Comparison of ultrasound transducers to determine rump fat thickness in mature horses at maintenance
Rump fat measurements are utilized to assess changes in subcutaneous fat thickness when conducting feeding trials in horses. Commonly, these measurements are taken using a transducer that is designed for use in the rectum to evaluate the reproductive tract. This study was conducted to determine if measurements obtained using a rectal transducer were comparable to measurements obtained using a transducer designed to measure carcass characteristics. Rump fat measurements were obtained from 30 mature horses (368 to 552 kg and 5 to 10 yr) that were part of a feeding trial. Measurements were taken at 28-d intervals for a period of 154 d at a point half way between the points of the hip (Tuber coxae and Tuber ischiadicum) and 6 cm from the midline of the horse with the transducer positioned perpendicular to the midline of the horse. Images were taken with an Aloka SSD 500V real-time ultrasound machine (Corometrics Medical Systems, Wallingford, CT) equipped with a 17.2-cm, 3.5-MHz linear transducer (carcass probe) and a 6-cm, 5-MHz linear transducer (rectal probe). All measurements using the carcass probe were conducted by a certified technician who captured images that were analyzed at the Centralized Ultrasound Processing Lab (Ames, IA). All measurements using the rectal probe were conducted by a non-certified employee using on-screen diagnostic tools on the scanner. Longissimus muscle (LM) area between the 17th and 18th ribs and fat thickness ¾ the length ventrally over the LM were also obtained using the carcass probe. Least squares means for rump fat thickness (RFT) and LM fat thickness (LMFT) were calculated using the GLM procedure of SAS. Pearson correlation coefficients were used to determine the relationship between RFT and LMFT. Mean RFT was greater (P<0.01) using the rectal probe (1.27±0.39 cm) compared to the carcass probe (1.00±0.36 cm); however, there was a strong, positive relationship between the two measurements (r=0.76, P<0.01). Measurements using both the rectal (r=0.48) and the carcass (r=0.33) probes also had a positive relationship (P<0.01) with LMFT with a greater correlation observed between LMFT and RFT measurements using the rectal probe. These data validate the utility of the rectal probe to measure RFT in horses. Although measurements were different between the rectal and carcass probes, measurements obtained using the rectal probe accurately assessed changes in RFT and exhibited a greater correlation to changes in fat thickness over the longissimus muscle.
horses, ultrasound, rump fat