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Geologic Column
Oink If You Love Coal
Edward R. Landa

There are some things you hear in a geology class that stay with you long after obscure mineral names have faded from memory: “Pigs eat coal.”

As a soil science graduate student in 1973 at the University of Minnesota, I heard my professor Walter Parham mention this curious fact in passing during a class on environmental geology. A decade later, Wally and I would renew our acquaintance in Washington, D.C., when he was at the Office of Technology Assessment in Congress, and I was at the U.S. Geological Survey. In our talks over Szechuan eggplant and Hunan chicken at lunchtime get-togethers over the coming years, Wally explained to me the events that led him to utter that memorable phrase back in Minnesota.

While working for the Illinois State Geological Survey in the late 1950s, Wally spent each summer with a field assistant collecting samples of “underclays,” layers that directly underlie coalbeds of various thicknesses. Along the Illinois stream banks in which he was searching, many farmers raised pigs. In a number of cases, pigs liked to dig in the underclay to make a cool place to rest during the hot summers. Rain accumulated in these disturbed sites, turning them into mud holes. These were favorite sites for pigs to gather.

Wally and his colleagues were surprised to see pigs eating pieces of coal from the outcrop. In Wally’s words: “There was a loud, crunching noise as the pigs bit the coal off the outcrop, and you could hear them grinding the coal in their mouths before they swallowed it. Pigs were useful to us in our work because they uncovered new exposures of underclay and coal for us to sample.”

After observing this behavior, Wally asked various farmers about this eating habit of pigs. They were not surprised to hear about it, saying that farmers commonly threw several shovels-full of coal into railroad cars in which pigs were being shipped to market.

Turns out that Wally and I were hardly alone in talking about this phenomenon. A look at Time magazine from April 6, 1936, shows that the question “do pigs eat coal?” has been discussed and debated by the general public for more than eight decades. The comment “Pigs eat coal, digest it with relish” was the opening of a short piece on nutrition research using laboratory animals.

But was this behavior well-known among field geologists? I recently queried state geological surveys in coal mining regions, and most respondents indicated that they had never witnessed or heard of it. However, Russ Jacobson of the Coal Section of the Illinois Geological Survey verified learning early in his career during the 1970s that pigs seem to like something in coal, and that their digging was useful in locating otherwise hard to find coal outcrops.

Pigs have a highly developed sense of taste and smell. The ability of sows to sniff out truffles is well-known. Wild boars and domestic pigs have the largest number of taste-bud-bearing structures (fungiform papillae, the mushroom-shaped bumps seen on our tongues) of any species on record. So perhaps their attraction to coal is based on a perceived good taste. However, pigs are also known to engage in non-specific chewing behavior — pigs raised in confinement pens chew the bars, and pigs raised outdoors chew stones found in the soil.

A recent study in England showed up to 1.2 kilograms of stones in the stomachs of slaughtered pigs. Stone chewing does not generally harm the pigs, although there are concerns about laceration of the gastrointestinal tract where flint is abundant.

The reported feeding of coal to pigs in railroad cars prompted me to query animal scientists and railroad historical societies. A member of the Wabash Railroad Historical Society who had seen hogs eating bituminous coal confirmed the practice, suggesting that the coal was either used as a bait to keep the animals occupied in the car until the loading was completed or to comply, at least minimally, with Interstate Commerce Commission requirements for feeding livestock in transit. In an era of coal-fired steam locomotives, coal would be a readily available “feed.”

Several animal scientists confirmed the past practice of farmers putting piles of coal in pens or pastures for pigs to eat. They also pointed out that distractions tend to reduce aggressive behavior in confined pigs, especially in unfamiliar groupings. It is not uncommon today to put paper feed bags, tires, bowling balls or other distractions in confined hog barn pens to reduce fighting and biting in newly mixed pigs. Some of the swine specialists also commented on the practice of farmers in the past of feeding ashes from coal and wood fires to pigs, and that the animals would enthusiastically eat these.

Pigs in the United States today are typically raised in confined feeding barns. Hence the coal feeding at outcrops observed earlier will rarely be seen today with domesticated swine. However, feral pigs may still be crunching at outcrops of coal. And naturally occurring humic materials are presently marketed as sources of iron and other trace elements for pigs and other farm animals. Research in Poland during the 1990s showed that using brown coal as a swine feed supplement improved weight gains and decreased incidences of anemia in the pigs, and created manure as a byproduct that can be dried, briquetted and burned as fuel.

My queries to these scientists produced both valuable information and some inspired puns. One of the latter came from a University of Kentucky swine nutrition specialist who confirmed that pigs eat coal if it is available in their pens. But as to the view that this proves that there is something in the coal that the pigs need or crave — “I’m inclined to say ‘hogwash,’ but I won’t.” Another came from a geologist with the Iowa Geological Survey who wrote: “Not only do pigs have a well-known fondness for coal, they seem to have the ability to locate coal with the properties best suited for certain foundry products. Hence the term ‘pig iron’.”

So it seems clear that pigs eat coal. Strange can be true. Thanks, Wally.

Landa is a research scientist with the U.S. Geological Survey in Reston, Va.

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