Heat production determined by the RQ and CN methods, fasting heat production and effect of the energy intake on substrates oxidation of indigenous Manchega sheep

Heat production (HP) from CH4 and CO2 production, and O2 consumption was determined by the respiratory quotient (RQ) method and by the C–N balance (CN method). Twelve dry Manchegas sheep were fed with a diet constituted by alfalfa hay and barley grain at three levels, approximately 1, 1.5 and 2× metabolizable energy for maintenance (MEm). Later, the sheep were fed close to maintenance and HP determined by indirect calorimetry after 3 d fasting. The sheep were allocated to metabolism cages. After 10 d of adaptation, feed intake, and total fecal and urine output were recorded daily during 5 d, as well as the body weight (BW) at the beginning and end of the sampling period. Gas exchange measurements were recorded by a mobile open-circuit respirometry system. Average HP measured by RQ method was in agreement with the HP determined by CN method accounting for 410 and 407 kJ/kg0.75 BW/d, respectively. Fasting HP was 268 kJ/kg0.75 BW. Proportion of energy associated to the oxidation of protein (OXP), carbohydrate (OXCHO) and fat (OXF) was calculated. In fasting condition most of HP associated to substrates oxidation (HPx) was due to OXF (0.931) and less to OXP (0.037) and OXCHO (0.032). OXP was increased with the feeding level and was greater (0.193) than in fasting. OXCHO was increased with the feeding level until a stable level (0.507).