As the days lengthen and temperatures rise, the signs of winter's end become more apparent. In certain regions of South Africa, this transition is marked by the emergence of lush greenery and abundant grazing opportunities. However, in other parts—particularly the summer rainfall areas—scarcity prevails. During this time, humans, animals, and the land itself eagerly anticipate the relief brought by rolling clouds and thunder, a respite from months of drought, dust, and hardship. It is during this time, when the pale brown veld has little to offer in terms of nutrition, that livestock impatiently search for any form of green grazing the veld has to offer. Regrettably, in their pursuit, animals often unknowingly consume the earliest growth after droughts: poisonous plants. Both native and non-native poisonous plants thrive in conditions of drought and would be some of the first to come up at the onset of spring, and despite the ability of livestock to generally discern and avoid poisonous plants, the scarcity of sustenance during these times makes these plants an enticing option.

 

Poisonous plants pose one of the most formidable threats to the financial viability of livestock farmers, giving rise to both direct and indirect losses within their operations. The economic repercussions of plant poisonings are staggering—stemming from the tragic loss of animals and extending to diminished productivity and reproductive setbacks, such as miscarriages, congenital anomalies, and infertility. These consequences have far-reaching implications, impacting not only the present but also the future of a farmer's livelihood. The financial strain is furthermore not just limited solely to animal losses. The expenses linked to treatment and control strategies, such as the installation of protective fencing, supplemental feeding, and increased labour demands, amplify the already substantial financial burdens faced by livestock farmers. The culmination of these factors creates a daunting challenge, underscoring the urgent need for proactive measures to mitigate the risks posed by poisonous plants.

 

 

Roughly 600 indigenous species of poisonous plants are found across South Africa, several of which can cause mortalities among livestock. Many invasive poisonous species, however, also occur and are of significance when talking about livestock poisoning. Various parts of these plants – including their leaves, pods or seeds – may be toxic. A study by Kellerman et al. in 1996 pinpointed the seven most significant instances of plant poisoning in South Africa affecting both cattle and sheep. Remarkably, four of these instances were shared between the two species, while two cases were distinct to either cattle or sheep. These are summarised in the tables below.

 

Plant Toxicity Syndromes of Large and Small Stock

 

Ingestion of the above-mentioned plants can have severe and permanent effects on various bodily systems and functions. Some toxins might affect a single organ, such as the heart or the liver, while other will target more than one organ – all depending on the type of plant and the type of toxin it produces.

 

 

Cardiac glycoside poisoning, gousiekte and gifblaar poisoning are all caused by plants that produce toxins affecting the functioning of the heart. This accounts for roughly a third of cattle mortalities arising from plant poisonings, and around 10% of those in small stock. The damage caused to the heart can result in sudden death when animals are chased or if they run towards feed or water. Affected animals tend to stand with a lowered head and stomach tucked in while often groaning or grinding its teeth. An increased heart rate, bloat, diarrhoea, and weakness of the hind limbs are also commonly seen in poisoned animals. Autopsies on affected livestock often reveal an enlarged or pale heart and fluid in the chest cavity, froth in the windpipe, stomach ulcers and swollen lungs filed with fluid. 

 

 

Vermeersiekte is caused by plant toxins that affect the gastro-intestinal system. Poisoning from these plants result in degeneration and necrosis of striated muscle, particularly of the skeletal and oesophageal musculature. This result is a reduction of peristaltic activity of the oesophagus and ruminal stasis – which subsequently interferes with normal eructation and regurgitation. What follows is the chronic vomiting of rumen content, often accompanied by a watery, green diarrhoea. These symptoms are more often seen in sheep, whereas goats are more frequently seen to develop muscle stiffness and tremors. A small number of animals also develop bloat from this disease and can die as a result thereof. Choking due to regular vomiting is also common, either resulting in instant death or causing pneumonia, which could eventually lead to death. 

