The maximum age for cows in the foundation herd was 11 yr, but cows were not culled until they were 12 yr old. The foundation herd was generated on the first day of the simulation. Starting values are theoretical quantities and must be estimated from literature values or other information. The starting values for all four traits are given by breed in Table 1. Hybrid Vigor .
Rows represent breed of sire, and columns represent breed of dam.
Diagonal elements are zero because hybrid vigor is not associated with purebred animals. We used these percentages for BW, YW, and MW and assumed no differences between reciprocal crosses to calculate the values for F 1 vigor in Table 2. Hybrid vigor for MP is assumed to change throughout lactation, increasing monthly by a fixed rate over an eight-mo period. 785.9 1,764.0 22.1 Table 2. We changed the sex ratios in the maternal and terminal parts by inputting different values for the expected proportion of maternal females born.