Abstract
Nine feeding experiments with dairy cows have been combined to assess liveweight change and its energy value at two stages of lactation. All animals were individually fed. Records of group average food intakes, milk yields and composition, and liveweights were available. Mean metabolisable energy (ME) intakes were calculated and estimates of energy balance (EB) derived by difference between ME intake and the ME requirement for maintenance and milk production according to the Ministry of Agriculture, Food and Fisheries (MAFF). Mean EB was regressed on mean liveweight change (LW) and highly significant ( P < 0.01) relationships found for heifers and cows in Weeks 1–10 of the lactation, the regression coefficients being 49.1 and 31.3 MJ ME/kg Δ for heifers and cows, respectively. The data could be pooled to give a coefficient of 39.5 ± 11.0 MJ ME/kg liveweight loss. This can be compared to the MAFF allowance of 28 MJ ME/kg (based on a net energy value of 20 MJ/kg liveweight change) and to the Agricultural Research Council (ARC) value of 34 MJ ME/kg liveweight change based on 26 MJ net energy/kg liveweight change. In mid lactation, when mean A W was small and positive, no significant relationship was found for heifers, but for cows a significant relationship ( P < 0.05) was found with a coefficient of 89.1 MJ ME/kg.
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