 

Geeldikkop occurs when sheep and goats consume a toxic form of Devil’s Thorn (duwweltjie), that then greatly affects the liver. When consumed, the steroidal saponins found in the plant are hydrolysed (the chemical breakdown of a compound due to reaction with water), hydrogenated (charged or combined with hydrogen), reduced (gaining electrons) and epimerized (converted from one epimeric form to another) in the rumen. Among others, two components - epismilagenin and episarsasapogenin glucuronides – are formed. These bind to calcium ions (Ca²⁺) to form an insoluble salt which then precipitates to form biliary crystals, leading to obstruction of the bile ducts in the liver. This obstruction results in jaundice and severe photosensitivity. Animals will seek out shade and rub their swollen heads against objects due to itching, mucous membranes around the eyes and mouth will appear yellow and difficulty eating and breathing due to severe swelling might also occur.

Other toxicity syndromes affecting the liver are Seneciosis and Lantana poisoning. Senecio plants contain large amounts of active alkaloids, a water dissolvable liver toxin that is easily absorbed and causes severe damage to the liver. Consuming large amounts in a short period often results in acute deaths, often with no precursing symptoms. When small amounts are consumed over an extended period, long term toxicity occurs. These animals will steadily lose body condition, appear lethargic and have decreased wool production. Lantana poisoning occurs due to the production of pentacyclic and triterpene acids which alters the permeability of liver cell membranes and leads to intrahepatic cholestasis, with possibly some damage to the liver cells. These animals develop rumen stasis, constipation or diarrhoea and a yellowish pigmentation of the sclera. 

 

 

Diplodiosis is a phytopathogenic fungus (Stenocarpella maydis) that causes stalk rot and cob rot in maize. This occurs mainly in areas where drought conditions prevailed in the early parts of the production season, followed by heavy rains during the later part of the season. When livestock feed on the diplodia-infected maize, the central nervous system is affected, resulting in neurotoxicosis. Symptoms may include restlessness, sensitivity to sounds and touch, difficulty in walking, muscle tremors, staggering or stumbling, pushing against objects, stargazing, blindness, convulsions or paralysis.

 

Considering the economic impact that plant poisonings pose to livestock farmers, it is important to know exactly why and how these poisonings come to be. There are a number of factors that contribute to the likelihood of livestock being poisoned; as mentioned before, poisonous plants are some of the first greenery to appear after winter or after a veld fire, and in their young stage often appear extremely attractive to stock. Some poisonous plants may be harvested as weeds along with fodder sources (such as hay and silage) and then fed when grazing is scarce, while rough winds can knock the pods, fruits or seeds of other poisonous plants to the ground, making them available to livestock. Furthermore, animals that are less selective, such as hungry, thirsty or pregnant animals, might not avoid poisonous plants or even seek them out, resulting in a higher intake thereof. Another factor is animal age; young and old animals are much more susceptible to poisoning, as their livers do not have the capacity to properly eliminate the toxins. Lastly, areas that have been fertilised will have plants with increased toxicity, which could exacerbate the effects of the plant toxins on livestock. 

 

In order to prevent or limit the incidence of poisonings, it is crucial to know which poisonous plants occur in a specific region and which of those are poisonous to cattle, goats and sheep. If possible, animals should be kept away from areas with poisonous plants, however this is not always a practical solution. Eradicating known poisonous plants will be very beneficial, though not necessarily possible. Keeping animals in good condition – supplementing with feed and licks during the dry season – and ensuring they have sufficient drinking water will limit the animals need to graze on dangerous vegetation.  Also make sure to take precaution when introducing from other areas as they will be unfamiliar with the poisonous plants in their new surroundings. When harvesting fodder and hay, make sure not to accidentally incorporate toxic plants, and never feed livestock mouldy feed. 

 

Unfortunately, the available treatment options for plant poisonings are limited in number, and any potential treatment would need to be tailored precisely to the specific poisoning incident. While some animals do manage to recuperate following exposure to plant toxins, the consequences thereof can be lifelong, potentially impacting their growth and restricting overall productivity. As we strive to safeguard livestock and ensure their well-being, it becomes imperative to further research, educate, and implement preventive measures to minimize the far-reaching effects of plant poisonings on animal health and agricultural sustainability.