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Eating Toxic Plants as Part of an All-Round Diet

How to help prevent toxic poisoning of your browsers [and grazers]

By D. de Treville.  Forthcoming in:  JOURNAL OF THE WORKING GOAT

Recent discussions with North American goat owners about shinnery oak – and other potentially toxic plants – led me to look back at the data – and experience – we have here in Africa on toxicity as well as on foraging behavior in general.  These two topics are central to livestock husbandry in Africa, because of the wide variety of indigenous plants consumed by goats, sheep and cattle about which little is know [i.e., their levels of toxicity and/or nutritive value vis-ΰ-vis different breeds, different age and condition of the animal, different ages of the plant, etc.].  And while we do have various plants out in the common lands where our herds browse/graze [supervised by herders who let them select plants they choose], we've had only one possible toxic reaction in the last 5 years.

Bucklings Browsing in Brush
Single-species browsing, as shown here, can be dangerous if the plant species has some level of toxicity and no alternative forage is available.

To begin with, where grazers/browsers are in areas with a wide variety of plants, they seem rarely eat enough of a plant with toxins to either kill themselves or suffer debilitating effects.  This is where keeping animals in small enclosures or on degraded and/or largely monoculture rangelands can bring trouble – when animals no longer have a choice.  And unfortunately, as land becomes degraded, toxic plants – such as locoweed or shinnery oak in North America – may predominate.  When this happens and livestock are forced to eat what otherwise they would avoid or eat only a few bites of, toxicologists have an old saying, something like:  'the toxin is in the amount eaten.'  And think about it.  If you've eaten too much chocolate, drunk too much red wine or tea, or indulged in a several helpings of an eggplant dish [all containing different kinds of tannins] you may feel 'off' – or possibly downright ill.  But eating a little of those and other toxin-containing foods can be mighty enjoyable and sometimes even good for you.

 

As well, an interesting question is, 'WHY are plants toxic?'  It's to protect themselves from both foragers and insects; an adaptive strategy.  BUT toxin-containing plants do continue to be successfully eaten by many animals.

 

So what's going on?  In free choice foraging, how do animals 'know' whether – and how much – of a toxin-containing plant to eat?  Is the behavior learned or inherited?  If learned, how is it learned?  If inherited, how?  The answers are multiple, not always clear-cut, hotly debated in many cases, and pretty fascinating.  Here are just a few high points to consider:

  • Animals can inherit some dietary preferences, such as juniper plants eaten by goats – an inheritance that has been found to be upwards of 30% in certain freerange goats in the Southwest [USA].  Meaning, if you use a buck from goats with such a preference, its kids stand to inherit a juniper preference, which is great if you have a lot of juniper you want to get rid of.  There are other heritable preferences, as well.
  • However, it appears that avoidance of toxic plants is not inherited, or at least the evidence is at yet not clear.  Many toxic plants have a bitter taste (such as eggplant), and certainly in the livestock world, most foragers seem to prefer sweet over bitter plants – but this is not always the case.  Furthermore, goats sometimes prefer bitter over sweet plants.  So, bitterness cannot be taken as an indicator of toxicity.
  • Learned avoidance – or preference – of particular plants does clearly take place and in several different ways.  Here are some of those ways:

    ΨThere is evidence that a fetus may acquire the taste of a particular plant eaten by its mother, since the taste may pass through the amniotic fluid. 

    Ψ Infants can learn from watching what their mothers' eat or avoid or what the herd eats or avoids

    Ψ It has been reported that herd members can also  learn from the herd queen, who may pointedly avoid potentially toxic plants through past experience

    Ψ Digestive feedback can be another method.  This is proposed to take place when - through the cud or through malaise a few hours after eating a plant - the animal falls sick or feels unwell or tastes 'badness' in the cud.  Studies suggest that herds will eat up to a certain portion of a given plant – then avoid it for a few days – then go back, again.  They are preventing the level of toxicity from interfering with eating a basically nutritious plant.  But again, just HOW they 'know' how much to take is the question that's still not well understood. Many plant species fall into this category – various leguminous species, shinnery oak, and others.  Really good nutrition for goats – but generally no more than 25% of the total diet – on a full-time basis – should be consumed.  As well, shinnery oak is more toxic the first 30 days of new foliage – and less should be eaten during that time. 

    Ψ Thinking of these toxic, but beneficial species as 'grain' rather than as plants can be a useful analogy:  grain is just fine – but in limited quantities.  Exactly is the same for a variety of quite nutritious plant species that have some level of toxicity.  Excellent in small amounts; can kill if over-fed.

    Ψ To repeat: when placed or tied in a confined area with little choice, animals will be forced to eat more than they normally might of a toxic plant.  Hence, consuming too much shinnery oak or Leucaena if there's nothing else available are two examples.

