Abstract
To study the effect of growth stage and environmental temperature on metabolizable energy requirement for maintenance (MEm) and efficiency for growth (kg) of heifers, 74 energy balance trials were conducted using 34 Holstein heifers. Seventy four energy balance trials were divided into three treatment groups. The first group is measured as early stage heifers under thermoneutral environment (n＝16, ET, 8～12 age of month, 18～20°C), the second one is measured as late stage heifers under thermoneutral environment (n＝21, LT, 16～20 age of month, 18～20°C), and the last one is measured as early stage heifers under hot environment (n＝37, EH, 7～12 age of month, 28～33°C). (1) Regression analyses of retained energy (RE, kJ/BW0.75) on intake of ME (MEI, kJ/BW0.75) gave the following equations : ET : RE＝0.456 (±0.070)×MEI－245 (±61) (R2＝0.927,) LT : RE＝0.522 (±0.038)×MEI－263 (±35) (R2＝0.965)，EH : RE＝0.564 (±0.028)×MEI－298 (±24) (R2＝0.806). Amounts of ME for maintenance (MEm) calculated as RE＝0 were 538±158, 505±77 and 529±50 kJ/BW0.75 for heifers of ET, LT and EH, respectively. MEm was not significantly different among three groups and was 524kJ/BW0.75. The kg were 46±7％, 52±4％, 56±3％ for heifers of ET, LT and EH, respectively. The kg of ET was lower than those of EH (P<0.001) and LT (P<0.06). (2) Retained energy as body fat of LT and EH were higher than that of ET. By use of all data (n＝74), the efficiency of ME to retained energy as body protein (REp) and fat (REf) is followed ; MEI－MEm＝2.77 (±0.18)×REp+1.21 (±0.10)×REf (R2＝0.963). The efficiency of ME to retained energy as body protein and fat are 36% and 83%.
