The objective of this study was to determine technician effects of live animal ultrasonic estimates of fat thickness (FTU) and longissimus muscle area (LMAU). Steers (n = 36) representing four breed-types (Brown Swiss, Average Zebu-cross Mexican, Corriente Mexican, and typical British crossbred) of commercial slaughter cattle were isonified to estimate accuracy and repeatability of fat thickness (FT) and longissimus muscle area (LMA) measurements by two experienced technicians. Repeated measures of FTU and LMAU were taken by technicians on two consecutive days with an Aloka 500V ultrasound unit equipped with a 3.5-MHz, 172-mm scanning width, linear-array transducer. Ultrasonic estimates of fat thickness and LMAU were taken at the 12th and 13th rib interface 48 h before slaughter; carcass fat thickness (FTC) and longissimus muscle area (LMAC) were measured 48 h postmortem. Means for FTU, FTC, LMAU, and LMAC were .91 +/- .36 cm, .82 +/- .40 cm, 70.7 +/- 9.43 cm2, and 72.4 +/- 8.9 cm2, respectively. Ultrasound and carcass measures of FT and LMA were different (P less than .01) among breed-types but were not different (P greater than .10) between technicians or for technician x breed-type interactions. Pooled simple correlation coefficients (P less than .01) were .87 and .86 between FTU and FTC and .76 and .82 between LMAU and LMAC for Technicians 1 and 2, respectively. Repeatabilities estimated by intraclass correlation methods were .91 +/- .03 and .81 +/- .06 for images repeated over 2 d and .95 +/- .02 and .83 +/- .05 for images repeated by two technicians for FT and LMA, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrasonic measures of backfat thickness (BFT) and longissimus muscle area (LMA) taken on Angus bulls (n = 805) and heifers (n = 877) ranging in age from approximately 8 to 20 mo in a production setting in western Nebraska were used to estimate genetic (co)variances. Further information used in the analyses, including weaning weight (WWT), postweaning ADG, and scrotal circumference (SC), was obtained from the American Angus Association, St. Joseph, MO, for these individuals and for animals from the same herd and contemporary groups. Data were analyzed using single-trait, sire model, restricted maximum likelihood (REML) procedures to estimate starting variances for later two-trait analyses. These two-trait analyses were done to estimate variance components for WWT coupled with BFT, LMA, SC, and ADG, also using REML procedures, but with a sire-maternal grandsire model for WWT and a sire model for the other trait(s). Heritabilities for BFT, LMA, WWT direct, WWT maternal, SC, and ADG were .56, .11, .19, .24, .56, and .51, respectively. Genetic correlations between WWT direct and WWT maternal, WWT direct and LMA, WWT maternal and LMA, WWT maternal and BFT were -.57, .42, .01, and -.69, respectively. Genetic parameters from this study were used to calculate possible genetic change with a typical selection scenario, and it was shown that among WWT, BFT, and LMA, BFT could be affected the most by selection, relative to its mean.

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