ARS | Publication request: THE NUTRITIONAL DYNAMICS OF OUR MAJOR RANGELAND GRASSES

Submitted to: Eastern Oregon Agricultural Research Center
Publication Type: Experiment Station
Publication Acceptance Date: January 1, 2005
Publication Date: January 1, 2006
Citation: Ganskopp, D.C., Bohnert, D.W. 2006. The nutritional dynamics of our major rangeland grasses. Eastern Oregon Agricultural Research Center. SR1057:46-47.

Interpretive Summary: Ranchers and forage managers need to be aware of the seasonal nutritional dynamics of forages to sustain adequate growth and reproduction of their animals. Our objective was to describe the seasonal and annual nutritional dynamics of 7 of the regions most prominent grasses. Annual precipitation for the 2 years sampled was 86 and 167% of average, with estimated forage yields of 484 and 1,121 pounds per acre. The protein content of grasses equaled or exceeded cattle requirements for 6 of 8 months sampled in the dry year and only 3 of 8 months sampled in the wetter year. Grasses were also more digestible for 5 of 8 months for the drier of the 2 years. In the better (wetter) growing seasons, grasses mature, produce a wealth of low quality seed stalks, go dormant, and do not respond to mid- or late-summer rains. During the drier growing season, grasses cure at an earlier and more nutritious stage of growth and do not produce many low quality seed stalks. Also, because grass stems have not completed their reproductive stages of development, plants can respond to mid- or late-summer rains with an additional flush of high quality growth. To assure adequate growth, reproduction, health of their animals, ranchers should be more concerned with supplementing cattle on rangelands with additional protein in years with good growing conditions than during drier years when plants produce less herbage.

Technical Abstract: Ranchers need to understand the nutritional dynamics of forages to sustain adequate growth of their animals. We addressed this issue via monthly sampling of 7 rangeland grasses during 1992, a drier year (86% of average, and 1993, with above average rainfall (167%). With high yields in 1993 (1,257 kg/ha), the period of adequate forage quality (CP>7.5%) was 83 days. In 1993 grasses did not respond to 97 mm of Jul./Aug. rain with renewed growth. During 1992, a drier than average year, grasses responded to midsummer (49 mm) and fall (69 mm) rains by maintaining greater than 7.5% CP for 185 days. Giant wildrye, a deeply rooted grass, supported high quality forage until mid August, but did not respond to late-season rain. In contrast, shallow rooted grasses like bottlebrush squirreltail, Sandberg's bluegrass, and the winter-annual cheatgrass responded to summer or fall moisture with herbage ranging from 10 to 16% CP, thereby supplying high quality late-season forage. With abundant moisture, managers will see rapid deterioration of forage quality that occurs as grasses advance through their reproductive stages of growth and generate a wealth of seed stalks. In drier years, timely precipitation may elevate desirable nutrient characteristics and the duration of adequate livestock/wildlife nutrition may expand.