Can sheep help manage feathertop Rhodes grass?
with Dr Asad (Md) Asaduzzaman, weeds researcher, Charles Sturt University
Feathertop Rhodes grass is an invasive weed that has rapidly spread across all grain-growing regions in Australia. It is notoriously hard to kill with single herbicide applications and is a prolific and rapid seed producer.
Dr Asad (Md) Asaduzzaman, a weeds researcher at Charles Sturt University, says there is good evidence that sheep grazing young feathertop Rhodes grass can effectively reduce seed set.
“Interestingly, feathertop Rhodes grass is very palatable and has high protein when the plants are young,” he says. “Integrating grazing and herbicide treatments can significantly impact the seed bank of this challenging weed.”
“The small seed is readily damaged as it passes through the sheep’s gut, rendering over 99 per cent of the mature seed consumed unviable. Making silage from feathertop Rhodes grass with mature seed heads also reduced seed viability by 65 to 95 per cent, depending on the population.”
The grazing trials have investigated the optimal timing and frequency of grazing and the chance of the weed spreading in animal faeces.
“Employing grazing as a non-herbicide tactic for feathertop Rhodes grass can make productive use of the weed and starve the plants of carbohydrates, which results in smaller seed heads and fewer seeds,” says Asad. “It is reassuring to know that only a minute number of mature seeds will remain viable after grazing or making and feeding out silage.”
The purpose of the WeedSmart Big 6 is to develop robust weed management strategies that help growers reduce the weed seed bank and grow profitable crops. Growers who run mixed farming operations have the opportunity to integrate grazing strategies to target weeds like feathertop Rhodes (FTR) grass.
What is the benefit if grazing does not kill the FTR grass plants?
FTR grass is now widely distributed and naturalised throughout Australian cropping and pastoral regions. It has a long growing period (September to June), and many populations are resistant to glyphosate. Group 1 herbicides are effective, but there is a high risk of evolving resistance to this mode of action group.
One of the features of FTR grass is its ability to produce multiple seed heads from very small plants shortly after minor falls of rain. Although grazing does not kill FTR grass, it can delay and reduce the number of seedheads produced on stressed plants while they are less susceptible to herbicide treatments. Digestion also renders any mature seed unviable.
When conditions are suitable, herbicide application will be more effective, and due to earlier grazing, less seed will have entered the seed bank.
Although digestion in the sheep’s rumen renders mature seed unviable, it is advisable to avoid grazing tall stands of FTR grass because the tiny seed can easily be distributed on feet, skin/wool and vehicles.
If sheep have consumed mature FTR grass seed heads, quarantine them for a week to allow the small amount of viable seeds to be excreted.
What effect did grazing have on FTR grass biomass and seed production?
The first of two grazing trials in 2020 showed that multiple grazing events in late March and through April reduced the number of spikelets per seed head from 12 in the no-graze control to 9 in the multiple-grazing treatment. This resulted in a 73 per cent reduction in seed production compared to the no-grazing control treatment.
In December 2020, another trial also showed that multiple grazing events prior to the summer fallow reduced seed production by 61 per cent compared to the no-grazing control treatment. Applying herbicide either before or after the late grazing event had a significant additive effect to further lower seed production. These plants produced only one seedhead per plant compared to 16 in the no-grazing treatment.
When the FTR grass was sprayed with glyphosate (Group 9 [M]) before the late grazing event, there was an 88 per cent reduction in seed production. Similarly, applying haloxyfop (Group 1 [A]) after the late grazing event reduced seed production by 87 per cent.
What is a strong and practical strategy to implement using sheep to manage FTR grass?
At tillering, FTR grass is a valuable feed source that offers 17 per cent crude protein and 10.7 MJ/Kg DM, both on par with annual ryegrass in its vegetative phase.
In southern grain cropping regions, FTR grass tillers in spring and early summer, so grazing soon after harvest is a good time to reduce weed growth and seed production.
Grazing moisture-stressed plants over summer is likely to be more effective on FTR grass than applying herbicide. Grazing can keep stressed plants under control and limit seed production until spraying conditions are suitable for maximum control efficacy.
A combination of grazing and herbicide application will help keep FTR numbers down over summer, and sowing a competitive winter crop will minimise in-crop germination of FTR, keeping downward pressure on the weed seed bank throughout the year.
Ecological weed management often relies on the combined impact of ‘many little hammers’, a principle that resonates through the WeedSmart Big 6 strategy.
Resources
- Asaduzzaman M, Piltz J, Koetz E, Hopwood M, Shephard A and Wu H (2022) Seed viability of feathertop Rhodes grass (Chloris virgata Sw.) reduced by silage, digestion, and sheep rumen digestion. Front. Agron. 4:954153. doi: 10.3389/fagro.2022.954153
- GRDC Update Paper: Key strategies for the management of Feathertop Rhodes grass (southern region)
GRDC project: DAN1912-034RTX FTR
Impact of grazing treatments of FTR grass biomass and seedhead production.
Sheep grazing plots of feathertop Rhodes grass.
