Nutritional dynamics of 7 northern Great Basin grasses.

Abstract

Land, livestock, and wildlife managers need to understand the nutritional dynamics of forages to sustain adequate growth and reproduction of their animals and/or assure equitable payment for forages. Despite a long history of livestock grazing in the northern Great Basin, seasonal and annual nutritional dynamics of many of the region's prominent grasses have not been described. We addressed this issue via monthly sampling of 7 cool-season grasses at 6 sites during 1992, a drier than average year having 86% of mean precipitation, and 1993, when above average precipitation (167% of average) occurred. With high yields predicted in 1993 (1,257 kg ha(-1)), the period of adequate forage quality [crude protein (CP) greater than or equal to 7.5%] was 83 days. In addition grasses did not respond to 97 mm of July-August rain with renewed growth. 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 period of adequate forage quality especially during the lower than average moisture year. Giant wildrye (Elymus cinereus Scribn. &Merr.), a deeply rooted grass, supported high quality forage until mid August, but did not respond to late-season moisture. In contrast, shallow rooted grasses like bottlebrush squirreltail (Sitanion hystrix (Nutt.) Smith), Sandberg's bluegrass (Poa sandbergii Vasey), and the winter-annual cheatgrass (Bromus tectorum L.) responded to summer or fall moisture with herbage ranging from 10 to 16% CP, thereby supplying high quality late-season forage. With most precipitation occurring in the northern Great Basin during colder months, livestock or habitat managers can, with a fair degree of certainty, predict yields from their pastures before turnout. With abundant moisture, managers will see the rapid deterioration of forage quality that occurs when grasses advance through their reproductive stages of phenology and generate a wealth of reproductive stems. The quandary arrives, however, when moisture accumulations are less than optimum. Fewer reproductive tillers develop, and our results show that timely precipitation may elevate desirable nutrient characteristics and expand the duration of adequate livestock/wildlife nutrition in the region. More long-term research is needed to decipher the mechanisms governing growth and development of rangeland grasses and to assess risks of various stocking alternatives when managers face uncertain yield and forage quality issues.

DOI:10.2458/azu_jrm_v54i6_ganskopp