Carbon dioxide levels in the atmosphere continue to increase, and while that stimulates plant growth, it also means that plants have less nitrogen, which is a key nutrient, according to an announcement from the U.S. Department of Agriculture's Agricultural Research Service (ARS).

ARS ecologist David J. Augustine in Ft. Collins, Colo., wanted to know if that means the forage grasses cattle eat are becoming less nutritious and harder to digest and if those changes affect the animals' ability to gain weight as they graze. 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 announcement said.

Global carbon dioxide levels have increased from 330 parts per million in 1976 to 405 ppm last year and could reach 600 ppm as soon as 2050, ARS said.

Augustine and his colleagues simulated the 600 ppm carbon dioxide levels on a mixed-grass prairie in the northern Plains that produces the native grasses that make up the forage consumed by cattle, ARS explained.

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 by the end of the grazing season, individual cattle would weigh about 30 lb. (12%) less, according to ARS. Trying to induce cattle to eat more of the available grass isn't a viable alternative because of its reduced nutritional content, the researchers said.

The study was published last year, and since then, Augustine and his colleagues at the Rangeland Resources & Systems Research Unit in Cheyenne, Wyo., and in Ft. Collins have been searching for new ways to help ranchers address the problem.

They reported 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, the researchers suggested.

The researchers remain optimistic. "There is certainly an important issue to be addressed regarding forage quality, but there also are a number of potential solutions," Augustine added.

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