more than one season.
Whether you manage a small block or a larger grazing property, the same approach applies. Spray at the right time, follow up on plants that return and keep your pasture healthy. Finding and treating new plants early is much easier than dealing with a large infestation later.
