What he's found is a trend in the nutritional quality of grasses that grass-fed cattle (and young cattle destined for grain-heavy feedlots) are eating. Since the mid-90s, levels of crude protein in the plants, which cattle need to grow, have dropped by nearly 20 percent. "If we were still back at the forage quality that we would've had 25 years ago, no less 100 years ago, our animals would be gaining a lot more weight," Craine says. He has a sneaking suspicion that rising carbon dioxide levels in the atmosphere are contributing as well. Increased CO2 levels have been linked to fewer nutrients in plants like rice, wheat and potatoes....Craine may have a point about rising carbon dioxide levels: The more carbon dioxide, the bigger the plant, but the amount of nitrogen, which makes plants nutritious for cattle, doesn't change.