Vet on Call: Human food that’s poison to cattle

Friday April 6 2018

Clara Chebet feeds hay to her dairy cows in her farm in Elburgon.

Clara Chebet feeds hay to her dairy cows in her farm in Elburgon. Most of the plant crops that are deemed palatable to humans are in essence poisonous to cattle. PHOTO | JOHN NJOROGE | NMG 

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Tomato is one of the most commonly farmed food crops in the country. Ripe tomatoes are greatly relished, especially in kachumbari that accompanies the mouth-watering roast meat (nyama choma) that Kenyans love.

Nonetheless, do you know that tomato stems, leaves and the green fruit are poisonous not only to humans but also to cattle and other domestic animals?

Kaloki from Machakos called me this week and shared that he fed his healthy pregnant dairy cow tomato stems soon after he had harvested them and the animal fell ill.

According to him, he thought the stems and few leaves were good for the cow. Therefore, he chopped the stalks and mixed them with other forage. The cow hungrily gobbled them up to Kaloki’s satisfaction.

But a few hours after eating the plant, the cow developed joint weakness and within two days it was unable to stand. It later gave birth but was never able to stand again.

The poor cow died after one week from the time it had ate the tomato stems.

Kaloki’s question was, “Are tomato stalks good for cows or was it another problem that killed my cow because the animal was very healthy before eating the stalks?” Before responding to his question, let us have a mental recall of the smell of tomato plants.

It is quite unpleasant especially when the plants are in their prime ready to start flowering.

As a small boy, I could not stand the smell when we started growing tomatoes. I would touch the plant when weeding and later thoroughly scrub my hands with a piece of rough sisal sack to eliminate the smell before using my hand to eat.

The bad tomato smell is part of the weaponry the plant employs to prevent itself from being eaten by animals. For sure, cows do not readily eat tomato plants. To add on to the smell, the tomato plant packs in a natural toxin called solanine and nitrate. Solanine is in the group of natural poisons known as steroidal glycoalkaloids.

The tomato plant belongs to a family of plants called the solanaceae or night shade. Other members of the family grown as food crops include the Irish potato, eggplant, bell pepper and capsicum. Most members of this family are known to contain the toxin solanine in the leaves, stem, tubers and green fruits but the solanine content in the fruit reduces to below toxic levels by the time the fruit ripens. Solanine is a bitter chemical and that is why green tomatoes are bitter.

The Irish potato, commonly grown for human food, has low levels of solanine in the tuber except when it has been exposed to a lot of sunlight and it turns green. Green potatoes should not be eaten as they are bitter and poisonous.


You will also notice that cattle are not very keen on eating Irish potato plants. Animals generally know poisonous plants because most have a bad smell.

However, in times when forage is scarce or if the poisonous plants are fed with palatable forage, a cow will eat the poisonous plants as in Kaloki’s case.

Solanine poisoning is generally difficult to diagnose because it has symptoms similar to poisoning by other plant toxins.

In addition to Kaloki’s observation, poisoned cattle will also show excessive salivation, lack of appetite, drop in milk production, diarrhoea, depression and weakness.

Some cattle may show prostration, dilated pupils and slow heart rate. Death occurs if the cow has eaten large quantities of the toxic material.

Animals that have eaten a small portion of the toxic plant mostly recover after a few days of showing mild signs of poisoning. Signs of nitrate poisoning are also seen in animals that have eaten tomato stems and leaves.

In the course of my work, I have seen cattle, sheep, goats and even dogs poisoned by plant materials that we regularly use as human food.

The fact that people eat the plants without any problem confuses animal keepers and they assume the plants are also safe for animals.

What most people forget is that humans normally will not eat the same part of the plant as animals. Further, human food is mostly cooked and this denatures most of the chemicals thereby detoxifying the food. Just as in people, ripe tomatoes are safe for cattle.

Other plants we grow as food for humans but may poison cattle include cabbage or any plant in the brassica family, onions, amaranthus and beans. Unfortunately, some of the plants like amaranthus, brassica and beans are also appetising to cattle and cows can consume them in large quantities.

Beans are well-known for causing bloat and diarrhoea in cattle and other ruminants. Raw beans contain saponins that cause bloating in the rumen.

The froth traps air, causing distention of the rumen, which exerts pressure on the lungs and causes difficulties in breathing and blockage of blood circulation. If not treated promptly, the cattle die of lack of oxygen.


The brassica family is made up of crops such as cabbage, spinach and sukuma wiki (collard greens). Others are broccoli and cauliflower.

The main toxin in these plants are a group of chemicals called glycosinolates, which interfere with the utilisation of iodine in ruminants and cause reduced growth rate and milk yield. They may also cause thyroid gland problems such as goitre especially in sheep.

The brassica plants also have a toxin that is transformed by rumen micro-organisms into another toxin that once absorbed into the blood, causes the bursting of red blood cells.

This is called haemolysis. Affected animals will show a condition known as haemolytic anaemia meaning blood loss due to bursting of red blood cells. Such animals void reddish brown urine and have pale mucous membranes.

Feeding excessive amount of cabbage may cause a highly acidic rumen environment, almost similar to feeding too much carbohydrate. This is because cabbage has a lot of highly digestible sugars.

In February this year, a charitable institution that keeps a few dairy cattle reported their only milking cow had gone down and could not wake up.

I was shocked by the amount of cabbage the cow had been fed on for the past two weeks. This had caused chronic malnutrition and poisoning making the cow recumbent. The animal never recovered.

In addition to toxins, cabbage has a high content of calcium. If pregnant cattle are fed large quantities of cabbage, then they accumulate calcium and they may fail to maintain the blood-bone calcium balance once they give birth, resulting into milk fever attacks.

Consumption of onions causes haemolytic anaemia too. The plant contains the chemical S-methylcysteine sulfoxacide (SMCO).

In the rumen, SMCO is converted to a chemical that acts in the same way as brassica poisoning. Affected animals will show reddish brown urine and breathing difficulties due to loss of blood.

Amaranthus, commonly known as pig weed, contains a lot of nitrate. In the rumen, the nitrate is converted to nitrite.

Once the nitrite is absorbed, it binds to the blood carrying molecule haemoglobin in the blood and blocks the uptake of oxygen. The animal dies quickly due to lack of oxygen in the blood.

For sure, all that glitters is not gold. Consult your veterinary doctor before choosing what to feed your animals.