The mean of SC365 and SC450 and AFC, BF, and RF were similar to those previously found in Nellore which indicates that although Polled Nellore is raised separately from common Nellore in Brazil, in general both herds have the same pattern of development (Barbosa et al., 2010; Boligon and Albuquerque, 2011; De Faria et al., 2015; Grossi et al., 2008; Regatieri et al., 2012; Yokoo et al., 2007; Yokoo et al., 2008).
The WBSF values reported here are comparable to the values commonly found for Bos taurus breeds (Wheeler et al., 2005), whose meat has good acceptability by the consumer for tenderness. On the other hand, Tizioto et al. (2013), reported greater values of WBSF in meat from Nellore cattle. Possibly, this divergence can be attributed to the intense meat quality selection conducted by OB for the last 15 years.
The AP per female averaged 130 ± 22.57 kg of calves weaned per year of life, like those reported for Nellore females by Schwengber et al. (2001) and De Faria et al. (2007). The value of AP is well below the average weaning weight (191 kg) of the animals participating of the Nellore Brazil Program because weaning weight does not include time factor as AP does (De Faria et al., 2007).
Heritability estimates for SC365 (0.31) and SC450 (0.37) indicated moderate magnitude, similar to those reported by Mercadante et al. (2000), but were lower than most reports in the literature, ranging from 0.49 to 0.65 (Corbet et al., 2013; De Faria et al., 2009). The present study was conducted using data from two Polled Nellore herds with a very similar genetic basis, therefore it had a lower population effective size and may present lower genetic variability for these traits. Moreover, SC was not a priority selection criterion for the herds analyzed in this study.
The 95% credible intervals and the posterior heritability estimates were similar for AFC and STAY (0.12 to 0.20 and 0.16, respectively), confirming several previous studies in Nellore cattle, ranging from 0.16 to 0.35 (Mercadante et al., 2000; Gutierrez et al., 2002; Regatieri et al. 2012) and 0.11 to 0.22 (De Vasconcelos Silva et al., 2003; (van Melis et al., 2010), respectively. Even with their low heritabilities, selection for these two traits can aid in genetic progress and maximize gains in reproductive efficiency of the herd (De Vasconcelos Silva et al., 2003; Eler et al., 2014).
The negative genetic correlation between AFC and STAY (-0.38) indicates favorable genetic correlations in terms of selection, that is, some of the genes that act to increase the scrotal circumference also act in favor of decreasing age at first calving, as well as in favoring the increase of the cow's ability to remain in the herd. Therefore, scrotal circumference can be considered a good criterion for the selection of sexual precocity for Nellore males and females, promoting genetic progress in important reproductive traits, as well as its positive and favorable genetic association with growth characteristics.
The posterior mean heritability for AP (0.25) lies within the 95% credible intervals (0.20 to 0.30) for AFC and STAY (both posterior mean heritabilities equal to 0.16). This suggests that AP may be adopted as a selection criterion by Nellore genetic improvement programs, favoring improved reproductive efficiency of females and males. Genetic correlations were favorable and high (Table 3) between AP and AFC (-0.69), STAY (0.83), SC365 (0.61) and SC450 (0.62). Therefore, when selecting for AP, we would also tend to increase the frequency of genes that also act to increase STAY, SC365 and SC450, and to decrease AFC. Considering the high and favorable genetic correlations found in this study, direct selection for any of these characteristics should favor improvements in the others.
Genetic correlations among characteristics measured in different animals (i.e., reproductive traits of males vs. females) may be estimated due to use of the kinship matrix to include genetic connections between the animals. By contrast, residual correlations can be estimated only for traits measured in the same animals (e.g., between SC365 and SC450 in males; AFC, STAY and AP in females). Residual correlations of the same direction with lower values, such as between AFC and AP and between AP and STAY (both equal to -0.24), indicate moderate non-additive genetic associations and/or environmental effects on these traits. By contrast, residual correlations were greater between SC365 and SC450 (0.74) and between AP and STAY (0.61).
The mean heritability estimates and the 95% credible intervals for BF (0.13, 0.09 to 0.19, respectively), RF (0.24, 0.18 to 0.31, respectively) and LMA (0.30, 0.25 to 0.36, respectively), were of low magnitude compared to values previously reported for Nellore cattle. Heritabilities for BF, RF, and LMA have ranged from 0.17 to 0.52 (Yokoo et al., 2008; De Faria et al., 2015); 0.23 to 0.65 (Zuin et al., 2012); and from 0.29 to 0.65 (Barbosa et al., 2010; Zuin et al., 2013; De Faria et al., 2015), respectively. However, despite of the lower magnitude, they indicated a response to direct selection, confirming their recommendation as selection criteria in the Nellore breed to improve carcass yield and quality.
