The direct and maternal genetic relationships between calving ease, gestation length, milk production and selected type traits
Focus in cattle breeding is shifting from traits that increase income, towards traits that reduce costs. As a result, an increasing number of functional traits, of which calving ease (CE) is an example, are included in national breeding indices. Yet, knowledge of genetic relationships between CE and other traits of interest is scarce. The same applies to gestation length (GL), a potential novel selection trait. Linear type traits often assumed to be associated with calving traits due to their function of describing the visual characteristics of an animal. Therefore, genetic relationships are estimated between CE, GL, selected type traits and milk production using a national dataset of 27,845 primiparous cow performance records separating genetic direct and maternal effects. Traits chosen cover type traits that have previously been associated with calving performance: udder depth (UD), chest width (CW), rump width (RW), rump angle (RA), mammary composite (MAMC), stature (STAT), and body depth (BD). The milk production trait chosen is the 305-d milk yield (MY-305). Multi-trait linear trivariate sire models were fitted using ASREML v3.0 where an additional random sire of the calf effect was included to account for the direct effect of CE and GL. Results show that high yielding individuals are genetically prone to spend a shorter time in utero of their primiparous dams (-0.19±0.09) and be born more easily (-0.45±0.14). Individuals experiencing a longer first gestation period are likely to be larger animals (0.48±0.14) with wider rumps (0.52±0.15) which have gestated shorter before being born (-0.30±0.13). And finally, heifers having a difficult calving are likely to be deeper individuals (0.47± 0.18) with wider rumps (0.41±0.20) and chests (0.55±0.20) and lower pin bones (0.51±0.20). This study further shows that it is feasible, and valuable, to separate direct and maternal effects when estimating genetic correlations between calving traits and other traits of interest. Differences in direct and maternal genetic correlations indicate that genetic relationships between CE, GL and type traits are clearly present but careful consideration is needed if these traits are utilised in national breeding indices.
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