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Assessment of equine body composition using objective measurements is difficult owing to the large size of the animals and the costs involved. Bioelectrical impedance spectroscopy (BIS), a technique widely used for the assessment of body composition in humans, was investigated for practicality of use in horses. BIS uses algorithms that require values for the apparent resistivities of body fluids and body proportion factors (Kb), currently not available for horses. Aims of the present study were to derive resistivity coefficients and body proportion factors and to validate their use for prediction of body composition horses. Validation of coefficients and predictive power using a split-sample agreement study design using correlation and limits of agreement analysis. Whole body impedance measurements were performed on 35 standardbred horses, yearlings to 14 yr, concurrently with determination of total body water volume (TBW) by deuterium dilution and extracellular water volume (ECW) by bromide dilution. Kb was determined in an independent group of 38 mixed-breed, age, and sex horses. Mean apparent resistivity coefficients were 511.4 and 1415.9 ohm.cm for intracellular water and TBW, respectively. Mean Kb was 1.52 ± 0.1. Using these coefficients, TBW and fat-free mass could be predicted with limits of agreement (2SD) of ± 11.6%; mean fat-free mass and fat mass were under- and overestimated by 3.1% and 14.1%, respectively, compared to measured reference values although these differences were not statistically significant. BIS is a practical technique for the assessment of body composition in equids, but the relatively wide limits of agreement, particularly for fat mass, may limit its usefulness for predicting body composition in individual horses. [ABSTRACT FROM AUTHOR]