Do Not Pin Your Hopes for Guilt-Free Hamburgers on Seaweed

The recent positive press about cows revolves around a red algae seaweed native to Australia called Asparagopsis taxiformis. Over the past half-decade, a number of studies have been published showing that, under small-scale and short-term experimental conditions, adding dried and processed asparagopsis to cows’ feed on feedlots and dairy farms can massively diminish the amount of methane they belch into the atmosphere. Two of the most widely cited studies report reductions of up to 80 percent and up to a stunning 98 percent. The news media has dutifully covered these studies, usually taking them at face value. The Guardian alone has run stories about the promise of feed additives in 2021, 2023, and this past August; since then asparagopsis has also been covered by The Wall Street Journal and on NBC. (I was a guest on the NBC segment, where I presented a very short version of the argument in this article.)

If feeding cows seaweed really did reduce their total emissions by 98 percent, and if asparagopsis production and distribution could be scaled globally, this sort of reduction would be a game-changer. But it’s not clear that the math actually holds up. Feed additives are only really effective mixed into processed feed, meaning they work best in industrialized systems where ruminants are confined and fed a prepared diet rather than when they’re grazing on pasture. Most beef cattle are slaughtered between 18 and 24 months of age and spend the majority of their lives on pasture, only being rounded up on feedlots for the last few months of their lives and fed corn, soy, and grasses like alfalfa. This feed is designed to fatten cattle to slaughter weight, but it’s also much easier to digest, meaning that most beef cattle will only emit about 11 percent of their lifetime emissions on feedlots. Dairy cows, meanwhile, live longer than beef cattle and are kept in barns for most of their lives, where they are inseminated, separated from their young, and milked for their productive lives. This means dairy cattle eat an almost all-feed diet, but it also means they emit relatively less over their lifetime than beef cattle because they’re not digesting roughage.

Back in 2021, seeing the discrepancy between the reduction promised by headlines and press releases and the basic facts of cows’ life cycles, New York University environmental science professor Matthew Hayek and I did some back-of-the-envelope calculations and came up with far less promising numbers. Assuming an 80 percent reduction of emissions on feedlots, where cattle emit 11 percent of their methane, we figured that would make asparagopsis lead to lifetime emissions reductions of about 9 percent. Even that bombastic 98 percent reduction estimate—which hasn’t been replicated in any other studies—would still only result in about an 11 percent total emissions reduction. Hardly game-changing numbers.