Poisons and Anti-Nutritive Factors

In addition to the nutrients that plants may provide to an animal, many of them also contain anti-nutritive factors or poisons that are designed - by the plant - to protect the plant from being eaten. These may reduce the palatability of the plant (making an animal less likely to eat it), impair the plant’s digestibility, or cause disease in the animal when it eats it.

Mycotoxins Plant Toxins Glucosinolates Trypsin Inhibitors Gossypol Oxalates Saponins Tannins References and Further Reading Research Projects

In addition to any anti-nutritive factors produced the plant, a feed may also contain contaminants that are harmful to the animal. The most usual contaminant affecting animal feeds in the tropics are fungal contaminants. Storage of feed in warm, damp condition encourages the growth of fungi on the feed. The fungal spores produced may in themselves be harmful, but in addition many fungi produce toxins (mycotoxins) that can be extremely harmful both to the animal eating the feed and to the people involved in handling it.

Some of the anti-nutritive factors can be reduced, removed or destroyed by treating the feed in some way before presenting it to the animal. Other anti-nutritive factors are not so easily degraded, and their presence either limits the amount of a particular feed that can be included in the animal’s diet, or prevents some potential feedstuffs from being used for animal feed at all.

Moulds may grow on feed (either in the standing crop or, more usually, during storage). The fungal spores that are produced when the mould reproduces may be ingested or inhaled by the animal and this may itself be harmful. Many fungi, however, also produce secondary compounds called mycotoxins. These are produced to assist the fungi in its competition with bacteria and other fungi for food and space. Some of these secondary compounds are antibiotics, and this can be beneficial to the animal. However, many of these secondary compounds are harmful to the animal as well (and to humans too). The toxicity of the mycotoxins depends on the actual mycotoxin (which will depend on the species of fungus that produced it). Different species of animal are susceptible to different mycotoxins to different degrees as well. For example, aflatoxins that may flourish on badly stored feed (such as groundnuts) are highly toxic to turkeys and ducklings, less toxic to cattle, while sheep appear to be resistant to them. Young animals are also more susceptible than adults.

Mycotoxins from different sources can can attack different parts of the body, including:

• the blood system (e.g. haemorrhaging into body tissues or from the lungs),
• the digestive system (e.g. diarrhoea, vomiting, intestinal haemorrhage),
• the respiratory system (e.g. bleeding from lungs),
• the nervous system (e.g. lack of coordination),
• the skin (e.g. rash, photosensitization),
• the urinary system,
• the reproductive system (e.g. infertility and changes to reproductive cycles),
• and may also cause suppression of the immune system.

Ensuring that feed is stored in clean, dry conditions with an adequate airflow should help prevent the development of moulds. If feed has gone mouldy, it should not be fed to livestock and care should be taken when disposing of it as the spores and toxins are often harmful to humans as well.

Mycotoxins of world-wide Importance include: the Aflatoxins, the Trichothecenes, Zearalenone, the Fumonisins, Ochratoxin A, and Patulin. Additional mycotoxins may be of regional importance, especially in stored crops.

For more on Mycotoxins see "Contaminants and toxins in animal feeds" by D’Mello, in FAO (2004)

Plant Toxins

Plants have evolved a range of different defences to prevent themselves being preyed on by bacteria, insects and to a lesser extent larger animals and birds. However, some of the defences that might have originally been evolved to prevent attack by insects are also harmful to larger animals such as livestock. A plant has one of two defence mechanisms, physical or chemical. Physical defences include thorns, spines, leaf hairs and highly lignified tissue. Chemical defences include a range of toxins, which are specific to different genera or species of plants. Susceptibility of animals to these toxins depends on the species and also on the age of the animal.

In general, grasses do not produce toxins and rely instead on rapid regrowth to overcome the effect of frequent grazing. However, grasses may be contaminated with a range of mycotoxins. Sorghum does produce a cyanogenic glucoside and many tropical grasses produce oxalates and saponins that cause bone demineralization and photosensitization respectively.

