Toxic weeds in fallow paddocks
By Tim Bartimote - Cropping Officer
As we progress further into summer, sporadic rainfall events can sometimes occur. Often this rainfall is not enough to justify planting a summer crop, but frequently it provides an opportunity for summer weeds to emerge in fallow country. Those with mixed enterprises may consider utilising stock mouths for weed control in an effort to ‘kill two birds with one stone’. This can be an effective solution, as it may assist in meeting feeding requirements while also controlling potentially hard to kill weeds due to stress or dust coverage. When considering this approach, keep in mind that though there is a low input cost compared to a spray application, grazing weeds may incur a greater loss in moisture and nitrogen. This is because stock take longer to achieve the same coverage across the paddock. Research from NSW DPI in conjunction with the GRDC has shown that for every dollar spent on spraying weeds, there is a $3 per hectare to $8 return on investment.
There are instances where producers may be able to incorporate both herbicide application and stock, a process called spray grazing, which is often used in pastures. Weeds are first sprayed with herbicides and then stock graze on survivors once grazing withholding requirements have been met. Spray grazing can be used as an unconventional ‘double knock’. For information on grazing withholding periods check your label. In some instances, for example with the use of phenoxy herbicides (found in Group I), such as 2,4-D, the palatability of weeds increases after being sprayed. These herbicides mimic the effect of plant hormones and cause broadleaf plants to mobilise energy reserves to produce lethally abnormal growth. Always make sure to check the label before application, as regulations for these chemicals can be updated.
Before putting stock on fallow paddocks to assist in weed control it is important to know the weed spectrum. As there are weeds which can be toxic when consumed by stock. This is particularly important if they have been sprayed by herbicides which may make them more palatable.
Species mentioned in this article
● Caltrop (Tribulus terrestris)
● Yellow Vine (Tribulus micrococcus)
● Common Heliotrope (Heliotropium europaeum)
● Blue Heliotrope (Heliotropium amplexicaule)
Tribulus sp. - Caltrop and Yellow Vine (Cat Head)
About the plant
Commonly known as Cat Head, Caltrop is an introduced annual summer broadleaf with a small yellow flower and spine covered fruit. Yellow Vine is an annual summer native species and has a larger yellow flower with spineless fruit. Both are found widely throughout our region, being well-known residents of fallows and degraded pastures, due to their propensity to be transferred by stock and machinery. Dusting is a major issue when attempting to control these weeds with herbicides, due to their semi-erect to prostrate growth habits. Dust can gather on leaves and create a physical barrier, reducing the efficacy of herbicide applications. In particular, glyphosate readily binds to soil particles present as dust or in dirty water, causing poor control.
Risks to stock
Tribulus sp. can pose a serious risk to animal health. They are capable of causing severe acute liver disease and secondary photosensitisation. Saponin is the primary toxin which causes crystals to form in the liver. The secondary toxin is currently unknown and could potentially explain the sporadic nature of the disease. Generally, risk is highest for 10 days after small rainfall events as Tribulus sp. often provide the first green pick. Goats and juvenile sheep appear to be the most susceptible.
Liver disease and photosensitisation
Symptoms include depressed stock with swollen floppy ears, swollen eyes and swelling under the jaw. Stock flicking their head continuously, as if to ward off flies, is another early warning sign. As the disease progresses, animals become more depressed, eventually go down, then die. Producers are advised to monitor stock closely for 10 days after summer rain and remove them from problem paddocks if symptoms present themselves. Placing affected stock in shade, such as a shed, can reduce the impact of photosensitisation. Survival can occur if liver damage is not too extensive.
Consumption of Caltop and Yellow Vine can each cause quite similar nervous disorders in stock. Chronic Tribulus staggers develops in sheep when they have consumed large quantities of Caltrop over an extended period. This can result in a progessive and irreversible weakness in the hind legs of affected animals. Interestingly, it impacts one side of the body more than the other which causes the animal to lean, and struggle to run straight ahead. Often these stock die of thirst, flystrike or pneumonia as their front legs weaken from the increased strain. Unfortunately treatment is limited and signs may not develop for several months. Average duration of the disorder varies from 1-15 months.
Tribulus ataxia hind leg disorder
Transient Tribulus ataxia is a slightly different hind leg disorder caused by grazing in paddocks dominated by Yellow Vine. In this case, both hind legs are affected equally and sheep rely on their front legs to move around. Common symptoms include sheep that crouch down in their hind legs and drag their body around. Again, affected animals often perish of thirst, fly strike or pneumonia but unlike Chronic Tribulus staggers, the disorder is reversible by removing stock from yellow vine dominated paddocks for 3 - 6 weeks.
For more information on animal health issues caused by Tribulus sp. Consider reading this helpful Primefact document:.
Heliotropium sp. - Common and Blue Heliotrope
About the plant
Common Heliotrope (Heliotropium europaeum) is a hairy annual forb, initiated by summer rain. The Common Heliotrope can grow up to 30cm high and has white, tubular flowers, which are less than 5mm in diameter. It is found in a variety of soil types but is often found in fallows and pastures with limited ground cover. Leaves also can acquire a large amount of dust making control sometimes difficult . Blue Heliotrope (Heliotropium amplexicaule) is an introduced, hairy, perennial forb, roughly 15cm high, multi-branched stems and a considerable taproot. Blue heliotrope can be found in paddocks with minimal competition and can quickly spread across a paddock. They are often found in lightly textured soil types across our region.
Although called blue, the tubular flowers are actually purple or lilac with yellow centres. These plants often flower in spring and autumn and this is the preferred growth stage to target this weed with herbicides, as control is often quite difficult.
Risks to stock
Stock will not target these two species for grazing when there are more palatable species available, however they will when there is little other choice. Both plants contain pyrrolizidine alkaloids which causes chronic liver damage in stock. The extent to which the liver is damaged depends on the quantity ingested and which species consumed it. While cattle and horses are more susceptible to pyrrolizidine alkaloids, sheep (in particular cross bred sheep) are more often affected due to being more likely to consume blue and common heliotrope.
Symptoms will not appear initially and are often seen after the stock has been removed from paddocks containing the weed. In sheep the damaged liver accumulates copper. When stock experience a stressful event later in the season, this triggers the release of copper from the liver, causing copper toxicity. This breaks down red blood cells, and impacted stock can be observed to wander off alone one day and dead the next. The colour of their skin is generally a bright yellow at this point.
A molybdate drench can be supplied as a treatment, as it binds to the copper, but this is only a short term solution as the liver is now permanently damaged. Preventing stock from ingesting these two heliotrope species is the best option. For more information on blue heliotrope, see this profile from NSW WeedWise.
If you require any more information about other potential toxic weed species which may appear over summer feel free to contact your Local Land Services Ag Advisor. If you have any stock which you suspect may be impacted by grazing toxic species our vet team can assist in diagnosing the cause of stock death.