Body fat of stock-type horses predicted by rump fat thickness and deuterium oxide dilution and validated by near-infrared spectroscopy of dissected tissues

Body condition score and percent body fat (BF; %) of horses are positively correlated with reproductive efficiency and are indicative of metabolic issues. However, BF in horses may be poorly predicted because current procedures are either subjective or dependent on one anatomical location. Therefore, the objectives of the current study were to compare 2 methods of predicting BF using rump fat thickness (RFT) and deuterium oxide (D₂O) dilution with actual tissue fat analysis by nearinfrared spectroscopy (NIRS) in stock-type horses and to identify the relationship between BF and BCS. Twenty-four stock-type horses were selected to be humanely euthanized based on 3 primary criteria: geriatric, crippled, and/or unsafe. Approximately 20 h before slaughter, horses were weighed and BCS assessed to be 1 (n = 1; 433 kg), 2 (n = 1; 415 kg), 3 (n = 1; 376 kg), 4 (n = 7; 468 ± 13 kg), 5 (n = 10; 455 ± 11 kg), and 6 (n = 4; 493 ± 12 kg) and RFT was measured using ultrasonography. Blood samples were collected immediately before and 4 h after D₂O infusion (0.12 g/kg BW). Deuterium oxide concentration of plasma was determined by gas isotope ratio mass spectrometry. Horses were housed in a dry lot overnight before being individually sedated (1.1 mg xylazine/kg BW) and anesthetized using a jugular venipuncture (2.2 mg ketamine/kg BW), and potassium chloride (KCl) solution was administered to cease cardiac function before exsanguination. After euthanasia, horse carcasses were processed and dissected and tissues were collected for NIRS analysis. Body fat predicted by D₂O dilution was correlated with BF measured by NIRS analysis on various weight bases (r = 0.76 to 0.81, P < 0.001), whereas no correlation was observed for BF predicted by RFT. A paired t-test indicated that both D₂O dilution and RFT overestimated BF by 2.48 to 3.26% (P < 0.001) and 5.81 to 6.59% (P < 0.001), respectively, compared with NIRS analysis. Body condition scores affected BF measured by NIRS analysis (P ≤ 0.038) and predicted by D₂O dilution (P < 0.001) and RFT (P = 0.042). The current study provided evidence that D₂O dilution was a better prediction method for BF than RFT and that BCS may serve as a BF predictor in stock-type horses.