RESUMEN
The objective of this study was to contribute to genetic improvement for carcass quality in Nellore cattle by estimating genetic parameters for carcass characteristics measured by traditional and novel ultrasound adjusted for body weight. Records of 19,910 Nellore breed animals born between 2001 and 2015 as part of a genetic improvement program were evaluated for this purpose. The (co)variance components and genetic parameters for yearling weight (YW), slaughter conformation at yearling age (SCY), ribeye area (REA), REA adjusted for 100 kg of body weight (REA100), REA adjusted for 450 kg of body weight (REA450), subcutaneous fat thickness over the loin (SFTL) and the rump (SFTR), intramuscular fat (IF), and height × width ratio of the sirloin steak (RATIO) were estimated using the restricted maximum likelihood (REML) method and the three-trait animal model. The estimated heritability values were 0.39 (YW), 0.33 (SCY), 0.31 (REA), 0.42 (REA100), 0.38 (REA450), 0.31 (RATIO), 0.17 (SFTL), 0.27 (SFTR), and 0.41 (MAR). The genetic correlations between REA and YW (0.43) and SCY and YW (0.78), respectively, suggested that selecting for REA or SCY could result in an increase in body weight. Because of its moderate and negative correlation with YW, REA100 was ineffective as a muscling indicator (- 0.38). The indicators REA450 and RATIO had a - 0.12 and 0.11 low correlation with YW, respectively. Thus, it is concluded that REA450 and RATIO were the best indicators of muscling, with heritability of medium magnitude, making the selection viable, as well as having a low correlation with body weight.