Metabolizable Energy Requirements for Maintenance and Efficiency of Energy Utilization in Azawak Bulls Using Indirect Calorimetry

Accurate estimates of maintenance energy requirements are essential for precision feeding, yet breed-specific values remain scarce for Azawak cattle. This study quantified net energy for maintenance (NEm), metabolizable energy for maintenance (MEm), and the efficiency of metabolizable energy use for maintenance (km) in Azawak bulls. Nine bulls were evaluated in a replicated 3 × 3 Latin-square design and assigned to three graded metabolizable energy supply levels, designated low, intermediate, and high. The feeding levels were established by varying daily feed allowance while maintaining the same basal forage-based ration. Indirect calorimetry was performed using the GreenFeed system to quantify respiratory gas exchange, urinary nitrogen was determined, and heat production was calculated according to Brouwer’s equation. Increasing energy supply significantly (p < 0.05) enhanced oxygen consumption, carbon dioxide and methane production, urinary nitrogen excretion, and heat production. The heat production (HP) and metabolisable energy intake (MEI) relationship was analysed using a linear mixed-effects model, with bull included as a random effect to account for repeated measurements. The resulting population-level relationship was ln(HP) = −1.1246 + 0.8985 × MEI. The estimated intercept was −1.1246 ± 0.0536 (95% CI: −1.2297 to −1.0195), whereas the MEI slope was 0.8985 ± 0.0850 (95% CI: 0.7320 to 1.0651; p < 0.001). Back-transformation of the intercept yielded an NEm of 0.325 MJ/kg BW0.75/day (95% CI: 0.292–0.361), whereas MEm, estimated at the point where predicted heat production equalled metabolizable energy intake, was 0.517 MJ/kg BW0.75/day (95% CI: 0.467–0.576). The corresponding km was 0.629 (95% CI: 0.566–0.688), indicating that 62.9% of metabolizable energy supplied at maintenance was converted into net energy. These results provide the first breed-specific maintenance energy coefficients for Azawak bulls and offer a robust basis for improving ration formulation, energy-use efficiency, and feeding precision in this important indigenous cattle breed.