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Effect of Crude Protein Level and Limestone Buffer in Diets Fed at Two Levels of Intake on Rumen pH, Ammonia-Nitrogen, Buffering Capacity and Volatile Fatty Acid Concentration of Cattle

US Department of Agriculture, Beltsville MD 20705

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 x diet interaction effects.

¹ Ruminant Nutr. Lab., Animal Science Institute, AR, SE, Beltsville, MD 20705.

² The authors gratefully acknowledge the assistance of R. L. Brocht, R. Spencer, F. E. Sweeney and K. De Cesaris for animal care; E. L. Yoder andT. B. Jacobs, Jr. for sampling and chemical analyses and P, C. Marcus for data handling.

³ Mention of a trade name, proprietary product or specific equipment does not constitute a guarantee or endorsement by the USDA and does not imply its approval to the exclusion of other similar products that also may be suitable.

⁴ Dr. Haaland passed away suddenly on March 29, 1982.

⁵ Formerly Ruminant Nutr. Lab., Beltsville, MD.

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