Some of the toxins can be rendered harmless by treating the feed before offering it to livestock. Others may be managed by controlling the amount of a particular feed included in livestock diets. There is often considerable indigenous knowledge on the detrimental effects of feeding different locally available feedstuffs to different classes of livestock, and this knowledge should be explored and used when advising on potential alternative diets for small livestock.

Plants also produce a number of secondary compounds, either as protection from predation or to contribute to the plant’s physical structure. Among these compounds are tannins. There is a whole range of different tannins produced by different plants. Some are harmful, while others have some benefits for livestock feeding. These include reducing the degradability of protein in the rumen, which may be useful for ensuring that some digestible, high quality protein is supplied to ruminant livestock without it being degraded to ammonia in the rumen by the rumen micro-organisms (provided such protein is included in the diet).

An area that has received a great deal of attention is the use of the anthelmintic properties of some tannins, as a means of controlling internal parasite infection in livestock. For more information, go to the following pages, which summarize the results of a number of recent research projects:

	Helping lambs to control stomach worms before they get sick
	Feeding tree fodder to beat liver fluke
	Farmers can use Tannins to kill parasitic worms
	Improved Productivity in Goats: Enhancing the performance of goats through feed supplementation and deworming by use of locally available products
	Deworming for improving the productivity in goats
	Sensible use of drugs and feeding to stop roundworms killing sheep & goats

Plant toxins include the following categories of compounds:

Glucosinolates

Glucosinolates (or thioglucosides) are found in plants of the Brassicacaea (Cruciferae) family, such as mustard. In themselves, they are inert but they are rapidly broken down to compounds that interfere with the thyroid gland, causing goitre. They can also damage the liver and kidney.

Glucosinolates do not appear to cause much damage to sheep and goats, but their bitter taste is likely to reduce intake of mustard seeds and cakes. However, only limited amounts can be fed to pigs and poultry because these animals are much more susceptible. Most of the glucosinolates can be removed by soaking the cake in water (and then straining off and throwing away the water). Boiling the feed will also denature the glucosinolates. However, the breakdown products of the glucosinolates from this treatment are at least as unpalatable as the glucosinolates themselves, and so animals offered feed treated in this way are likely to refuse it or greatly reduce their intake. The reduced feed intake can be more of a problem than the goitre in some cases when mustard cakes or meals are fed to pigs.

Trypsin and other protease inhibitors are found in many legume seeds such as soya bean (Glycine max), pigeon pea (Cajanus cajan) and velvet bean (Mucuna spp.). When fed to pigs and poultry, they reduce protein digestibility by inhibiting the action of the animal's protein digesting enzymes. The animal tries to compensate for this by producing more of the enzymes, but as the enzymes are rich in limiting, sulphur amino acids (cystine and methionine), a deficiency of these amino acids quickly develops and the animal's growth is severely retarded. Trypsin inhibitors are usually denatured in the rumen by the rumen micro-organisms and so are not as much of a problem when feeding sheep and goats.

The trypsin inhibitors do not develop until the seed matures, and so feeding soyabeans and soyabean forage when it is at the "milk stage" does not cause any problems. The inhibitors are also destroyed by heat, so cooking the seeds (or the meals) before feeding them to pigs and poultry also overcomes the problem. However, the heat treatment needs to be harsh enough to destroy the trypsin inhibitor (124 °C for 15 min in the case of pigeon pea), but mild enough so that the protein in the seed or meal is not destroyed.

Gossypol

Gossypol is a yellow pigment found in cotton seeds, which is toxic to pigs, poultry and rabbits. The micro-organisms in the rumen of ruminant animals (sheep and goats) appear to neutralize the gossypol as ruminant animals do not show ill-effects even when they eat large quantities of cottonseed meal. However, in pigs and poultry, loss of appetite and body weight, laboured breathing and an irregular heartbeat are typical signs of gossypol toxicity. Death results from reduced oxygen-carrying capacity of the blood, breakdown of red blood cells and heart failure. On post mortem examination, there is usually extensive swelling in the body cavities. Acute toxicity is uncommon, but prolonged consumption of small amounts over a long time can be lethal.

