Genetic correlations between carcass traits measured by ultrasonography and semen quality traits in Nellore cattle incorporating genomic information

Nellore cattle (Bos taurus indicus), the predominant beef cattle breed raised in Brazil, has substantial economic importance in the global beef market. Ultrasonography is commonly used to measure carcass traits, which facilitate the genetic evaluation of live animals and provides indicators of meat quality. Male reproductive efficiency assessed by semen quality and scrotal circumference traits directly influences farm profitability. This study estimated variance components and genetic correlations between ultrasound-based carcass and semen quality and scrotal circumference traits in Nellore cattle. We used a dataset of 116,011 animals with phenotypic records, 13,632 genotyped animals, and a pedigree with 635,175 animals. Heritability estimates for scrotal circumference at 18 and 24 months were moderate (0.39 ± 0.008 and 0.38 ± 0.02, respectively). Semen quality traits, including motility, vigor, and mass activity were found to be lowly heritable with estimates ranging from 0.03 ± 0.01 to 0.05 ± 0.01. The heritability estimates for sperm defects were also low and ranged from 0.01 ± 0.007 to 0.12 ± 0.02. Carcass traits, including ribeye area and rump fat thickness, were moderately heritable (0.34 ± 0.05 and 0.39 ± 0.06), while subcutaneous fat thickness had a heritability of 0.18 ± 0.04. Genetic correlations between carcass with semen quality and scrotal circumference traits were of low to moderate magnitude and unfavorable. These findings indicate that carcass traits can be improved through genetic selection and the generally low genetic correlations with sperm quality traits and scrotal circumference traits suggest that selection for improved carcass traits, measured using ultrasonography technology, is expected to have minimal impact on semen quality. Therefore, simultaneous selection for both trait groups is recommended to ensure optimal long-term genetic improvement in Nellore cattle breeding programs.