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ABSTRACT: Animals vary in the way in which they use energy due to diet, genetics, and management. Energy consumed by the animal supports milk production, but considerable variation among animals in energy utilization is thought to exist. The study objective was to estimate the among-animal variance in energy utilization in data collected from Jersey cows using indirect calorimetry. Individual animal-period data from 15 studies (n = 560) were used. The dataset included 115 animals from 44 to 410 DIM producing 11.5 to 39.1 kg/d of milk. On average, the 63 treatments in the dataset ranged 14.8% to 19.5% CP, 21.4% to 43.0% NDF, 16.2% to 33.3% starch, and 2.21% to 6.44% crude fat. Data were analyzed with the GLIMMIX procedure of SAS (9.4) with random effects of cow, treatment nested within period, square, and experiment. The percentage of among-animal, dietary treatment, and experimental variance was calculated as the variance associated with each fraction divided by the sum of variance from animal, dietary treatment, experiment, and residual, which was considered the total variance. The percentage of among-animal variance was characterized as high or low when the value was greater than or less than the mean value of 29.2%. Among-animal variance explained approximately 29.3% to 42.5% of the total variance in DMI, gross energy (GE), digestible energy (DE), ME, and NEL in megacalories per day. When energetic components of feces, urine, heat, and lactation in megacalories per day were expressed per unit of DMI, the among-animal variance decreased by 20.4, 4.82, 9.55, and 3.80 percentage units, respectively. However, among-animal variance explained 4.80 and 5.02 percentage units more of the total variation for methane energy and tissue energy in megacalories per day when expressed per unit of DMI. Variance in energetic efficiencies of DE/GE, ME/GE, and ME/DE were explained to a lesser extent by among-animal variance (averaging 17.8 ± 1.95%). The among-animal contribution to total variance in milk energy was 28.8%. Milk energy was a large proportion of the energy efficiency calculation, which included milk energy plus corrected tissue energy over net energy intake, which likely contributed to the 22.2% of total among-animal variance in energy efficiency. Results indicate that among-animal variance explains a large proportion of the total variation in DMI. This contributes to the variance observed for energy fractions as well as energy components when expressed in megacalories per day. Variation in energetic loss associated with methane was primarily explained by differences among animals and was increased when expressed per unit of DMI, highlighting the role of inherent animal differences in these losses. |
