Variance component estimation on female fertility traits in beef cattle

The purpose of this review is to define possible breeding objectives for Southern African beef cattle farmers and to review different means of expressing genetic reproductive merit. The breeding objective considered was to maximize the number of calves born or weaned for a given number of cows in a herd under prevailing environmental and management conditions. This is a complex trait that has many components. While this trait is clearly a function of the reproductive ability of each cow, it is also affected by the age structure of the herd. A number of auxiliary or index traits are used to assess this objective and their merits, shortfalls and requirements in terms of data collection are discussed. It was evident that the trait of choice was influenced by the management system. Fewer component traits can be measured and incorporated as variables in a genetic model in more extensive systems. Since South African beef cattle production systems tend to be semi-extensive or extensive, traits to consider include calving rate, calving success, calf survival, days to calving, age at first calving, calving date, calving ease and calving interval. _______________________________________________________________________________________