Evaluation of Batch Culture Approaches That Can Be Used to Screen Release Rates of Ruminal Buffers

¹ Animal Science Department, Oklahoma State University, Stillwater 74078

Our objectives were to evaluate in vitro characteristics of ruminal fluid from several diets and from several substrates that can be used to evaluate release rates of ruminal buffers. Ruminal fluid was collected from three cows fed diets of concentrate and sorghum silage in three ratios: 70:30, 60:40, and 50:50 (DM basis). Ruminal fluid was incubated in a shaking water bath with either purified corn starch, the same grain mix that was fed to the cow that provided the ruminal fluid for incubation, or the TMR fed to the cow that provided the ruminal fluid. In addition to this substrate, incubation flasks received either .5 g of a 2:1 mixture of NaHCO₃ and MgO or no buffer. A flask representing each substrate and buffer combination was removed every hour for 5 h and analyzed for pH, buffering capacity, buffer value index, and VFA content. Each of the substrates yielded temporal alterations in ruminal fluid acid-base status similar to those observed previously in vivo. However, because ruminal fluid acidity tended to develop more rapidly with TMR as a substrate, we recommend that TMR be used as the substrate in batch culture to evaluate release rates of ruminal buffers. Addition of the NaHCO₃ and MgO yielded an increase in ruminal fluid pH similar to that observed previously in vivo. Ruminal fluid acidity was highest for fluid from cows consuming the 70:30 diet; however, the high gas content of this fluid prevented accurate volume measurement for the incubations. Based on handling characteristics and temporal acid generation, we suggest that donor cows be fed a 60:40 grain to forage ratio to provide ruminal fluid for batch culture incubation; a combination of 75 ml of this fluid, .5 g of this TMR, and .5 g of the test buffer provided a model that was acceptable for rapidly screening the release rates of ruminal buffers in vitro.

Key Words: in vitro • buffers • ruminal fluid • substrate

Submitted on March 25, 1992
Accepted on June 18, 1992