Combating pimelea toxicity

This research project update article by Agforce Biosecurity Policy Officer Marie Vitelli first appeared in the AgForce weekly Action enewsletter on 30 April 2020. 

Laboratory studies with field-collected rumen fluid samples

During 2019, twelve different fermentations of up to 63 days duration have been run through the artificial, anaerobic rumen at the Ecosciences laboratory in Brisbane (Figure 1). This artificial rumen is fed daily with Pimelea plant material or a 50:50 mix of Pimelea and Buffel grass hay. Daily rumen fluid samples are chemically analysed for presence and degradation of Pimelea material and toxic simplexin. To date, the most promising toxin decrease was from the rumen fluid sample collected from Dorper sheep.

Microbial diversity in rumen fluid samples

Recent scientific advances in gene technology has enabled studies of bacterial populations to look for differences in composition. This research looked for any differences in rumen bacterial populations between livestock showing mild, moderate or severe Pimelea poisoning. Results of this genesequencing analysis showed no difference in rumen microbial diversity between affected and nonaffected cattle. The main effect on microbial diversity (microbiome) was related to food supplement intake such as hay, grain, cottonseed, faba beans and a variety of licks. All 110 rumen fluid pellet samples from 82 cattle, 11 goat, nine sheep and eight kangaroos have been tested for microbial diversity. The kangaroo samples showed the greatest difference in types and abundance of rumen microbes.

Developing a rumen inoculum (drench) with bacterial isolates that can degrade simplexin

The gene-sequencing technique is also used to identify microbes from the rumen fermentation samples which are successfully breaking down the simplexin toxin. To date, 102 bacterial isolates from 21 bacteria species have been obtained from six laboratory fermentation experiments. Seventeen isolates were further tested for their ability to break down simplexin. Three bacterial isolates are showing some promise. These tests are ongoing and other remaining bacterial species are still to be tested, including the recent isolates from the Dorper sheep fermentation.

Rumen adsorbent studies

All three tested adsorbents had some effect in binding the simplexin toxin in laboratory experiments using rumen fluid. The most effective was sodium bentonite (Truefeed®) at 12mg/ml, which bound 95 per cent of the simplexin toxin (Figure 2). Other adsorbents Nutrilick® biochar and Elitox® bound 30 to 60 per cent of simplexin, depending on the concentration of adsorbent present. Both sodium and calcium bentonite remained bound to simplexin for the longest period. Tests are on-going and animal trials are planned to commence soon.

Biodegradable biopolymers as a rumen bolus toxin trickle-feed system

The retention of rumen bacteria that could detoxify the simplexin toxin when cattle are not ingesting Pimelea is important to consider. This would enable rapid build up of the effective bacteria when Pimelea is consumed. The concept of a biodegradable rumen bolus impregnated with small amounts of Pimelea to trickle-feed small amounts of the plant material into the rumen, could potentially sustain the effective bacteria. A range of biodegradable, porous biopolymers and biopolymer composites have been tested in the artificial rumen fermenter. To date, the most promising bolus combination for sustained release is a polyhydroxyalkanaoate (PHA) / Pimelea/ sugar mix (Figure 3). The next step is to trial the bolus for three months (without the Pimelea toxin) in fistulated cattle. Animal ethics approval has been granted. These biopolymer boluses may also be useful for slow-release drug delivery to livestock.

Other research and activities 

Pimelea ecology research Rashid Saleem is a PhD research student from University of Queensland Gatton campus studying Pimelea ecology to ascertain optimal trigger points for forecasting field germination of the toxic native plant. To date, Rashid’s results have shown:-

• Optimal soil temperature range for Pimelea germination is between 10oC and 25oC.
• Pimelea seeds have a dormancy period. The embryo has a maturation process and the hard seed coat of fresh seed (Figure 4c) requires at least three months weathering to become permeable enough to allow seed germination.
• When seeds were treated with the stimulant, gibberellic acid at 1.15mM concentration, approximately 80 per cent of seed germinated after 50 days. Gibberellic acid breaks seed dormancy.

Residual herbicide pot trial to suppress Pimelea germination

During 2020, Rashid is conducting a pot trial at UQ Gatton to assess the residual activity of three herbicides to suppress Pimelea trichostachya seedling emergence. Treating a paddock or graded tracks to suppress Pimelea during high risk seasons may be a useful option to free an area of this toxic plant. The first residual herbicide product being tested is:- • Tebuthiuron pellets (Graslan) at a product rate of 0, 0.25, 0.5, 0.75, 1 and 2 grams per square metre. Figure 5 compares the commercial formulation of a 20 per cent (coarse) pellet and 10 per cent (finer) experimental formulation of tebuthiuron, when applied at a rate of 2g /m2 .

Results from these pot screening trials will help ascertain follow up experimental field plot trials for residual herbicide efficacy.

Russell Mocker from near Begonia, Southern Inland Queensland delivered large loads of red sandy clay soil to UQ Gatton. The soil was required for these herbicide pot trials. Other residual herbicides for consideration in future experimental trials include metsulfuron-methyl at 30, 40 and 50 grams per hectare and picloram granules (Tordon granules) at 50, 75 and 100 grams per square metre.

Why dry Pimelea stalks are toxic to cattle?

Cattle tend to avoid grazing dense stands of odorous, green Pimelea plants. However, cattle cannot distinguish dry Pimelea stalks amongst Buffel and other pasture tussocks. Producers have reported  cattle losses from ingesting dry Pimelea stalks. Please be aware these dry stalks often contain a fluffy tuft of very toxic, immature seeds! Flowers and seeds contain the highest levels of simplexin.

What will Pimelea do this year?

Pimelea is often abundant after dry summers, followed by cool, wet winters. Rainfall events greater than 40mm during April to August may trigger Pimelea germination. Flowering can occur most months, in response to rainfall. Visible, standing dead plant material can occur anytime of the year and is a risk to grazing cattle. Livestock trampling, heavy rain and wind tend to flatten dead stems. Cattle poisoning is more frequent between September to January when Pimelea is abundant. Cattle losses have occurred this year in the Jericho region due to Pimelea growth after Oct 2018 rainfall.

Current herbicide management options for Pimelea

In small areas, where herbicide management is a feasible option, seven herbicides are registered for control of Pimelea as per APVMA Minor Use Permit 13549 in NSW and QLD. Spray young Pimelea plants to ensure no dead toxic stalks remain standing amongst pasture tussocks. These herbicides will also affect broadleaf legumes and forbs in treated areas.

What you can do to assist research into combating Pimelea?

1. Stay in contact with Pimelea research updates from MLA, DAF, UQ and AgForce.
2. Ensure Pimelea remains a priority amongst your Regional Beef Research Committee https://www.nabrc.com.au/regional-committees/, which in turn influence MLA’s research funding allocations.
3. Applications to MLA’s Producer Demonstration Sites https://www.mla.com.au/extension-training-and-tools/producer-demonstration-sites/. Requires at least 10 producers across three properties to demonstrate farm level benefit of Pimelea management?
4. If DAF’s Grazing Futures Project is active in your region, inquire if the Drought and Climate Adaptation Program would host Pimelea update sessions https://www.longpaddock.qld.gov.au/dcap/grazingindustry/.
5. Share your Pimelea management observations with AgForce and the rumen research leaders at UQ and DAF. Your valued input may help combat Pimelea toxicity and management.

Contact: Associate Professor Mary Fletcher, Centre for Animal Science, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland T: +61 7 344 32479 or E: mary.fletcher@uq.edu.au