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, especially during spring (July to October), at the onset of rainfall activities, or during periods of drought, particularly 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. 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, season, plant growth stage, plant part eaten, and plant moisture content, amongst others. Poisonous plants have different effects on animals and different clinical signs. They are classified as plants either causing heart problems, nervousness, diarrhoea, liver damage, obstruction in the gut, skeletal and skin problems, reproduction problems, and plants causing a taint in meat and milk.
There are some valuable fodder plants with toxic effects when overconsumed, eaten at a certain growth stage, and when certain plant 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 when green, but becomes poisonous at wilting.
Others include grasses such as Cynodon dactylon (quick grass, ongwena, kweekgras) and Panicum maximum (guinea grass). Fodder plants like lucerne, maize and others can also cause poisoning (nitrate poisoning), for example when hay is spoiled and mouldy.
In many rangelands, poisonous plants mostly emerge during spring months 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:
➢ Hunger - animals are supposed to satisfy their daily feed requirements. Thus, if valuable feed is depleted, then the animal will be tempted to eat any available plant to satisfy their daily need, and in the process also eat 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, and they are thus not familiar with the local forages, or cannot distinguish the valuable plants from the poisonous plants. It is thus advisable to avoid introducing animals to a strange area during the time/season when poisonous plants are active.
Accident -an animal familiar with local fodder is also vulnerable to plant poisoning as it may ingest it by accident. This is when a poisonous plant has grown close to or blends in with a valuable plant and are ingested together. Also, 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 to allow 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 when the other valuable plants dominate.
In the event of a 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 and not stressed. These are ways to limit or slow the circulation of poison through the entire body.
Additionally, there are remedies that are used to neutralise the poison in animals’ bodies. These include o Hypo, a crystal-like content that should be mixed with water. It can be added to the water in the drinking holder, and all animals can drink as they arrive from the veld.
Charcoal, which can either be activated charcoal sold commercially, or the normal charcoal from the fire.
The normal charcoal is the easiest home-based remedy to use. It should be well-grinded into a fine texture and mixed with water, and given orally to the poisoned animal. These remedies and others are readily available in agriculture 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 and to assist with the identification of these plants. Such books include “Toxic Plants of Veterinary Importance in Namibia”, and “Grasses of Namibia”, amongst others.
-Erastus Ngaruka is Agribank Technical Advisor: Livestock & Rangeland