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Four equations were used to compare alternative procedures to adjust ultrasonic estimates (y) of backfat thickness (BF) and LM area (LMA) for BW using data from a series of 7 scans on 24 Suffolk ram lambs born in 2007. Equations were linear, linear + quadratic, allometric (y = αBWβ), and allometric + BW (ABW; y = αBWβeγW). Goodness of fit was very similar between equations over the range of the data. Resulting adjustment equations were tested using 3 serial scans on winter-born Suffolk (n = 150), Hampshire (n = 36), and Dorset (n = 43) rams and 52 fall-born Dorset rams tested at the Virginia Ram Test in 1999 through 2002. Partial correlations (accounting for the effect of year) between predicted and actual measures ranged from 0.78 to 0.87 for BF and 0.66 to 0.93 for LMA in winter-born rams and from 0.70 to 0.71 for BF and 0.72 to 0.78 for LMA in fall-born rams. No significant differences in predictive ability existed between equations for BF or LMA (P > 0.05), and there was no indication that the allometric equation was a better predictor than linear within the range of the data. Adjustment equations were also tested using serial scan data from 37 Suffolk ewe lambs born in the same contemporary group as the rams used to derive the prediction equations but fed for a substantially slower rate of BW gain. Correlations between predicted and actual values of BF and LMA indicated lambs were too young and small at the first scan (77 d, 32.4 kg) to reliably predict carcass measures at typical slaughter weights. For prediction using data from the 2 subsequent scans, at mean ages >96 d and mean BW >39 kg, correlations between predicted and actual values were 0.72 to 0.74 for BF and 0.54 to 0.76 for LMA. Little difference existed between equations for predicting BF. For LMA, the ABW form was a weaker predictor than the others, and the linear equation was slightly superior to allometric. Therefore, it appears the linear and allometric forms are both suitable for use in central

Market lambs from the state fair of Virginia (n = 172) were ultrasonically evaluated by 4 scan technicians and 3 image interpreters to determine accuracy of ultrasonic estimates of loin muscle area (ULMA), backfat thickness (UBF), and body wall thickness (UBW). Lambs were initially scanned at the preferred magnification setting of each technician; 2 chose 1.5× and 2 chose 2.0×. Lambs were then scanned a second time for ULMA and UBF with machine magnification settings changed from 1.5 to 2.0×, or vice versa, midway through the second scan. Lambs were then slaughtered, and analogous measurements [carcass loin muscle area, carcass backfat thickness, and carcass body wall thickness (CBW)] were recorded on chilled carcasses. Pooled, residual correlation coefficients within technicians and interpreters between ultrasonic measurements from the first scan and carcass measurements were 0.66 for loin muscle area, 0.78 for backfat thickness, and 0.73 for body wall thickness, but were reduced to 0.43, 0.69, and 0.50, respectively, by inclusion of linear effects of carcass weight in the model. Mean bias for technicians and interpreters ranged from −1.30 to −2.66 cm2 for loin muscle area, −0.12 to −0.17 cm for backfat thickness, and 0.14 to −0.03 cm for body wall thickness; prediction errors ranged from 1.86 to 2.22 cm2, 0.12 to 0.14 cm, and 0.35 to 0.38 cm, respectively. Pooled correlations between repeated measures were 0.67 for ULMA, 0.79 for UBF, and 0.68 for UBW at the same magnification and 0.73 for ULMA and 0.76 for UBF across different magnification settings. Mean differences between repeated measures were more variable among technicians and interpreters than statistics comparing ultrasound to carcass measures. Standard errors of repeatability ranged from 1.61 to 2.45 cm2 for ULMA, 0.07 to 0.11 cm for UBF, and 0.36 to 0.42 cm for UBW. The effect of changing magnification setting on technician and interpreter repeatability was small for UBF and ULMA. The accuracy of predic