Scientists Feed Cows Onions to Make Them Stop Farting So Much Methane

Image: Paramount
Image: Paramount

Napoleon Dynamite takes a sip from a tall jar of milk, then sets it down next to two other jars. He points. “The defect in that one is bleach.” “That’s correct,” says a judge from FFA, the agricultural education organization. He sips another. “This tastes like the cow got into an onion patch.”


Almost the exact same scene took place in Spain last year. Except the scientists fed the cows onion extract intentionally to reduce the methane in their farts and burps.

Methane gas makes up 11 percent of America’s greenhouse gas emissions, although its up to 84 times more potent than carbon dioxide, according to the Environmental Defense Fund. As we’ve reported before, around twenty percent of our methane emissions comes from ruminants, like cows, belching, farting and pooping. Chemicals found in onions and garlic have been shown to decrease methane emissions by changing the chemistry of the rumen, according to a few studies. The problem: “Several studies have demonstrated the transmission of onion flavor to milk after the intake,” write the authors of a new study published in the Journal of Agricultural and Food Chemistry a few weeks back. “Milk must have a good flavor and an attractive appearance, among other nutritional parameters, to be acceptable to consumers.” No kidding.

The scientists, determined to get the onion into the cow without creating onion milk, figured they could track propyl propane thiosulfonate (PTSO), the chemical that gives onions their scent, in the milk. They fed 100 cows normal cow feed, and spiked 100 cows’ food with PTSO-containing onion extract, increasing by five grams per cow per day for five days, up to 25 grams. They then continued to feed the cows 25 grams of extract for two months.

The team also found a panel of 16 milk tasters from Granada, Spain to taste the milk from the cows, as well as other milks spiked with PTSO, in the lab. The milk from the cows eating a regular diet tasted fine, while milk containing a high level of PTSO—10 milligrams per kilogram—definitely tasted like onions to 88 percent of the tasters. The milk from the onion extract-eating cows contained lower levels of PTSO, and also tasted fine to all but 12.5 percent of the tasters.

From the tests, the scientists determined that 25 grams per day was a fine amount of onion extract to feed cows, and that 2 milligrams per kilogram of PTSO was the maximum concentration to avoid making the milk taste like onions. They also proposed a method using high performance liquid chromatography (HPLC), which can determine the presence of different chemicals in a liquid, to measure the amount of PTSO in milk.

The scientists concluded by recommending their 25 gram-per-day onion extract strategy as a good way to reduce methane emissions without compromising the taste of the milk. And it’s probably a good assumption that, if Napoleon Dynamite was a true story, adult Napoleon would be tasting milk spiked with onion extract for the sake of science and planet Earth.


[Journal of Agricultural and Food Chemistry]

Former Gizmodo physics writer and founder of Birdmodo, now a science communicator specializing in quantum computing and birds


Just feed the onions to beef cows. Milk and onions, not so good. Steak and onions, delicious.