Global CO2 levels have increased from 330 parts per million (ppm) in 1976 to 405 ppm last year and could reach 600 ppm as soon as 2050. That could be good news in terms of stimulating plant growth, but it also means less nitrogen, which could rob cattle of a key nutrient when they graze.  

Agricultural Research Service (ARS) ecologist David J. Augustine in Fort Collins, Colorado explored what happens when the forage grasses that cattle eat become less nutritious and harder to digest, and if it affects the animal's ability to gain weight as it grazes.

Augustine and his colleagues simulated the 600 ppm CO2 levels on a mixed-grass prairie in the Northern Plains that produces the native grasses that make up the forage used by cattle. Yearling cattle in rangeland systems in the Northern Plains typically are grazed on forage grasses from May to October before they go to finishing lots where they are fed grain-based diets.

The researchers collected data for seven years and found that the amounts of forage increased, but the decline in nutritional quality was consistent across the years and that by the end of the grazing season, individual cattle would weigh about 30 pounds, or 12 percent, less.

The study was published last year and since then, Augustine and his colleagues at the Rangeland Resources and Systems Research Unit in Cheyenne, Wyoming and in Fort Collins, have been searching for new ways to help ranchers address the problem. They say that trying to induce cattle to eat more of the available grass isn't a viable alternative because of its reduced nutritional content.

Instead, Augustine and his fellow researchers say that ranchers may want to consider shortening the traditional grazing season because by late summer forage grasses are less nutritious and the cattle gain only negligible weight from early September to mid-October. Net returns also are consistently higher for cattle removed from grazing and sold in early September, according to another ARS study.

Other options include feeding cattle nutritional supplements, seeding pastures with legumes to add nitrogen to the soil, applying low levels of fertilizer, stimulating new grass growth with prescribed burning and adjusting stocking rates in response to changing weather and climatic conditions (e.g., wet periods and drought) that affect grass production.

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