The Strange Case of the Cyanide Grass

In 2012, a pasture full of cows were killed by cyanide poison. But this was not the result of a malicious human act–the poison came from the grass they were eating. And the “killer cyanide grass” became an Internet sensation.

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Dead moo-cows led to an enduring Internet myth.

In June 2012, a Texas cattle rancher named Jerry Abel released a herd of 18 cattle onto a patch of pasture. At first, nothing seemed out of the ordinary. Then, the cows started bellowing, staggering around, and falling to the ground, convulsing. Within minutes, fifteen of them were dead. Necropsies on the dead cows revealed something startling and completely unexpected–they had all died from cyanide poisoning. But where had the poison come from?

It has long been known that many varieties of plants produce toxins as a method of protecting themselves from herbivores. Since plants cannot run away from animals that want to eat them, they must resort to chemical warfare, and manufacture an enormous variety of compounds in their tissues that discourage herbivores (especially insects) from chewing on them. These include chemicals that are extracted and used in some of our most common foods and medicines: peppermint, spearmint, digitalis, aspirin, caffeine, opiates, nicotine, vanilla–all of these are basically chemical insecticides, designed to sicken or kill bugs that try to eat the plant’s leaves, stems, or seeds. For larger herbivores such as deer or cattle or goats, the plants have more potent chemical weapons–such as urushiol (the irritant found in poison ivy), or oxalic acid.

One very common plant weapon is hydrogen cyanide. This is found most often in the sorghum family of plants, but is also common in other groups. The cyanide is manufactured and stored in the plant’s cells, where it is connected to a sugar molecule that makes it inert. When eaten, the sugar molecule is broken, releasing the poison. In most instances, the amount of toxin that is released is just enough to sicken the animal and make it stop eating the plants, but it is not unknown for animals (and humans) to sometimes get a fatal dose. Among the everyday plants that store cyanide precursors are wheat, barley, oats, rye, French beans, kidney beans, lima beans, apples, peaches, plums, apricots, cherries, almonds, Macadamia nuts, quince, papaya, passion fruit,  bamboo shoots, cassava root, and taro root. And grass.

Once it was determined that the cows in Texas had been killed by cyanide, attention immediately turned to the grass they had been eating. Rancher Abel’s pasture had been planted with a variety of grass seed known as Tifton-85. This is a variety of bermuda grass that was bred by researchers at the University of Georgia in 1990. Like virtually all agricultural plants, Tifton-85 is a hybrid. Hybridization, used for hundreds of years to produce new varieties of all sorts of plants, from roses to wheat, is simple “”cross-breeding”, in which two varieties of plant are bred together to move desirable characteristics from one breed to the other. In the case of Tifton-85, it was intended to produce a high-protein grass for use in cattle pastures that could withstand the heat and humidity of the southeastern United States. University researchers took an existing bermuda grass and crossed it with a closely-related variety of star grass. The resulting hybrid produced more grass per acre, had deeper root systems, and was more tolerant of southern climates. It became commercially available in the 1990s: Jerry Abel had been using it on his ranch for 15 years.

As it happens, several varieties of grass, including bermuda grass, star grass, sudan grass, rye grass and crab grass, naturally produce cyanide precursors in their tissues as defenses against herbivores. Under normal conditions, the amount is small and the dosage of cyanide is so tiny that it is harmless (just like in the almonds that we eat). However, it has long been known that when the plants suffer environmental stress, they  sometimes respond by increasing their production of poison precursors. And when the grasses in Jerry Abel’s ranch were examined, they turned out to have elevated levels of cyanide–as did other ranches in the area. Texas had, for the past two years, been undergoing a serious drought. It was concluded that the grasses, under the prolonged stress, had responded by manufacturing unusually high levels of cyanide precursors in their tissues, and when the cows ate the grass they unknowingly released the fatal toxin.

The dangers presented by potentially toxic cyanide in pasture grasses and other plants had long been known and discussed in both the scientific literature and in animal-management texts, and incidents like that at Jerry Abel’s ranch had already happened before. But Abel’s cows had dropped dead in the Age of the Internet, and as a result they became the center of a media sensation that still echoes years later.

On the day the cows died, a local CBS affiliate picked up the story and ran it on its website. Unfortunately, the initial story contained information that was simply not true. The Tifton-85 hybrid was erroneously identified as a “genetically-modified organism” or GMO. Since GMOs are the subject of much fear and hysteria amongst a certain segment of people, websites all over the Internet began screaming the “Poisonous GMO Plants Kill Cows!” story, many of which added details that were simply wrong, such as that the cows had been killed by a cloud of “poison gas” released by the plants, and that humans were in potential danger from cyanide-releasing grasses on golf courses or lawns. None of this was accurate (there were no GMO varieties of grass–they were all simple hybrids, and the cyanide was only dangerous if the grass itself was eaten), and CBS ran an immediate correction, but the sensationalized stories about the “killer GMO grass” can still be found today on anti-science advocacy websites, passed around by people who did not bother to do any fact-checking.

The real lesson of the Texas cow massacre is a simple one that we often tend to forget: in the evolutionary struggle for survival, Mother Nature has already produced a variety of poisons and biological weapons that are far more sophisticated than anything humans have come up with–and they can appear at the most unexpected of times.

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One thought on “The Strange Case of the Cyanide Grass”

  1. The irony is that GMO technology could quite possibly actually eliminate the cyanide problem… 🙂

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