Oh s**t

People who do research on agriculture and nutrients inevitably end up dealing with a lot of manure. Animal manure is one of the largest sources of nutrient pollution to our waterways. I'm accustomed to reading lines in scientific papers like “Assuming 1.5 g N/ kg of manure, the manure inputs total 40 million kg/year, or 13% of the nitrogen inputs to the river basin” (though, just once I’d like to see the author have the guts to add “man, that’s a lot of shit”.

Staring at the staggering nitrogen in manure numbers from a recent study of the Mississippi River Basin got me thinking… forget the nitrogen, just how much manure is out there? Ah, what a scientist does for fun.

Here’s how you can do the calculation. First, you need to look at the number of farm animals. According the data from the UN Food and Agriculture Association and the U.S. Department of Agriculture, there are 96 million cattle, 61 million hogs and, egad, 1.95 billion chickens in the US (two billion of the birds, it is like a Hitchcock movie out there). With that, you can use typical rates of manure production per animal in the U.S., available from the USDA’s Natural Resources Conservation Service, to get a rough estimate the total amount of manure produced.

The result: U.S. cattle, hogs and chickens produce about 4.37 billion kg of manure a day. Yes, each day. No joke.

Cattle alone produce 3.7 billion kg of manure a day – that works out to 1.34 trillion kg a year. Keep in mind, though there may be way more chickens, a chicken is a lot smaller than a cow, so chickens produce much, much less manure than cows.

Now, this estimate may be high for a variety of reasons (e.g. many of the reported millions of animals are not mature, produce less per day than the reported data). But it should be on the right order of magnitude.

Either way, it is staggering. Let’s put the total in perspective. The Empire State Building weighs approximately 0.33 billion kg (ah, the wonders of Google). That means cattle in the U.S. are producing an Empire States Building’s worth of manure about every few hours.

Globally, the same method gives an estimate of 51.7 billion kg/day produced by cattle, or almost 19 trillion kg/year. Again, the number is just an estimate. The point is the answer is on the order of trillions of kilograms.

Let me know if you can find something that weighs in the ballpark of 19 trillion kg. The best I could come up with is the ejecta from a small volcanic eruption or maybe a minor comet. Either way, not a nice image.