    Ψ As well, various toxin-containing  plant species don't outright 'poison' an animal, but can impact to varying degrees on the animal's overall growth, reproduction, or other performance indicators.  For example, some plant species contain a kind of tannin that binds to proteins so that they cannot be used by the animal.  Growth stunting, hoof and hair abnormalities, intestinal problems, or other detrimental effects can occur.  But these detrimental effects generally occur with continued and excessive eating.

    Ψ When the animal over-consumes such tannin-containing plants, the kidney has to over-work trying to neutralize the toxin so it won't kill the animal, and in the process can itself become diseased.  That's why, if you see bloody urine or blood diarrhea, your animal's kidneys and intestines already have serious lesions and it may be too late to treat.  (Not all tannins act in this way; tannins are as complicated as 'toxicity' and not a topic to be taken up here.)

    Ψ As for the 'happier' side of toxins, there are a few plants that are addictive – such as locoweed that you have in the States– or our marula tree that grows in various parts of Africa.  This is quite an interesting tree, because of the differences within the tree parts:

     

When the marula gets around to shedding its fruits once a year EVERYONE comes to have an extended cocktail party!  Elephants, chimpanzees, various birds – and whatever gazelle-type creatures might be about.  I've not seen this myself, but I have seen a video showing a variety of these creatures in the same forest weaving about after over-consumption and many collapsing – only to awake [to a massive headache?] and eventually scarping more down the hatch.  The fruit contains nicotic acid, and it is the nicotic acid that goes to work, as the fruit ferments in the animal's stomach.  Fortunately, marula fruit is only in season once a year. 

But the nuts are highly nutritious – being 54-70% protein – and widely eaten by goats and wild animals.  However, the leaves of the marula are toxic if eaten long-term – but during droughts will be gratefully eaten by animals and lopped by farmers for their stock.  Short periods – seasonally – of consuming a toxic plant is often not detrimental, and in fact can save the animal's life.  By way of byproducts, the fruits are made into a delicious jam, and in South Africa there is a lovely apιritif made from the fruit, called 'Amarula' – similar in taste to Baileys.  So, truly a multi-purpose tree:  intoxicating fruits; nutritious nuts; potentially toxic leaves; enjoyable aperitif …

 

Back to the topic at hand ….

    Ψ Here where we are in central Africa, goats, sheep and cattle roam the common lands, followed by their herders.  There are a multitude of plant species, and you might be surprised at the variety of grasses, forbs and shrubs that 'even' cattle and sheep eat, when presented with them.  This observation holds for other areas of Africa and the Middle East where I have traveled or worked.

BUT the bottom line is, do think 'variety' – use well-planned enclosed areas with a lot of choice, and your goats are not likely to poison themselves.  For goats, think shrubs and weeds – not just grasses. And don't forget: goats like their occasional, after-dinner chocolate and Amarula apιritif, too!

 

Summary

 

In this article I have been talking about goats and other browsers/grazers who are foraging on largely  undegraded and multi-species freerange - not small pastures with the odd plant, tree, weed that has toxic properties or large, degraded areas.  Nor am I talking about drought periods, when stock may be forced to eat a species of plant that is toxic but that they  otherwise might avoid.

 If you have plants in a small acreage that have some level of toxicity, and your animals have no other source browse, then you may be asking for trouble.  Or, if you have your animals on rangelands where a few plant species predominate and one or several are toxic - you may also be asking for trouble. Remember, too, that multiple-species stocking has been shown to reduce toxic reactions in North American rangelands.

The smaller your area or the more an area is propagated by only one or two plant and/or animal species, the more important it is to be knowledgeable of plant species in that area, and their relative toxicity.

To get back to shinnery oak in the Southwest of the States – where there are shinnery-dominated rangelands, and animals have no other or few feeding options, poisoning can take place and indeed has cost cattle producers in Texas millions of dollars.

The problem has more to do with poor rangelands management than with the plant species itself:  overstocking and/or using degraded lands in which toxic plants predominate creates the problem.  Or, confining the animal to a small area where toxic species grow.  Animals will generally not consume toxic levels of plants if presented with adequate and diverse forage. 

For example, I am confident that if I were to plant azalea bushes – which are toxic to goats – out in our commonlands along the shores of Lake Tanganyika in Burundi, our goats would take a few nibbles and - in their goat-wise manner - move on to a few more nibbles of another plant.  That's what they do with other toxic species in the common lands - that's how they've evolved to behave.

 

Goats, sheep, cattle share common lands

In most areas of Africa, goats, sheep & cattle graze/browse a wide mixture of grasses, forbs, brush and trees thereby reducing the possibility of toxic reactions to one plant species.

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