The heritability estimate obtained in this study for WBSF (0.16) was of similar magnitude to Tizioto et al. (2013) (0.16) and De Castro et al. (2014) (0.11). However, direct measurement of WBSF is laborious and expensive, besides requiring the slaughter of the animals. Due to the difficulty of obtaining the phenotypic measures for WBSF, there are few reports in the literature regarding shear force in Nellore cattle, as well as its relationship with other characteristics of economic interest. More studies are necessary to investigate and identify other methodologies and tools to obtain new tenderness criteria.
Among carcass traits, it was observed that the posterior means genetic correlation and its 95% credible interval between BF and RF (0.49, 0.26 to 0.69, respectively) were high and positive, indicating that these traits are influenced by many of the same genes. However, the mean genetic correlations between LMA and BF and between LMA and RF were lower. Therefore, a small proportion of the genes responsible for LMA expression may be common only with BF, indicating that selection for increased LMA may also result in a slight increase in BF. Genetic correlations reported in the literature (Yokoo et al., 2008; Caetano et al., 2013; Zuin et al., 2013) for these same characteristics are small, indicating little genetic association and suggesting that selection for increased LMA should not necessarily influence fat thickness.
Residual correlations for the carcass characteristics studied indicated small non-additive genetic associations (dominance, overdominance and epistasis) and/or environmental effects, except for the residual correlation between BF and RF (0.34).
Genetic correlations between reproductive and carcass characteristics were of low to moderate magnitude, except between LMA and AP (0.70) (Table 5). These results suggest a possible association between AP and higher carcass yield, however further studies are needed to confirm this and elucidate the nature of this association.
Genetic correlations involving LMA with SC365, SC450 and STAY were positive and moderate, around 0.35, while with AFC it was slightly lower, however negative and favorable (-0.25). However, the correlations involving BF or RF were of low magnitude, although they were all favorable. Of particular note were the genetic correlations of BF with AFC (-0.28) and AP (0.23) and of RF with AFC (-0.23) and STAY (0.22). Thus, inclusion of reproductive characteristics such as scrotal circumference in males and the AP and AFC in females as selection criteria in genetic improvement programs in Nellore cattle should improve reproductive and productive efficiency, as well as carcass traits due to favorable genetic associations.
The residual correlations between most reproductive and carcass characteristics were close to zero. That is, there were little or no non-additive genetic effects and/or a common environment between them. Exceptions include residual correlations involving LMA with STAY (-0.49), SC365 (0.21) and SC450 (0.22), between BF and AP (0.35) and between RF and STAY (-0.28).
The 95% confidence intervals of the genetic correlation estimate of WBSF with SC365, SC450, LMA, BF and RF were very broad (Table 6), indicating imprecision of the estimates. The distribution of these estimates showed high standard deviations and the posterior means of WBSF with SC365 and SC450 were quite different. The estimated means for these genetic correlations were of low magnitude and could be considered null. These results indicate that selection to improve scrotal circumference and WBSF should be conducted independently.
Except for WBSF with SC365 (-0.84), residual correlations were close to zero, that is, the characteristics were not associated by common environmental effects. This is not surprising, since meat shear force is subject to multi-factorial environmental influences pre- and post-mortem, therefore, is much more susceptible to changes than those related to morphological characteristics such scrotal circumference and even carcass traits. In this data set, the environmental influences favorable to increased SC365 contributed favorably to decreased WBSF, suggesting that the environment interfered in sexual precocity and finishing in a positively.
Genetic correlations of WBSF with AFC, CAP and STAY were calculated based only on the genetic association contained in the kinship matrix, since reproductive efficiency measures were obtained only for females and that WBSF were obtained only in slaughtered males. Due to this limitation, and the inherent difficulty of obtaining measurements, biological interpretation of these values must be approached with caution.
Despite the crucial importance of the variables associated with the reproductive efficiency of the females for the sustainability of the beef production system, they presented low estimates of heritability, which should result in slower genetic progress. This low heritability for female reproductive traits is known to be strongly influenced by environmental factors and management.
Based on the results obtained in this study, it is expected that in the medium term, animals with greater sexual precocity will also have greater accumulated productivity and longer stay of females in the herd, along with superior carcass traits. However, due to the low heritabilities and small genetic associations with reproductive traits, fat thickness characteristics (BF and RF) will still require direct selection.
Due to the heritabilities of the scrotal perimeter measured at 365 and 450 days of age, as well as their genetic correlations with the reproductive characteristics of females, direct selection for them will indirectly result in improvements in accumulated productivity, stayability, and age at first calving.
Accumulated productivity proved to be an attractive selection criterion to be adopted by breeding programs, indirectly favoring rapid genetic progress for reproductive characteristics of females and males (SC365, SC450, AFC and STAY), as well as for LMA.