This study was conducted to determine the energy balance of beef cattle by indirect animal calorimetry utilizing a ventilated head box respiration system. Fifteen native Thai bulls were randomly allocated to one of three dietary metabolizable energy intake (MEI) levels (1.1 × maintenance, 1.5 × maintenance and 1.9 × maintenance) in a completely randomized design for a 116 day feeding trial. Animals were allocated to individual metabolic cages for the determination of digestibility and energy

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... ance. Heat production was determined from oxygen consumption, carbon dioxide and methane production. The results showed that dry matter, organic matter and crude protein intake were increased (P < 0.01), but digestibility of all nutrients, except neutral detergent fiber, was not significantly affected (P > 0.05) by MEI levels. The energy loss in feces and urine (% of gross energy intake) were not different (P > 0.05); however, enteric methane conversion rate (% of methane energy loss per gross energy intake) and heat energy production loss (% gross energy intake) were linearly decreased (P < 0.01) with increasing MEI levels. Methane conversion rates ranged from 8.4 to 10.0% and appeared to have been underestimated by the Intergovernmental Panel on Climate Change 6.5% default values set for cattle fed low quality crop residues and by-products. The estimate of metabolizable energy requirements for maintenance was measured using linear regression analysis derived for native Thai cattle was 520 kilojoules per kilogram of metabolic body weight per day. Increased dietary intake levels reduced enteric methane emissions in beef cattle fed on tropical feedstuffs. The results of the present study indicated that greater dietary intake feeding strategy in cattle fed above the maintenance level resulted in improved energetic efficiency utilization, and thus improved energy retention because of the reduction of enteric methane energy emission and heat production. Discipline: Animal industry Additional key words: cattle, energy requirements, enteric methane, tropical This paper reports results obtained in the international project on "Establishment of feeding standard of beef cattle and feedstuff database in Indochina." The Japan International Research Center for Agricultural Sciences, Department of Livestock Development, Ministry of Agriculture and Cooperatives, Thailand, and Khon Kaen University took responsibility for all subjects picked up in the project, implemented main subjects jointly and managed the project. Since it is essential to construct a regional research cooperation network for the efficient achievement of the targets, other research organizations such as