When you see the "grass-fed" label on beef or milk, what comes to mind? Free-roaming cattle grazing on the wide-open range, most likely.

That may — or may not — be true.

What "grass fed" means

All cattle and dairy cows grazed on hay and grass until the 1950s, when producers started moving livestock onto feedlots and feeding them grain. Grain-fed cattle require far less land than grass-fed animals, and they fatten up faster. Grain feeding also produces meat that's nicely marbled with fat, which makes it tender.

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The trouble is that cows are ruminants, and grass, hay and the like are their natural diet. Munching on grain often makes them sick, so they need to be treated with antibiotics. Widespread use of antibiotics in livestock has contributed to the rise of antibiotic resistance (germs that antibiotics can’t kill). Feedlots also generate pollutants that contaminate the air and groundwater (and create poor conditions for the animals), according to the Environmental Protection Agency.

So is switching to grass-fed meat and dairy from grass-fed cows the answer? Maybe, but as with many food issues, it's not as straightforward as you might like.

The truth is, all cattle graze at least some of the time. Even in conventional feedlots, the diet is usually 15 percent ground hay, silage (fermented grass or other green fodder), straw and such.

It's hard for consumers to know what the "grass-fed" really means. The U.S. Department of Agriculture's voluntary Grass-Fed Marketing Claim Standards stipulate that “grass-fed” ruminants (including cows, bison, goats and lambs) are fed grass and forage for their lifetime (except for milk before they're weaned). These animals cannot be fed grain or grain byproducts, and they must have access to pasture during the growing season.

You'd think that means livestock grazing on the open range, but those standards leave a lot of room for interpretation. Animals may have limited access to pasture and spend most of their lives in feedlot conditions. Some private certification programs, like the one from American Grassfed, guarantee livestock are raised on forage, in pastures, with no antibiotics and under humane conditions.

A nutritional edge

What livestock eat changes the composition of their meat or milk. Grass-fed meat is lower in calories and fat and higher in protein. For example, ground grass-fed beef has 16 percent fewer calories, 27 percent less fat and 12 percent more protein than conventional. Grass-fed dairy has similar nutritional advantages.

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Several studies have found that grass-fed cattle are higher than feedlot counterparts in healthful fats such as omega-3 fatty acids and conjugated linoleic acid. Grass-fed meat also tends to have a healthier ratio of omega-3 to omega-6 fatty acids.

Like omega-3s, omega-6 fats are essential to your health (recent research suggests omega-6s may help lower harmful LDL cholesterol). But the standard American diet generally has a ratio that's too high in 6s compared to 3s, which can promote disease-causing inflammation. Choosing grass-fed meat and dairy may help improve that balance (although if your goal is to load up on omega-3s, you're better off opting for salmon or nibbling chia seeds, both of which are a far richer sources than any beef).

Weighing the cost

Because grass-fed beef costs more to produce, you'll pay more at the supermarket. Grass-fed beef can cost more than twice as much per pound as conventional beef, and grass-fed dairy can be three times more expensive than regular milk. Is it worth it? That depends on your goals when it comes to nutrition, the environment and animal welfare.

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Alison Ashton is a freelance food writer, recipe developer and Cordon Bleu-trained chef based on Los Angeles.