Using serial ultrasound measures to generate models of marbling and backfat thickness changes in feedlot cattle

Serial ultrasound estimates of backfat thickness and marbling score were obtained on two groups of steers fed high-energy rations for averages of 166 and 148 d. These measurements were used to develop mathematical models that described the changes in those two attributes as functions of days on feed. An exponential model best described the increase in backfat thickness, and the rate coefficients were .0117 and .0096 for the two groups (doubling times = 59 and 72 d, respectively). A modified power function was better than either a linear or exponential model for describing the development of marbling. In this model, marbling increased very slowly, approximately one degree every 100 d, in the interval from low Select to low Choice but then increased at faster rates as initial marbling scores became higher. Ultrasound backfat measures could be used to predict days to reach a target carcass backfat level with an average error of 30 d or less when backfat thickness averaged more than 3 mm at evaluation. Projections from measurements taken when cattle were started on feed were not accurate from one group, which averaged only 1.7 mm backfat at that time. Projections of carcass marbling improved as evaluation date neared slaughter date. Projections from the initial evaluation date allowed tentative categorization of candidates for Choice or not Choice but were only 64% accurate. Projections from evaluations made later in the feeding period exceeded 75% accuracy in distinguishing Choice from Select. Only small relationships (r2 = .07 or .16) existed between carcass backfat thickness and marbling score. Results indicate that ultrasound estimates of backfat and marbling made during the feeding period can be used to predict carcass merit at slaughter.