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The present study aimed to estimate variance components and heritability coefficients (h2) in Santa Inês sheep by single- and two-trait analyses for the traits loin eye area (LEA), length (LEL) and maximum depth (LED), in the longissimus dorsi muscle, subcutaneous fat thickness (SFT), fat thickness over the biceps femoris muscle (FTBF), croup height (CH), thoracic circumference (TC), thoracic depth (TD), body length (BL), and adult live weight (ALW). The restricted maximum likelihood (REML) was used to estimate variance components in an animal model. In both types of analyses, we estimated moderate heritability for all traits, with the exception of SFT, CH, TC, BL, and ALW (all of which yielded low h2 estimates). In two-trait analysis for body size, only TD yielded a lower h2 estimate, when compared to single-trait analysis. On the other hand, CH, TC, and BL yielded higher h2 estimates. Most estimates for variance components and h2 in two-trait analysis were higher than those in single-trait analysis. This finding suggests the possibility of a moderate response to selection for improvement in the carcass of meat sheep using the specific carcass traits evaluated (except SFT), highlighting LEA. The fat thickness over the biceps femoris muscle showed higher potential for response to direct selection for fat deposition than SFT. Thoracic depth showed higher potential for response to direct selection for body size than the other morphometric measurements. In the present study, adult live weight was not an effective criterion for direct selection.

The objective of this study was to verify the impact of inclusion of initial age and weight as covariable in the analysis of body weight at 550 days of age, average daily weight gain, weight linear regression coefficient in relation to age, scrotal circumference, testicular volume, and visual scores for Nellore young bulls' body structure, precocity, and muscularity under pasture performance tests (PT). Single-trait analyses for the estimation of the parameters were carried out through four statistical models. In the first model, the fixed effect of PT and random additive genetic value were included. In the second and third model, the effects of initial age and weight were also included as covariates, respectively. In the fourth model, the effects of the two covariates were included. Growth and reproductive traits have moderate heritability and selection can contribute to changing the means of these traits. Morphological traits have low heritability, which suggests that selection is not as efficient as to modify its means. Covariates inclusion result in alterations in the variances, heritability and genetic values. Models with initial age and, or, initial weight as covariates are parsimonious for the genetic evaluation of Nellore young bulls' growth, reproductive and visual scores under pasture weight gaining tests. [ABSTRACT FROM AUTHOR]