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

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 organisation. 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.

Image: Paramount

Methane gas makes up 11 per cent of America's greenhouse gas emissions, although its up to 84 times more potent than carbon dioxide, according to the Environmental Defence Fund. In 2008, 21 per cent of Australia's greenhouse gas emissions came from methane. As we've reported before, around 20 per cent of global methane emissions come from ruminants, like cows, belching, farting and pooping. Chemical 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 flavour 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 flavour 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 5g per cow per day for five days, up to 25g. They then continued to feed the cows 25g 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 — 10mg/kg — definitely tasted like onions to 88 per cent 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 per cent of the tasters.

From the tests, the scientists determined that 25g per day was a fine amount of onion extract to feed cows, and that 2mg/kg 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 25g-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]

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