On a gray December day, Kent Davidsen left his dairy farm in rural Jutland, Denmark, for the neo-Baroque palace in Copenhagen that houses the Danish parliament. Unlike many of his fellow farmers, Davidsen had welcomed an initiative by the government that went into effect last year, to reduce the agricultural sector’s greenhouse gas emissions. But now, as he would testify to a small group of parliamentarians, he was convinced that one of those measures had produced terrible effects.

In January 2025, Denmark became the first country in the world to require that dairy farmers give their herds an additive called Bovaer or, alternatively, feed them a high-fat diet. The additive is meant to cut the animals’ production of methane — a potent greenhouse gas. Davidsen began adding Bovaer to his herd’s feed in October, but not long after the cows fell ill. “It’s not normal for a full herd of a thousand cows to have diarrhea, all of them,” he said in an interview. Within three days, he added, “they dropped in milk production with almost three kilos per cow.”

After 10 or 12 days, he said, some of the cows couldn’t stand up. Within a month, 10 were dead.

Hundreds of other Danish dairy farmers have now reported similar effects. Yet Bovaer, which its manufacturer says has been administered to an estimated 500,000 dairy cows in more than 25 countries, has never before been associated with bovine health problems. As the effects of the case ripple through Denmark’s barns and milk processing plants, the questions behind that conundrum are multiplying. Can more than a hundred peer-reviewed studies on Bovaer’s safety and efficacy be wrong? And if not, are there other ways of understanding what is happening on Danish farms?