Refinements in Determining the Energy Value of Body Tissue Reserves and Tissue Gains from Growth

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Abstract
Energy density of weight gain increases with age and weight for age. Gains by more mature cattle contain more fat, less protein, and less water than gains by less mature cattle. The energy density of body tissue reserves mobilized during negative energy balance in early lactation is greater than that in the apparently contributing weight gains. The data reviewed had 140% as much fat and 60% as much protein in mobilized reserves as in gains. These observed differences justified the requantification of the energy density of tissue reserves mobilized for milk synthesis. The energy of gains was estimated by using equations published by the Agricultural Research Council to predict the fat and protein contents of the gains and multiplying them by their respective energy densities. The energy of mobilized reserves (weight losses) was estimated by predicting the fat and protein per kilogram of the contributing gains with the Agricultural Research Council equations, multiplying those fractions by 1.4 and .6, respectively, and multiplying the adjusted fractions by the respective energy densities of fat and protein. There was a close agreement between the predicted and observed energy values of both weight gains and weight losses.