Precise methods for measuring livestock body and carcass composition are essential for both animal and meat scientists. The aim of this study was to calibrate the 11th rib cut dissection method for the estimation of crossbred beef-on-dairy bull empty body (EB) and carcass compositions against reference tissue and chemical postmortem measurements. Sixty-six (66) crossbred bulls from Angus, Limousin and Simmental sires (n = 22 each) crossed on Brown Swiss dams were serially slaughtered along growth from 58 to 534 kg BW. The muscle, adipose tissue and bone contents of the left 11th rib were determined by physical dissection. Linear regressions followed by leave-one-out-cross-validation were tested between rib dissection variates (with or without additional ones: BW or carcass weight, carcass grading or postmortem linear measurements) and reference EB or carcass chemical (water, lipids, proteins, minerals and energy) and tissue (muscles, adipose tissues and bones, only for final slaughter group of 514 ± 12 kg BW, n = 30) compositions. When all bulls are considered (serial slaughter group, n = 66), the inclusion of rib dissection variate together with BW or hot carcass weight allowed precise estimations of EB and carcass masses and proportions of water [R2 ≥ 0.91, residual CV (rCV) ≤ 3.1%], lipid (R2 ≥ 0.88, rCV ≤ 14.0%), protein (R2 ≥ 0.23, rCV ≤ 3.7%) and energy (R2 ≥ 0.89, rCV ≤ 7.7%). Slight further improvements in precision were achieved when carcass grading conformation or fat scores was added to the multiple estimative regressions. Crossbreed effect was significant on the intercept of most of the predictive equations. Especially ×Angus had higher intercepts for lipids, energy and adipose tissues and lower ones for water, proteins and muscles, when compared to ×Limousin and ×Simmental. Further developments using for example rib imaging analysis rather than physical dissection may contribute to large scale and high-throughput phenotyping of body and carcass compositions.