Demand for grass-fed beef has been rising for quite some time, even though it can be twice as expensive as conventional beef. Many consumers prefer it over conventionally raised beef for several reasons, including that it comes from cows that were raised on diets more suited to their biology and were allowed to graze. It's also tastier (in my opinion) and contains more beneficial omega-3 fatty acids and antioxidant vitamins.

But the added benefits that grass-fed beef offers may be in danger if a scientific analysis of cow pies is any indication. NPR reports that between 1994 and 2016, the poop collected from grazing areas in many states shows the grass is losing nutritional value.

During that time about 50,000 samples were studied and research shows "levels of crude protein in the plants, which cattle need to grow, have dropped by nearly 20 percent. As a result, cows aren't growing to the size they once were."

What's behind the nutrition loss?

beef stew The price of grass-fed beef for dishes like this stew may increase. (Photo: Bartosz Luczak/Shutterstock)

There are several theories why the grass has lost some of its nutrient content. Joe Craine, one of the researchers who studies the cow pies, believes it could be because the cows don't spend all their time on pasture. When they're taken to the feed lots, they stop dropping manure on the pasture, which adds nutrients to the soil. Less poop could be leading to fewer nutrients in future poop.

He also suspects climate change could have something to do with it. It's already been shown that the increase in carbon dioxide in the atmosphere has led to plants like rice and potatoes containing fewer nutrients. The same thing could be happening to grass on the prairie.

Another theory by Jerry Voleski, a professor and range and forage researcher at the University of Nebraska-Lincoln, thinks CO2 may have something to do with for a different reason. It makes plants bigger but the amount of nitrogen doesn't change so there's less nitrogen per volume. The cows are eating the same volume, but there's less nitrogen in that volume, even though the amount of nitrogen in total hasn't changed.

No matter which theory proves to be true, it's likely the price of grass-fed beef will go up.

If that happens, cutting back on grass-fed beef is one option. You can also replace beef with chicken, fish, or better yet, plant-based proteins like quinoa or lentils.

Robin Shreeves ( @rshreeves ) focuses on food from a family perspective from her home base in New Jersey.