Erastus Ngaruka
A significant number of livestock mortalities in many farming areas can be attributed to plant poisoning, as many farmers report symptoms that relate to poisoning.
This occurrence is most common during the spring months at the onset of rainfall activities or during periods of drought, especially in degraded rangelands.
Many rangeland plant species contain chemicals that are poisonous to livestock when eaten.
Poisonous plants vary in their toxicity and the symptoms displayed by affected animals.
Generally, the level of toxicity can be classified into two: plants that are extremely toxic and those with low toxicity levels.
The extremely toxic plants, such as Dichapetalum cymosum (Poison leaf/Gifblaar), will only have to be ingested in small amounts to cause harm or for animals to show signs of poisoning.
Whereas plants with lower toxicity, such as Geigeria ornativa (Vermeerbos), show their effects after being consumed in larger amounts and over a longer period.
The toxicity of poisonous plants is influenced by several factors, such as soil type, climatic factors, seasons, plant growth stage, plant part was eaten and plant moisture content, amongst others.
Poisonous plants have different effects on animals and different clinical signs; they are classified as plants that cause heart problems, nervousness, diarrhoea, liver damage, obstruction in the gut, skeletal and skin problems, reproduction problems and plants, causing discolouration in meat and milk.
Moreover, some valuable forage plants exude toxic effects when over-consumed, eaten at a certain growth stage, and when certain parts such as flowers are eaten.
For example, Tribulus terrestris (devil’s thorn, ohongwe, oshosholo, nhonho, duwweltjie) is a common weed in many areas; it is a valuable forage plant that is well utilised by livestock when green but becomes poisonous at wilting.
Others include grasses such as Cynodon dactylon (quick grass, ongwena, kweekgras) and Panicum maximum (guinea grass), amongst others.
Fodder plants like lucerne, maize and others can also cause poisoning (nitrate poisoning), for example, when hay is spoiled and mouldy due to poor storage when it gets wet.
In many rangelands, poisonous plants mostly emerge during the spring months (July to October) and when pastures are in poor condition or overgrazed.
In some rangelands, however, they form part of the plant composition throughout the year.
Animals are vulnerable to poisonous plants due to the following basic factors, amongst others:
Hunger: animals are supposed to satisfy their daily forage requirements; thus, if the valuable forage is depleted, the animal will be tempted to eat any available plant to satisfy their daily needs and simultaneously consume poisonous plants.
This is more likely to occur during drought periods and when the rangeland is degraded.
Inexperience: this can be attributed to animals being new in the area or young; thus, they are not familiar with the local forages or cannot distinguish between valuable plants and poisonous plants.
It is, thus, advisable to avoid introducing animals to a strange area during the time/season the poisonous plants are active.
Accident: an animal familiar with the local forage is also vulnerable to plant poisoning, as it may ingest it by accident.
This is when a poisonous plant has grown close to or blended in with a valuable plant and ingested together.
Moreover, it may be that a poisonous plant may have been harvested together with grass hay.
The basic means of preventing and treating plant poisoning include: avoiding overgrazing and not allowing animals to graze in areas where poisonous plants tend to grow.
Such areas can be camped off and grazed only when the poisonous plants disappear and the other valuable plants dominate.
In the event of suspected poisoning, the animal should not be allowed to drink water for at least two days, especially when poisoning is suspected to be from an extremely toxic plant (e.g., Poison leaf), and the animal should be handled with care.
These are ways to limit or slow the circulation of poison through the entire body.
There are remedies that are used to neutralise the poison in the animal’s body.
These include Hypo, which is a crystal-like content that should be mixed with water.
It can be added to the water in the drinking trough, and all animals can drink as they arrive from the veld.
Charcoal: this can either be activated charcoal sold commercially or the normal charcoal from the fire.
Normal charcoal is the easiest home-based remedy to use.
It should be well ground into a fine texture, mixed with water and given orally to the poisoned animal.
These remedies and others are readily available in agricultural shops and their application should follow the instructions on the labels.
In conclusion, it is advisable that farmers familiarise themselves with their rangeland and develop an inventory of local valuable plants and poisonous plants.
Moreover, there are guidebooks for more information that assist with the identification of these plants.
Such books include ‘Toxic Plants of Veterinary Importance in Namibia’ and ‘Grasses of Namibia’, amongst others.
– Agribank
* Erastus Ngaruka is AgriBank technical advisor for livestock and rangeland.