One of the greatest risks to livestock in South Africa is plant toxicity. This can result in poor condition, poor production (for example, low milk yield), reproductive failure (abortion, stillbirths, birth defects and failure to conceive), and even death. Farmers can suffer further financial losses through the cost of treatment, losing the use of veld due to toxic plants, and a drop in the value of the infested land.
Animals usually avoid toxic plants, but poisoning can occur when they are introduced to a new area or there is a shortage of feed and they are forced to eat what is available.
The dangers
Toxic plants are usually among the first green plants to sprout after the dry season or a veld fire. A number of these are at their most toxic in the young stage when they are most attractive to stock. Similarly, some are highly resistant to drought and may be the only green plants available during drought.
Where and what to look out for:
- Toxic plants are often found as weeds in harvested lands and along the roadside (areas frequently used for grazing in times of scarcity).
- Certain poisonings occur after a sudden change in the weather, usually after an unseasonable frost or when wet, cool conditions are suddenly followed by a warm, dry spell.
- Wind or hail can knock poisonous acorns or pods to the ground, making them available to animals.
- The use of fertiliser may increase the toxicity of some plants.
- Fodder such as hay, silage, stover or concentrates may contain toxic plants, fungi or chemicals.
How animals can be affected
When livestock have been kept in an area for some time, they are often familiar with the toxic plants and will not eat them unless forced to do so. Animals that are moved from familiar areas to new areas tend to graze less selectively and are more likely to get poisoned. Other factors to bear in mind:
- Different species of animals are affected by different types of toxic plants and to varying degrees. Exotic breeds of livestock tend to graze less selectively, are more susceptible and are poisoned more frequently than indigenous breeds.
- Some plants affect males and females in different ways.
- Young and older animals are more susceptible. Their livers do not have the capacity to eliminate the toxins and young animals also have not yet learnt to avoid toxic plants.
- Hungry animals graze more greedily and are less selective and therefore more likely to be poisoned. This can occur during drought, after veld fires or when the veld is overgrazed.
- Thirsty animals look for plants with a high moisture content, grazing on those which they would normally avoid. Some of these plants may be toxic.
- Pregnant animals tend to be less selective and eat more than normal; they may therefore be poisoned.
- Animals in poor condition are at a greater risk of poisoning than animals in good condition.
- The skin colour of animals will determine the extent to which certain toxic plants will affect them. For example, in the case of photosensitivity (poisoning that results in damage due to sunburn), unpigmented white areas on the skin may become red and swollen.
In the next issue, we shall discuss symptoms of plant poisoning.
Source: ‘Poisonous plants’, Animal Health for Developing Farmers Programme, ARC-Onderstepoort Veterinary Institute.