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Eight ruminally fistulated steers were observed on native range from 4 May 1981 to 5 Nov. 1981 to determine effects of advancing forage maturity on rumen fermentation, fluid passage, fluid volume, and forage intake. Effects of these factors are poorly defined for cattle on the Northern Great Plains but are essential for developing management strategies for optimum animal production. On 6 different dates, the steers were given an intraruminal dose of cobalt ethylenediaminetetraacetate (CoEDTA), and samples of rumen fluid were drawn at 4-hour intervals over a 24-hour time period. Rumen fluid samples were analyzed for volatile fatty acid, ammonia-N, cobalt concentration, and pH. CoEDTA was used as a marker to estimate rumen fluid passage and volume. Forage intake was determined by total fecal collection and in vitro digestibility of the forage. Total ruminal volatile fatty acid, molar proportions of individual volatile fatty acid, pH, and ammonia-N concentrations varied (P<0.01) within each of the six 24-hour periods, but the changes were dependent on date. Advancing forage maturity was associated with reduction in individual and total ruminal volatile fatty acid, ammonia-N, pH, and fluid dilution rate. Rumen fluid volume increased with increasing forage maturity. Variation in organic matter intake was small (P>0.05) over the range of forage maturities studied. We concluded that variation in rumen fluid passage, volume, and fermentation depended on forage maturity, and protein supplementation may be beneficial during late summer-early fall to increase or sustain animal production. This material was digitized as part of a cooperative project between the Society for Range Management and the University of Arizona Libraries. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020