Sheep poop may spread poisonous weed | Science News for Students

Sheep poop may spread poisonous weed

Teen shows that using sheep to halt plant’s spread may do the opposite
May 19, 2017 — 6:50 am EST
Jade Moxey

Jade Moxey with some of the sheep on her parents’ farm in Australia. The teen studied the animals’ poop to see what happened after they ate an invasive plant.

Anne-Marie Moxey

LOS ANGELES, Calif. — Fireweed is invading Australia. The bright yellow plant, native to Africa, is poisonous and can harm cattle and horses. Sheep are resistant, though, and often are used to eat away at the problem. But are the sheep coming away poison-free? Jade Moxey, 17, decided to find out. And the findings by this senior at Sapphire Coast Anglican College in Australia turned up some surprises.

Although sheep may consume fireweed in one spot, they also spread the plant around, she found. And while the sheep may not suffer ill effects from the poisonous plant, its chemical weapons may end up in the sheep's meat.

Jade shared her results here at the Intel International Science and Engineering Fair (ISEF). Created by Society for Science & the Public and sponsored by Intel, the competition brings together almost 1,800 high school students from more than 75 countries. (The Society also publishes Science News for Students and this blog.)

Fireweed (Senecio madagascariensis) looks like a bright yellow daisy. Sheep love to eat it. “When we put the sheep in a new paddock, they automatically go for the yellow flowers,” Jade says. The plant, also known as Madagascar ragwort, has spread as far as Australia, South America, Hawaii and Japan. But its pretty appearance hides a toxic secret. It makes chemicals called pyrrolizidine alkaloids (PEER-row-LIZ-ih-deen AL-kuh-loidz). They can cause liver damage and liver cancer in horses and cattle.

Senecio madagascariensis is known as Madagascar ragwort or fireweed. The little yellow flower packs a poisonous punch.
Pieter Pelser/Wikimedia Commons (CC-BY 3.0)

Sheep resist these poisonous effects, though, so they have seemed an ideal way to control the problem. Farmers let the animals loose in places where fireweed is a problem. And the sheep gobble it up.

But plant seeds can sometimes survive the digestion process. And Jade wondered what might be happening after the fireweed passed through the sheep’s gut. She collected manure twice from 120 sheep on her parents’ farm. She laid that poop out on the ground, protected it from stray breezes that might blow in seeds and waited. Sure enough, 749 plants grew. Of these, 213 were fireweed. So the sheep might be eating the weed, she concludes, but they’re also probably spreading its seeds.

Jade also was curious whether it was true that sheep are immune to the fireweed’s poison. Working with her local veterinarian, she tested blood samples from 50 sheep. She also examined the livers from 12 sheep to see if that organ had been damaged. Jade now reports that sheep have no need to fear the fireweed. Even animals that had grazed on fireweed for six years showed little sign of harm

That didn’t mean the poison wasn’t present, however. Very low levels of it turned up in the animals' liver and muscle (that is, the meat), Jade found. Though the fireweed poison can be toxic to people, “the levels aren’t cause for concern,” she says. Indeed, she still eats local mutton (sheep meat) without worry.

But she might have reason to change her mind if those sheep were to eat more of the weed. “The fireweed on my property where the sheep were sourced from [has a density of] 9.25 plants per square meter [about 11 plants per square yard]. And in other areas of Australia there are densities up to 5,000 plants in a square meter [5,979 plants per square yard].” In those cases, sheep may eat a lot more of the plant. And then, Jade says, more testing should be done to find out how much ends up in the meat people eat.

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Power Words

(for more about Power Words, click here)

cancer     Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.

cattle     Also known as bovines (because they’re members of the subfamily known as Bovinae), these are breeds of livestock raised as a source of milk and meat. Although the adult females are known as cows and the males as bulls, many people refer to them all, generally, as cows.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

density     The measure of how condensed some object is, found by dividing its mass by its volume.

engineering     The field of research that uses math and science to solve practical problems.

immune     Able to ward off a particular infection. Alternatively, this term can be used to mean an organism shows no impacts from exposure to a particular poison or process. More generally, the term may signal that something cannot be hurt by a particular drug, disease or chemical.

Intel International Science and Engineering Fair     (Intel ISEF) Initially launched in 1950, this competition is one of three created (and still run) by the Society for Science & the Public. Each year, approximately 1,800 high school students from more than 75 countries, regions, and territories are awarded the opportunity to showcase their independent research at Intel ISEF and compete for an average of $4 million in prizes. 

invasive species     (also known as aliens) A species that is found living, and often thriving, in an ecosystem other than the one in which it evolved. Some invasive species were deliberately introduced to an environment, such as a prized flower, tree or shrub. Some entered an environment unintentionally, such as a fungus whose spores traveled between continents on the winds. Still others may have escaped from a controlled environment, such as an aquarium or laboratory, and begun growing in the wild. What all of these so-called invasives have in common is that their populations are becoming established in a new environment, often in the absence of natural factors that would control their spread. Invasive species can be plants, animals or disease-causing pathogens. Many have the potential to cause harm to wildlife, people or to a region’s economy.

liver     An organ of the body of animals with backbones that performs a number of important functions. It can store fat and sugar as energy, break down harmful substances for excretion by the body, and secrete bile, a greenish fluid released into the gut, where it helps digest fats and neutralize acids.

manure     Feces, or dung, from farm animals. Manure can be used to fertilize land.

Society for Science and the Public     A nonprofit organization created in 1921 and based in Washington, D.C. Since its founding, SSP has been not only promoting public engagement in scientific research but also the public understanding of science. It created and continues to run three renowned science competitions: the Regeneron Science Talent Search (begun in 1942), the Intel International Science and Engineering Fair (initially launched in 1950) and Broadcom MASTERS (created in 2010). SSP also publishes award-winning journalism: in Science News (launched in 1922) and Science News for Students (created in 2003). Those magazines also host a series of blogs (including Eureka! Lab).

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

veterinarian     A doctor who studies or treats animals (not humans).

weed     (in botany) A plant growing wild in, around — and sometimes smothering over — valued plants, such as crops or landscape species (including lawn grasses, flowers and shrubs). Often a plant becomes such a botanical bully when it enters a new environment with no natural predators or controlling conditions, such as hard frosts. (in biology, generally) Any organism may be referred to as a “weed” if it enters an environment and begins to overwhelm the local ecosystem.