Peaches, a brown-and-white Jersey cow weighing 1,200 pounds, was amiably following Edward Towers through a barn on a sunny March morning when the 6-year-old dug in her front hooves.

Mr. Towers, a 28-year-old-farmer whose family owns Brades Farm, near Britain’s rugged Lake District, slapped Peaches gently to move her along. She didn’t budge. Already muddy from a morning herding hundreds of cows to a milking session, Mr. Towers leaned all his weight into Peaches’ ample backside, until she finally stepped through a metal gate that would hold her head still for an exam.

Deepashree Kand, a scientist studying animal nutrition, stepped forward with a device about the size of a grocery-store scanner. As David Bowie’s “Changes” played on a radio, Ms. Kand pointed a green laser at the cow’s nostril and waited for Peaches to belch.

Ms. Kand’s employer, a Swiss company called Mootral, is studying whether an altered diet can make cattle burp and fart less methane — one of the most harmful greenhouse gases and a major contributor to climate change. If they were a country, cows would rank as the world’s sixth-largest emitter, ahead of Brazil, Japan and Germany, according to data compiled by Rhodium Group, a research firm.

It is a well-known problem that has had few promising solutions. But in the last five years, a collection of companies and scientists has been getting closer to what would be an ecological and financial breakthrough: an edible product that would change cows’ digestive chemistry and reduce their emission of methane.

Several companies are pursuing a seaweed-based compound, and a Dutch firm, DSM, is testing a chemical supplement with promising results. Mootral is one of the furthest along. By mixing compounds from garlic, citrus and other additives into a pellet that’s mixed with a cow’s regular diet, the start-up has surprised scientists by significantly and consistently cutting the toxic output of animals like Peaches.

At Brades Farm, Ms. Kand kept her laser steady. Changes in the light beam would measure the methane in Peaches’ burps, which she produced about once every four minutes. Soon, there was a subtle flex in the cow’s neck, and Ms. Kand’s device put out a few readings: 32 to 38 parts per million.

“That’s good,” Ms. Kand said. “A reduction of about 30 percent.”

The drop was consistent with the findings of several peer-reviewed studies of Mootral’s food supplement. Additional trials are underway in the United States and Europe. The product is being tested at dairy and meat farms, including a Dutch farm used by McDonald’s for studying new techniques in its supply chain. The venture capitalist Chris Sacca, who became a billionaire with early bets on Uber and Twitter, has invested.

Many questions of viability remain. Mootral must prove that its product works on different breeds of cows and in different climates. It has had success in areas with mild weather, like Northern Europe, but is now conducting experiments in hotter locations.

Most urgent, the company must find its place in the coronavirus economy. An investment round that was scheduled to close in March fell apart because of the crisis. The start-up’s business model depends on convincing typically conservative livestock and dairy companies that they will receive credits they can sell in the unpredictable and largely unregulated carbon-offset market for using what is basically Gas-X for cows.

But if Mootral or one of its competitors can withstand the challenges of the coronavirus era and hold up at scale, the result could be one of the simplest and fastest ways to cut a major source of greenhouse-gas emissions.

“It is something, to be honest, that I never expected,” said Gerhard Breves, a longtime livestock researcher in Germany who performed one of the first independent tests of Mootral’s product and is now an unpaid member of its advisory board.

‘An existential threat’

Cows are a digestive miracle. Inside their stomach is an oxygen-free environment with a steady temperature, similar to the fermentation tanks used to make beer. Microbes decompose and ferment materials like cellulose, starch and sugars. Cows can eat just about anything — grass, hay, cornstalks, rapeseed — and turn it into energy for producing milk and meat.

“They could live on wood,” said Mootral’s director of science, Oliver Riede, a molecular biologist who started his career studying vaccines and infection management.

But just as a midnight pizza can come with a gaseous cost, a cow’s digestive system has a way of retaliating. Methane is a main byproduct of the enzymes that help break down the food. The gas can’t be turned into energy, so as it builds up, a cow must burp, sending little puffs of pollution into the atmosphere. (A small amount is released by farting.) Up to 12 percent of a cow’s energy intake from food is lost this way.

There are about 1.4 billion cattle globally, each emitting the equivalent of 1.5 to 2.5 metric tons of carbon dioxide each year, roughly half the output of an average American car.

As awareness of cattle’s environmental impact has reached the mainstream, thanks to compelling media campaigns by environmentalists and Netflix documentaries, the meat and dairy industries have felt the effects. Sales of alternative milks and meat substitutes have soared. Vegetarianism and veganism have spread.

“This is an existential threat,” said Joe Towers, Edward Towers’s older brother, who also works at Brades Farm. “Farmers are keen to improve and show they aren’t the bad guys.”

