Predicting the composition of lamb carcases using alternative fat and muscle depth measures

Hot carcase weight (HCW) and the GR tissue depth (total tissue depth over the 12th rib, 110mm from the midline) were measured on 312 crossbred (Poll Dorset×Merino) lambs with a mean live weight of 47kg. At a position 30mm distal to the lumbar-sacral junction, the depth (RMD) of the rump muscles was measured along with subcutaneous fat depth overlaying the m. gluteus medius (RFD). The subcutaneous fat depth over the m. longissimus (loin) at the 12th rib was measured (Fat C), as was the depth and width of the loin cross section (EMD and EMW, respectively). The right side of each carcase was scanned by a dual energy X-ray bone densitometer and fat and lean weight determined and the percentage of lean and fat content in the carcases calculated. HCW alone accounted for 26% of the variation in percentage lean content (LC), with GR measurement being the single best predictor of LC (R(2)=48.1; r.s.d.=1.89). RFD provided a less accurate and precise estimate of LC (R(2)=37.3; r.s.d.=2.07) than either GR or Fat C (R(2)=46.5; r.s.d.=1.92). Inclusion of HCW with either GR or Fat C provided a small improvement in the amount of variation in LC which could be explained. The measurement of breeding animals by ultrasound includes the terms weight, Fat C and EMD. These predictors when combined explained 49% of the variation in LC whereas by comparison measures of weight, RFD and RMD only explained 42% of the variation in LC. The current method of measuring fat depth over the 12th/13th rib is practical and measurement over the rump does not present an advantage based on the results of this study.