|Bohnert, David - OREGON STATE UNIVERSITY|
Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 29, 2000
Publication Date: November 1, 2001
Citation: GANSKOPP, D.C., BOHNERT, D. NUTRITIONAL DYNAMICS OF 7 NORTHERN GREAT BASIN GRASSES. JOURNAL OF RANGE MANAGEMENT. 2001. 54(6):640-647 Interpretive Summary: Despite a history of cattle grazing in the northern Great Basin, seasonal and annual nutritional patterns of the region's prominent grasses have not been described. In a wetter year, protein content and digestability of grasses declined throughout the growing season. Grasses provided less than adequate forage for livestock by late June. Acceptable forage quality was available for only 83 days because grasses grew and withered quickly, and the forages did not respond to high levels of mid-summer rain. In the drier year, less forage was available, but adequate forage quality was sustained for 185 days. This happened because plant growth was limited by dry soils, but grasses tried to keep growing when mid-summer rainfall occurred. Fields with a mixture of deep and shallow rooted grasses provided the longest period of adequate forage. Shallow rooted grasses began growth earlier and grew again in the fall if rains occur. Deep rooted grasses supported high quality forage in the middle of the growing season when shallow rooted plants had already withered. To assure sustained weight gains and reproductive success in their animals ranchers should start providing supplements for cattle by late June in moist years. This information will help ranchers sustain good weight gains and reproduction in their cattle during wet and dry years.
Technical Abstract: Despite a long history of livestock grazing in the northern Great Basin, seasonal and annual nutritional dynamics of many of the region's grasses have not been described. We addressed this via monthly sampling of 7 cool season grasses at 6 sites during 1992, a drier than average year (86%), and 1993, when more precipitation (167%) occurred. With high yield expectations in 1993 (1,257 kg/ha), the window of adequate forage quality (CP>7.5%) was 83 days. In addition grasses did not respond to 97 mm of Jul.-Aug. rain. During 1992, a growing season beginning with less than average moisture, grasses responded to midsummer (49 mm) and fall (69 mm) rains by maintaining greater than 7.5% CP for 185 days. A diversity of grasses expanded the window of adequate forage quality especially during the lower than average moisture year. A deeply rooted grass, giant wildrye, supported high quality forage until mid August, but it did not respond to late-season moisture. 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 increased forage production but rapid deterioration of forage quality.