‘Want to smell it? It smells like fart’

Mootral’s main research lab is at the base of a lush valley, in a former coal-mining region of Wales. The company’s work on cows dates to 2010, when a group of researchers participated in a European Union research effort to explore ways to reduce methane from cattle.

The team, working for a company called Neem Biotech, had studied garlic’s antimicrobial properties in humans. In lab trials, the scientists found that it also reduced methane in cows thanks to allicin, the same strong-smelling compound that’s produced when a garlic clove is cut with a knife. But the company was small and didn’t see a business case for the finding, so the work didn’t go any further.

In 2012, Neem was sold to a life sciences company, Zaluvida, that developed over-the-counter diet and allergy supplements. One product, derived from compounds found in prickly pears, gave people the sensation of feeling full. Another helped with digestion.

Zaluvida’s founder, Thomas Hafner, bought Neem intending to work on drugs for people, but during a review of past research, a colleague found the methane work in a computer file named “Mootral.” It explained how allicin interacted with microbes inside a cow’s stomach.

After becoming rich by manipulating the human digestive tract — he sold the supplements business for about $150 million in 2014 — Mr. Hafner saw an opportunity in doing the same with cows. By 2016, he put a team of scientists to work testing different combinations of garlic extracts.

The challenge, they learned, was finding the right balance between delivering the maximum amount of allicin without triggering adverse effects. The chemical targets enzymes in the cow’s gut that create methane. Too much could harm the cow’s ability to process food, or give the milk and meat a garlic flavor.

“The first thing the farmer will ask is, ‘What will this do to my animal?’” said Mr. Riede, the Mootral science director.

Allicin is volatile, and the team struggled at first to come up with a consistent blend that would work across members of a herd of cattle. In the lab, researchers used bacteria from the stomachs of sheep — members, like cows, of the ruminant family — to see how certain combinations would change methane levels.

They’re still tweaking the formula. Every few weeks, Daniel Neef, a biochemist, travels to a nearby butcher in Wales to buy a stomach from a freshly slaughtered sheep. He cuts it open to extract a wet, tangled ball of grass and other feed. He squeezes the substance through cheesecloth to extract a liquid that he puts in glass milk jars — making what looks like a green vegetable drink available at Whole Foods.

“Want to smell it?” Mr. Neef asked one day at the Mootral lab, opening the lid. “It smells like fart.”

The juice was filled with scores of different kinds of bacteria, which interact in ways we don’t fully understand. At one point, Mootral’s scientists improved results by adding a trace amount of citrus from Spanish oranges. New additives like seaweed and other different kinds of garlic are being tested.

Mr. Neef combined the bacterial juice with droplets of extracts in medical vials, which he then moved to a machine that sucked out the oxygen and reported how much methane was produced.

“You overlook plants and think they are quite simple,” said Robert Saunders, a Mootral scientist whom colleagues call Mr. Garlic, “but when you realize the complexity going on inside them, you can exploit them and make products from this.”

He added: “We’re not just buying garlic and putting it in a pellet. Chemistry is at the center of it.”

The benefits of garlic breath
Mootral leases farmland in China’s Gansu and Shandong Provinces, where garlic is picked by laborers, stuffed in bags and stored in a warehouse. It is peeled, dried and milled into a fine powder at a plant in China before being sent via train to Germany and trucked to Wales, where it is mixed with other food extracts. The company recently installed a shower at the facility so staff don’t have to go home reeking of garlic.

By 2017, Mootral was confident enough in its work to ask outside scientists to perform their own trials. That year, researchers in Denmark and Germany published findings saying the company had reduced cows’ methane emissions more than 50 percent in lab simulations. In Mootral’s first tests in dairy cows on a fully functioning farm, Brades, methane emissions fell 38 percent. A California study found a reduction of about 20 percent in meat cattle.

Sixteen tests and studies are scheduled once work stoppages from the coronavirus lifts, including at Purdue University and the University of California, Davis, Mr. Hafner said. The Swiss and Irish governments are funding Mootral research. In one testing technique, the cow is put inside a tent — a little like the ones that pro football players enter when injured — that is outfitted with methane-detecting sensors.

There have been unexpected results. Researchers have shown an increase in milk production, possibly because cows that expend less energy expelling methane produce more dairy. The farmers at Brades said flies weren’t bothering their cows as much, perhaps as a result of garlic breath.

“I was skeptical when we started,” said Professor Breves, director of the Physiological Institute of the Veterinary University of Hannover, who has spent three decades studying livestock biology and emissions. “I do not remember any other compounds having such a pronounced and significant effect without any negative effects.”

Many scientists need more convincing. Hanne Hansen, who performed an early lab test on Mootral and is an associate professor at the University of Copenhagen’s department of veterinary and animal sciences, said more published research was needed to prove the food additive would work on different breeds and in various climates. Much of the research, she said, has been performed in labs that only simulate the chemistry of a cow. Mootral also hasn’t been tested on cows at large industrial farms, like those in the United States, which are notorious hubs for methane emissions.

