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Abstract Rumen pH, ammonia-N (NH3 -H), buffering capacity and volatile fatty acid (VFA) concentration were measured with four rumen-fistulated cows fed a corn silage and corn meal based diet with (WL; 2.5% of dry matter) or without limestone (WOL) at maintenance or two times maintenance levels of intake. Three crude protein (CP) levels (11, 14 or 17%) also were fed, resulting in 12 treatments. Each animal was fed all treatments. Limestone increased (P<.01) rumen pH at 11% CP (6.30 WOL vs 6.53 WL), while increasing protein increased (P<.01) rumen pH only in diets without limestone (6.30 at 11% CP vs 6.44 at 14% CP vs 6.49 at 17% CP). Fecal pH values, determined with another set of animals subjected to the same treatments, were increased (P<,01) by increased protein (5.98 at 11% CP vs 6.20 at 14% CP vs 6.36 at 17% CP) and by limestone (5.75 WOL vs 6.61 WL). Rumen ammonia values reflected dietary N intake (10.5 mg NH3 -N/dl at 11% CP vs 17.8 at 14% CP vs 29.9 at 17% CP). The concentration of VFA increased with each increase in protein, but was not affected by limestone. Rumen fluid buffering capacity evaluated between pH 7.0 and 5.5 followed a pattern of significant differences similar to that observed for rumen pH values. Rumen fluid buffering capacity of limestone treatments evaluated from pH 7.0 to 3.0 by units of .5 (data collected only at 14 and 17% CP levels) was increased (P<.01) between pH 5.0 and 4.5. Increasing CP from 14 to 17% increased (P<.01) the buffering capacity of rumen fluid between pH 6.0 and 4.5. Neither factor slowed rate of pH decline below pH 4.0. Level of intake had a significant effect on nearly all traits measured, but there were no significant level of intake × diet interaction effects. Copyright © 1982. American Society of Animal Science . Copyright 1982 by American Society of Animal Science