Hopes were running high for cow 401, and cow 401 serenely bore the weight of expectations. She entered the cattle chute obligingly, and as the vet searched her uterus, making full use of the plastic glove that covered his arm up to his shoulder, she uttered nary a moo. A week ago, Cow 401 and four other members of her experimental herd at UC Davis were in the early stages of pregnancy. But now, following a string of disappointing checkups, it was all down to her. Alison Van Eenennaam, the animal geneticist in charge of the proceedings, kept watch from off to one side, galoshes firmly planted in the damp manure, eyes fixed on a portable ultrasound monitor. After a few moments, the vet delivered his fifth and final diagnosis. “She’s not pregnant,” he said. Van Eenennaam looked up. “Ah, shit,” she muttered.

Cow 401 and her herdmates were the product of two and a half years of research, Van Eenennaam’s attempt to create a strain of gene-edited cattle specially suited to the needs of the beef industry. Had everything gone as planned, all the calves in this experiment would have been born male—physiologically, at least. Like humans, cattle carry two sex chromosomes; those born XX are female, and those born XY are male. But it isn’t the Y that makes the man. It’s a single gene, called SRY, that briefly flickers to life as an embryo grows and instructs it to develop male traits. Using Crispr, Van Eenennaam’s team added a copy of SRY to the X chromosome too. That way, even if a cow was born genetically female, she’d be expected to appear male all the same. Since beef ranchers generally prefer males to females (more meat for the money), Van Eenennaam believed there could someday be a market for these Crispr’d animals.

April 2019. Subscribe to WIRED. Christie Hemm Klok

More than that, though, the project was a proof of concept. One of Van Eenennaam’s goals is to make the raising of livestock not only more efficient but also more humane. If a calf’s sex could be altered with a copy-paste of a single gene, that might pave the way for all kinds of experimentation—and not only in the beef business. Although ranchers may prefer male animals, their colleagues in the egg and dairy industries favor females. Since bulls can’t make milk and roosters can’t lay eggs, it’s cheaper to destroy them than raise them to adulthood. But if you could ensure that only heifers and hens are born, the carnage wouldn’t be necessary.

The Davis team wasn’t yet sure what had gone wrong with the pregnancies. They’d done their work with such care. First they located a target area on the bovine genome and created a custom set of Crispr scissors to cut the DNA and insert the new gene. Then they took a trip down the interstate to a slaughterhouse in Fresno, where they purchased a fresh batch of ovaries. Back in the lab, they aspirated the eggs, fertilized them, and set their Crispr scissors loose. They let the resulting embryos grow for a week, biopsied them to make sure the edits had gone as planned, then froze them until the cows were ready for implanting.

Perhaps, Van Eenennaam thought, the arduous process had simply knocked the life out of the embryos. “Science is a bitch,” she said with a shrug. But there was a more troubling possibility—an issue with the gene edit itself. On a map of the bovine X chromosome, the location where they’d inserted SRY seemed to be within a stretch of extraneous code, far from any life-critical genes. But then again, the map they currently had was about as accurate as a 16th-century atlas of the New World, full of unknown and mislabeled territories. Maybe, by tinkering in the wrong place, they had arrested development in the womb.

Alison Van Eenennaam at the UC Davis Beef Barn. Christie Hemm Klok

Twenty-five years ago, Van Eenennaam was a student at Davis in the early days of the GMO craze. Animal scientists, long limited by the pace of traditional trial and error breeding, could now mix and match genetic traits from different organisms, giving their livestock strange new powers. At Davis, for instance, they engineered a line of goats that carried a human protein called lysozyme in their milk. (Later on, researchers realized that, when fed to children in the developing world, that milk could prevent diarrhea.) As a young faculty member at Davis in the mid-2000s, Van Eenennaam explored a method for modifying cows to produce milk with extra omega­-3s. Then, just as she prepared to begin experiments in actual cattle, she says, the money dried up.