Selenium - poison or miracle?


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Is selenium a poison, or a miracle benefit? The answer depends on your use of it - too much or too little is a problem.

Selenium (Se) was deemed an essential nutrient only in 1957, but was discovered in 1817. Before that, explorer Marco Polo referred in his journals to "a poisonous plant ... which if eaten by (horses) has the effect of causing the hoofs of the animal to drop off." What Marco Polo found in the 13th century is now thought to be a plant known as a selenium accumulator, which stores toxic amounts of selenium.

Such plants are not thought to exist in New Zealand, but are prevalent in the range areas of the north-central United States, and one, Morinda reticulata, is reported in some areas of Queensland.

Scientists at the University of Wyoming discovered that most plants - even those on soils containing selenium - contained 10 parts per million or less of selenium, whereas a selenium level of 14,990 ppm was found in a sample of Astragalus racemosus, which is a selenium accumulator.

In New Zealand, toxicity is caused by overdosing, or by using more than one selenised product at a time. Symptoms of selenium toxicity include abnormal movement, dark watery diarrhoea, high temperature, weak and rapid pulse, laboured respiration, bloating and abdominal pain, pale and blue mucous membranes, and dilated pupils. Most animals with selenium toxicity are found dead, and there is no antidote for those found alive.

Selenium deficiency can lead to White Muscle Disease, which causes degeneration of the skeletal muscles, stiff gait, and other problems.

Veterinarians spoken to about selenium toxicity and deficiency were reluctant to give precise figures of the recommended levels of supplementation, because of area and farm variations in the levels. Some studies report that a third of New Zealand soils are selenium deficient.

A North Canterbury veterinarian, who did not wish to be named, said the level of supplementation needed depended on blood and liver levels. "It is not something that you can extrapolate to all farms - it is an individual farm and an individual animal problem, so you need to determine what the selenium status is before you start supplementing, and the status of the plants, as the selenium is coming up from the soil. You first need to determine that there is a deficiency, then go with a supplementation programme based on that."

He pointed to the latest NZ Veterinary Association update, which has reported animal deaths from selenium toxicity, caused by double dosing. The NZVA reported several pockets of deaths in younger and older animals.

Labnet Invermay's report for May 2002 ascertained that low selenium levels were diagnosed as causing ill-thrift in four groups of yearling cattle, and six herds were found through blood testing to have selenium deficiency.

The North Canterbrury veterinarian says three or four blood or liver samples can be used to determine the level or the herd or flock. "In dairy cattle we'll go to, say, a 400-cow herd and take three samples, and that will give us a good idea about the status of that herd."

Delivery methods of selenium depend on the management of the farm: "For animals that are handled daily, such as dairy cattle, selenium can be added to the drench so that they get a daily dose. The level will depend on the deficiency." In one case, a 1ml daily dose of a 0.5 per cent solution was given on one North Island farm.

Beef herds can be supplemented with slow release bullets in the rumen, which last from nine months to a year.

Injections can also be given. "Often selenium is combined with a Vitamin B12 injection. Selenised prils on the ground give a 12-month level of selenium, "but you've got to be careful not to use any other selenised product without consulting a vet," he said.

Farmers should also be aware that some drenches contain selenium, so care must be taken when using a selanised vitamin B12 injection in conjunction with drenching.

Horses and selenium are a hot topic after the recent case of a North Island racehorse trainer who was found to have several horses in his care with selenium toxicity. The horses have difficulty in moving, and in come cases their hooves appear to be coming off.

A Christchurch horse veterinarian, who also did not wish to be named, said that selenium was dispensed only to clients of the clinic, and in cases where the vets had seen the property and were aware of the situation of the horses. "There are basic guidelines for feeding selenium, but everyone's situation is different," he says.

"Racehorses who are in stables usually get specially balanced feeds with selenium in it. If the trainer is feeding only chaff and grain then they'd probably be supplementing with selenium, but if they're feeding a complete feed they would not be. It also depends on how much work the horse is doing and what situation they're in."

In cases where horses are turned out on grass, with no extra feed "some sort of supplementation is advisable, certainly in the Canterbury area, as it is renowned to be selenium deficient. But, again, it's an area thing. And some people may be on a farm and they know they're putting selenium on the pasture, so they don't necessarily need to be supplementing the horses."

He advised horses owners to talk to their vet for recommendations. "There are so many varied ways in which people feed their horses, that you can't give a general guideline. The situation would be different for a lifestyle block compared to a high-country farm, where they're putting a lot of selenium on their pasture, whereas on a lifestyle block maybe they don't. These different properties have all got different aspects that need to be looked at."

Recommended nutritional requirements:

* Dairy cattle 0.3 mg/kg
* Beef cattle 0.2 mg/kg.
* Sheep 0.10 - 0.20 mg/kg
* Pigs 0.10 - 0.30 mg/kg.
* Horses 0.10 mg/kg
* Poultry varies with age but 0.10 mg/kg for 6 weeks old +​