Steers from research crossbreeding projects (n = 406) were serially scanned using real-time ultrasound at 35-d intervals from reimplant time until slaughter. Cattle were evaluated for rump fat depth, longissimus muscle area (ULMA), 12th-rib fat thickness (UFAT), and percentage of intramuscular fat (IMF) to determine the ability of ultrasound to predict carcass composition at extended periods before slaughter. Additional background information on the cattle, such as live weight, ADG, breed of sire, breed of dam, implant, and frame score was also used. Carcass data were collected by trained personnel at "chain speed," and samples of the 12th-rib LM were taken for ether extract analysis. Simple correlation coefficients showed positive relationships (P < 0.01) between ultrasound measures taken less than 7 d before slaughter and carcass measures: ULMA and carcass LM area (CLMA, r = 0.66); UFAT and carcass 12th-rib fat thickness (CFAT, r = 0.74); and IMF and carcass numeric marbling score (r = 0.61). The same correlation coefficients for ultrasound measures taken 96 to 105 d before slaughter and carcass values (P < 0.01) were 0.52, 0.58, and 0.63, respectively. Steers were divided into source-verified and nonsource-verified groups based on the level of background information for each individual. Regression equations were developed for the carcass measurements; 46% of the variation could be explained for CLMA and 44% of CFAT at reimplant time, 46% of the variation in quality grade and 42% of the variation in yield grade could be explained. Significant predictors of quality grade were IMF (P < 0.001), natural log of 12th-rib fat thickness (LUFAT, P < 0.001), and ADG (P < 0.01), whereas LUFAT (P < 0.001), ULMA (P < 0.01), live weight (P < 0.001), hip height (P < 0.001), and frame score (P < 0.001) were significant predictors of yield grade. Regressions using ultrasound data taken 61 to 69 d before slaughter showed increasing R2. Live ultrasound measures at reimplant time are a viable tool for making decisions regarding future carcass composition.

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and Implications While the Iowa 4-H & FFA carcass contest cattle are not completely representative of all Iowa cattle, they can indicate trends for the Iowa cattle industry and represent the calves locally available for projects. With nearly 60% of these cattle receiving an $8/cwt premium, cattle feeders have the potential to improve net revenues by marketing cattle into a grid market.

and Implications Real-time ultrasound can be used to make marketing decisions for fed cattle. Of primary importance are evaluations for intramuscular fat, subcutaneous fat, and live weight. This information can then be used to stratify cattle into marketing groups ranging from need/ready to sell immediately (i.e. too heavy, too fat, or already reached Choice grade) to not ready for market until after feeding for several additional days (i.e. 35 days). Real-time ultrasound can also be used to identify cattle that are approaching the threshold of a higher quality grade. These cattle should benefit from additional days on feed, and should warrant further premiums in most market situations. Conversely, cattle that are near the upper end of the fat cover limit for a particular yield grade may move to the next higher yield grade with additional time on feed and be discounted.