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Development and Validation of a Prediction Equation Estimating Heat Production by Carbon Dioxide Entry Rate Technique¹

University of California, Davis 95616^,2 and U.S. Department of Agriculture, Clay Center, NE 68933^,3

Abstract

A series of experiments was conducted to validate the CO₂ entry rate technique (CERT) for prediction of heat production (HP) of sheep. Finnsheep cross wethers were used. Carbon dioxide production was estimated by continuous infusion of NaH¹⁴CO₃ intraperitoneally and collection of saliva. Times required for ¹⁴C to equilibrate with the body CO₂-bicarbonate pool and excretion of ¹⁴C in feces and urine were determined in four wethers (45.5 ± 1.7 kg) infused for 3 d. Retention of radioactivity was measured for wethers (29.0 ± 1.9 kg) slaughtered 3 h and 3, 10 and 15 d postinfusion. Using an indirect respiration calorimeter, CO₂ production estimates were compared to values derived by CERT for six wethers (45.0 ± .4 kg) fed at low, medium and high levels of intake. Further data on feed intake level and CO₂ production were obtained from 24 wethers in two weight groups (29.5 ±1.1 and 42.3 ± 1.4 kg) fed at three levels of intake. From 12 to 20 h were required for equilibration of NaH¹⁴CO₃ and the body CO₂-bicarbonate pool. Radioactivity of the saliva samples declined rapidly after cessation of infusion. Fecal and urine excretion of ¹⁴C was minimal. No detectable ¹⁴C was found in tissue of animals slaughtered after CERT. Estimates of daily CO₂ production did not differ between the calorimetry and CERT measurements (20.6 vs 20.3 liters/kg body weight ^.75). Although feed intake levels were different, HP and respiratory quotients (RQ) did not differ between the methods. In the second calorimetry experiment, feed intake level was correlated with estimated HP. Respiratory quotient values did not differ among intake levels. A linear prediction equation for HP based on CO₂ production fit the calorimetry data very well (r² = .97). The equation was validated with calorimetry data on 96 fed and 31 fasted sheep. The equation predicted HP very closely for fed sheep (slope = 1.06 ± .06), but consistently underestimated HP of fasted sheep (slope = .47 ± .01). The CERT provides accurate predictions of CO₂ production and HP for fed sheep over a wide range of intake levels.

¹ Mention of a trade name, proprietory products or specific equipment does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products that may also be suitable.

² Dept. of Anim. Sci.

³ Roman L. Hruska US Meat Anim. Res. Center.

⁴ Present Address: American Inst, for Goat Research, Langston Univ., Langston, OK 73050.

⁵ Present Address: USDA-ARS, US Dairy Forage Res. Center and Univ. of Minnesota, St. Paul 55108.