Sources of variation in partitioning of intake energy for lactating Holstein cows

Variation in energy partitioning was quantified. Data were from 34 multiparous Holstein cows using indirect respiration calorimetry during the 6th, 10th, and 14th wk postpartum. For each period, cows were placed in digestion stalls for a 6-d excreta collection, followed by two consecutive 11-h determinations of methane and heat production. Sources of variation among cows were analyzed for gross energy consumed; for fecal, urinary, CH4, heat, milk, and maintenance energies; for tissue energy balance; and for tissue partitioning. Ratios of heat production, energy balance, milk energy, and maintenance energy to intake energy were also analyzed for sources of variation. A within-period model was used that contained fixed effects of treatment, parity, season, and covariates for metabolic BW and SCM. None of the diet or parity effects were a significant source of variation in any of the variables. Season effects were only significant for milk energy and water intake. As a covariate, SCM was highly significant for all variables except energy for heat production and energy for maintenance. However, metabolic BW as a covariate was a significant effect for heat production at wk 10 and 14 postpartum. The significance of metabolic BW differed between periods for most energy partition and efficiency measures. The most significant proportion of the variation in energy utilization was attributable to the energy in the milk.