Effect of ruminal CO2 on gas exchange and ventilation in the Hereford calf.

The contribution of ruminal CO2 to gas exchange measurements and ventilation was determined in four rumen-fistulated Hereford steers at rest and during exercise. The calves were exercised at 1.4 and 2.2m X s-1 under three treatments: 1)full rumen with fistula sealed, 2) full rumen with fistula open, and 3) empty rumen. Measurements also were made at rest while flushing the empty rumen with either 100% N2 or a mixture of 50% CO2-50% N2. O2 consumption, CO2 production (Mco2), and ventilation were measured by collecting the expired gas. Absorption across the ruminal epithelium during rest increased Mco2 by 3%, whereas absorption and eructation together increased Mco2 by 15%. The respiratory exchange ratio (R) was significantly different among the three treatments at rest, but no differences were observed in R among the treatments during exercise. No changes were observed in minute ventilation among the three conditions, but a decrease in respiratory frequency and an increase in tidal volume occurred when the rumen was empty. These changes in ventilatory pattern may have been due to a decrease in body temperature when the rumen was empty. When the empty rumen was flushed with 50% CO2, Mco2 was increased 21% over the value observed when flushing with 100% N2. CO2 of fermentation origin is added to the expired gas by both eructation and absorption and has a significant effect on R in the resting animal, but no effect on R during exercise.