Cottonseed and cottonseed meal can be made safer to eat by pigs and poultry if it is mildly heat treated (achieved by using a screw press in the removal of oil), or by supplementing the diet with iron (II) (ferrous) sulphate (1 to 4 parts to 1 part gossypol). However, care is still needed when feeding cotton to laying hens as even small doses will cause a green discoloration to the yolk during storage.

Oxalates

Oxalates are salts of oxalic acid that bind to calcium in the gut and reduce the absorption and therefore the availability of calcium to the animal. They may also form extremely painful stones in the urinary tract when the animal tries to excrete them. Pigs, poultry and rabbits are particularly susceptible, and a diet containing 20g/kg oxalate can be lethal to chickens.

Oxalates are broken down by the rumen micro-organisms and so feeds containing oxalates are less of a problem for sheep and goats. However, even with sheep and goats, if oxalates are eaten in large quantities they will cause digestive disturbances, and even kidney failure and death.

Sheep and goats appear to be able to adapt to diets with relatively high oxalate contents because of the ability of the microbial population of the rumen to adapt to breaking down oxalates. However, if sheep and goats are not generally fed diets containing oxalates, and are then taken to browse or graze an area with plants rich in oxalates, or are suddenly provided with feed and forages with a high oxalate content, then the oxalates will prove toxic to them. If oxalate containing feeds are going to be introduced to sheep and goats, it is important that this is done very gradually over a number of days to give them time to adapt to the oxalates.

Plants with high concentrations of oxalate include rice (found in the straw), the leaves of Acacia spp., and grasses such as buffel grass (Cenchrus ciliaris), elephant grass (Panicum spp.), kikuyu grass (Pennisetum clandestinum) and setaria grass (Setaria sphacelata).

Soaking the feed in water and then straining off the water will reduce the oxalate content of the feed and help to reduce its toxicity.

Saponins are compounds which break down the membranes of red blood cells. They are produced by a range of different plants and are designed to combat plant pathogens. In some cases the saponins occur as pre-formed compounds in the plant tissues. In other cases they are rapidly produced from inactive precursors when the plant tissues are damaged, e.g. when leaves are browsed and eaten.

Saponins are widely distributed throughout the plant kingdom (at least 400 species from more than 80 different families), although they are not usually a problem in tropical legumes apart from soya. They are found in different parts of the plant, such as the leaves, stems, roots, seeds, flowers and fruit. They have a bitter taste - and so cause a reduction in feed intake. There is a wide range of different saponins, and some of them are more toxic than others. They are particularly toxic to pigs, but in common with many anti-nutritional factors they can be tolerated by adult ruminant animals. Indeed, they can increase the efficiency of the rumen fermentation by reducing the amount of methane produced and increasing the supply of microbial protein to the animal. This can then result in increased growth and, potentially, increased resistance to disease.

The toxicity from saponins can be reduced by repeatedly soaking the feed in water. This removes the saponin, and increases the palatability of the feed by reducing its bitter taste.

Bhat, R.V. and Vasanthi, S. (2003). Mycotoxin Food Safety Risk in Developing Countries. Focus 10, Brief 3, September 2003, International Food Policy Research Institute, Washington.

D'Mello, J.P.F. (1992). Nutritional potentialities of fodder trees and fodder shrubs as protein sources in monogastric nutrition. In: Speedy, A. and Pugliese, P. (eds.). (1992). Legume trees and other fodder trees as protein sources for livestock. FAO Animal Production and Health Paper 102. FAO, Rome.

FAO (1998). Animal feeding and food safety. Report of an FAO Expert Consultation Rome, 10-14 March 1997. FAO Food and nutrition paper – 69. FAO, Rome.