The most important factor that determines beef tenderness is its proteolytic activity and the balance between calpain1 protease activity and calpastatin inhibition is especially important, while contributions could arise from calpain2 and possibly calpain3. These processes are however affected by the meat aging process itself. To determine whether genotypes in the calpaincalpastatin system can enhance tenderness throughout a 20 day aging period, South African purebred beef bulls (n=166) were genotyped using the Illumina BovineHD SNP BeadChip, through genebased association analysis targeting the cast, capn3, capn2 and capn1 genes. The WarnerBratzler shear force (WBSF) and myofibril fragment length (MFL) of Longissimus thoracis et lumborum (LTL) steaks were evaluated between d 3 d 20 of aging, with protease enzyme activity in the first 20 h postmortem. Although several of the 134 SNP associated with tenderness, only seven SNP in the cast, capn2 and capn1 genes sustained genetic associations, additive to agingassociated increases in tenderness for at least three of the four aging periods. While most genomic associations were relatively stable over time, some genotypes within SNP responded differently to aging, resulting in altered genomic effects over time. The level of aging at which genomic associations are performed is an important factor that determines whether SNP affect tenderness phenotypes.