“What happens in the laboratory is not always what happens in real life,” Professor Hansen said. “Mootral has potential, but we need to see more proof.”

Fund-raising during a pandemic
Mr. Hafner, who is German and has a buttoned-up manner that is more boardroom than barn, puts an optimistic spin on Mootral’s prospects. If the world economy opens up in the coming months, he expects to have roughly 300,000 cows taking its supplements by next year, and 7.5 million by 2024.

Yet he is realistic about the challenges. In March, agreements with several investors were put on hold as the coronavirus spread. One group had pledged to put in 6.5 million euros (about $7 million) and another €6.5 million if certain scientific targets were met.

“Has that put us in a pickle? Of course,” Mr. Hafner said recently by phone from Austria, where he owns a home and spent parts of March and April recovering from what was diagnosed by a doctor as coronavirus. (He did not receive a test.) Having already put more than $20 million of his own money into the business, he added, “We have a plan to weather the storm and come out the other end.”

Eventually, Mootral’s plan is to sell its food additive for about €50 per year per cow. Mr. Hafner, whose first job after dropping out of college was at Burger King, said it would add only a few pennies to the cost of meat or dairy. He figures that grocery stores, restaurant chains, and large milk and livestock companies will be willing to bear the cost because they are under increasing pressure to appeal to eco-minded customers and satisfy sustainability mandates from investors and governments. If Mr. Hafner hits his 2024 goal, he will have annual revenue of €375 million.

An important financial incentive for companies to use Mootral are the carbon credits it would generate. The credits could offset the companies’ own emissions levels or be sold to others that have pledged to cut theirs. In December, the manager of the world’s largest voluntary carbon offset program, Verra, said Mootral would be the first company able to sell credits for reducing methane from cows.

The approval means a grocery chain or fast-food brand could require meat producers in its supply chain to use Mootral, then use the resulting carbon credits to meet its corporate sustainability goals. The credits could also be sold to companies, such as Microsoft, Royal Dutch Shell and Delta Air Lines, that have pledged to buy credits to offset their carbon footprint.

The problem is that carbon markets are still voluntary in most industries, and the system’s credibility has been hampered by concerns that many offsets are tied to projects that don’t have a measurable effect on climate change. In 2018, the entire voluntary carbon market was about $300 million, according to Forest Trends, a research group.

Mr. Hafner is convinced demand will grow as more governments mandate reductions, particularly to meet the targets of the international Paris climate agreement. In Europe, countries have pledged to cut greenhouse gas emission levels from 1990 by 40 percent by 2030 — commitments that will affect every industry, including agriculture.

“We are working from the assumption that down the line every cow will be regulated to be on a methane reducer,” Mr. Hafner, 56, said over a steak dinner in Wales in early March. “This is going to come.”

That is a risky bet. Meat consumption continues to rise globally as a result of an emerging middle class in countries like China. And national leaders have been reluctant to impose tough rules on politically influential agriculture and farming industries. Many fear climate change will take a back seat to getting the global economy back on track after the coronavirus pandemic.

“Are we going to offset our way out of the problem? No,” said David Antonioli, the chief executive of Verra, referring to climate change. “If we all continue to eat as much meat as we do, no matter what we do with Mootral or other products, we are probably not going to address the problem.”

Mr. Hafner is frustrated that Mootral and its competitors have products that could help address sea-level rise and other perils but are hamstrung by financial and political constraints.

“There isn’t enough urgency,” he said. “The scale of Covid is nothing like the climate crisis.”

A dating-show strategy

In Britain, Brades Farm has seen hard times before. Five years ago, it nearly closed after milk prices collapsed. Documentaries detailing the environmental harm of cattle farming — like “Cowspiracy,” produced by Leonardo DiCaprio — didn’t help. At one point, the Towers brothers got so desperate that in a bid for attention, Edward became a contestant on a dating show, “Love in the Countryside.”

“We didn’t sell any milk,” Edward Towers said of the experience, “but I’ve been with my girlfriend for three years.”

Mootral provided a lifeline. Marketing its cows as low methane, Brades Farm has found a niche selling climate-friendly milk to cafes and artisanal baristas around Britain, in bottles labeled “Less CO₂W Burps.”

In March, behind the barn where the cows eat and rest, the smell of garlic wafted from piles of Mootral feed. Twice a day, it is mixed with grass, maize, wholecrop and rapeseed. The additive accounts for about 1 percent of the 75 to 110 pounds of food a cow eats every day.

“Just feeding this to 400 cows isn’t going to change the world, but by setting an example, and being first, that can have an impact,” Mr. Towers said. “That’s what’s cool about our little farm.”