Previous experiments have demonstrated the effects of diet composition, energy substrate and stage of lactation on the energy balance of lactating Friesian cows (Beever et al 1989). The purpose of the present experiment was to extend these studies by measuring energy balance and respiratory exchange in parallel with direct measurements of changes in body composition by slaughter. (See accompanying summaries).
Nine cows were selected on the basis of age, parity, live weight and previous lactational performance to be similar to those in an associated feeding trial (Sutton et al 1992). They were allocated to 3 (L), 6 (M) and 9 (H) kg DM/day of concentrates (206g CP and 12 MJ ME per kg DM) with grass silage ad libitum (206g CP and 10.4 MJ ME per kg DM). Measurements of energy exchange were made at five stages of lactation, weeks 5, 10. 16, 22 and 28. Gaseous exchange was measured during each stage of lactation for 4 days using open-circuit respiration calorimetry followed by a 6 day collection of faeces and urine.