When you buy a pound of hamburger in the grocery store, you’re likely to be bombarded by an incredible assortment of labels. With all-natural, grass-fed, free-range, pastured, sustainably sourced, and certified organic options to choose from, it’s not easy to parse which beef is actually the best.

In recent years, demand for grass-fed beef has grown rapidly, thanks to the popularity of high-protein diets and growing consumer awareness about the overuse of antibiotics on farms and other related concerns. Grass-fed beef is also seen as nutritionally superior to its corn-fed counterparts, thanks to the omega-3 fatty acids that cows ingest when they graze on clover and other grasses. Grass-fed burger chains are popping up all over the country, and even Carl’s Jr. began offering a grass-fed burger earlier this year.

But what exactly do we mean when we say “grass-fed”? And is all grass-fed beef the same?

It’s All in the Finishing

“All cattle are grass-fed at one time in their life, until most end up in a feedlot where they’re finished on grain,” says Texas rancher Gerry Shudde. Indeed, most cows spend at least six months eating grass, before they are “finished,” or fattened up, with grain. The National Cattlemen’s Beef Association puts that number at 12 months, but most grain-finished beef cows don’t live beyond 18 months.

According to rancher and the author of Defending Beef Nicolette Hahn Niman, the real number likely falls somewhere in the middle. “On average, the cattle in the U.S. that is going through feedlots is slaughtered at 14-16 months,” she says. “They do grow fatter and faster if they’re being fed grain, so they’re going into feedlots at younger ages to shorten that time as much as possible.” In a feedlot environment, grain causes cows to put on about one pound for every six pounds of feed they eat. In contrast, grass-fed cows are slaughtered anywhere between 18-36 months.

“When you keep cattle on grass their whole lives, and truly have them forage for a diet that their bodies have evolved to eat, you allow them to grow at a slower pace,” says Niman. Not surprisingly, caring for the animal for so long can be expensive for ranchers and consumers.

Many informed eaters will tell you that this slower process results in a signature flavor and distinct leanness that sets it apart from its corn-fed counterpart, but the fact is that beef producers can label their product “grass-fed,” even if the animal is fed grain over the course of its lifetime. Unlike the lengthy auditing process involved in U.S. Department of Agriculture’s (USDA) organic certification, the use of “grass-fed” is only regulated under the agency’s “marketing claim standards.”

According to these standards, grass-fed cows are supposed to be given continuous access to rangeland, and they cannot be fed grains or grain by-products. In the event of drought or other “adverse weather conditions,” farmers are allowed to bend these rules if the animal’s wellness is in jeopardy, but they must maintain meticulous records. Unfortunately, these regulations are, for the most part, a paper tiger.