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Relationship of Rumen Fluid Dilution Rate to Rumen Fermentation and Dietary Characteristics of Beef Steers^1,2,

New Mexico State University, Las Cruces 88003-0009

Abstract

Data from seven beef steer trials were compiled and regression analyses used to evaluate relationships among molar proportions of acetate (Ac), propionate (Pr) and butyrate (Bu), total concentration of volatile fatty acids (VFA), rumen ammonia (NH₃), rumen pH, rumen fluid dilution rate (FDR), rumen fluid volume (FVOL), body weight (WT), dry matter intake (DMI) and dietary concentration and intake of crude protein (CP and CPI), acid detergent fiber (ADF and ADFI), ash (ASH and ASHI) and metabolizable energy (ME and MEI). Of the six fermentation variables, Pr (negative regression coefficient, β) and pH (positive β) were related (P<.05) to FDR, but only 3 and 12% of the variation in these two variables, respectively, was explained by FDR. When FDR was described by dietary characteristics, ASHI was positively related to FDR (R² = .16). The best two-variable model for FDR contained DMI (positive β) and WT (negative β) and increased R² to .36. Fluid volume was best described by ME (positive β; R² = .20). The two-variable model for FVOL added ASH with a positive partial β (R² = .23). When fermentation variables were regressed on dietary characteristics, Ac was best described by ADF (positive β; R² = .71). The variable that best described Pr proportion was ADF (negative β; R² = .50), and addition of CP (negative β) and MEI (positive β) into the Pr model improved R² to .70. Molar proportion of butyrate was related to CP (positive β; R² = .23), and addition of ME (positive β) to the model improved the R² to .31. Total VFA concentration was best described by ADFI (positive β; R² = .14). An R² of .29 resulted when ME (positive β) and CPI (negative β) were included in the total VFA model. Rumen pH was related to ADF (positive β; R² = .45), and addition of CP (positive β) to the rumen pH model increased R² to .55. Crude protein concentration was related to ruminal NH3 level (positive β; R² = .42), and inclusion of ADFI (positive β) into the model improved the R² to .47.

¹ Journal Article 1108 of the New Mexico Agr. Exp. Sta., Las Cruces.

² Dept. of Anim. and Range Sci.