GENETIC ANALYSES OF COW LIFETIME PRODUCTION UP TO 12 MATING YEARS IN CROSSBRED BEEF-CATTLE

A total of 1088 females of 14 breed groups (Angus and Hereford purebre ds, and 12 first-cross groups) were evaluated over two locations for l ifetime survival, numbers of calvings, numbers of calves weaned and co w lifetime records of calf survival. These traits are known to be rela ted to a cow's lifetime productivity. The animals were part of the Rua kura Beef Breed Evaluation, designed to compare the growth and carcass es of steers, and the reproductive and maternal performance of females of different breed groups. Data were from 4 birth years of females an d 11 breed-groups at location 1, and from 5 and 10 respectively at loc ation 2, with seven breed-groups common to both locations. Females wer e first mated as yearlings. Culling at ages 2.5 to 4.5 years was based mainly on females that were non-pregnant on two occasions, whereas in subsequent years any non-pregnant female was culled. At location 1, t here was a maximum possible number of mating years of 12 for females i n the 1st birth year, declining to a maximum of 9 for those in the 4th birth year; for each age group at location 2 there was a maximum of 9 mating years. The average cow survival (number of mating years) was 7 .26 (s.d. 3.02) at location 1 and 5.81 (s.d. 2.31) at location 2, with a coefficient of variation similar at both locations and averaging 0. 41. The performances from the poorest to the best breed groups had a 1 .5-fold range for number of mating years and a 1.8-fold range for numb er of calvings and number of calves weaned. The heritability of number of mating years (no. = 150 sire groups) was 0.13 (s.e. 0.08), number of calvings 0.11 (s.e. 0.08), number of calves weaned 0.15 (s.e. 0.08) , and calf survival as a cow trait 0.027 (s.e. 0.018). This last herit ability increased to 0.093 if adjustment was made to the underlying li ability scale. There was no significant effect of breed of cow on numb er of mating years, nor on number of calves weaned per 100 calvings at either location, whilst the effect was significant for number of calv ings and for number of calves weaned per cow (P < 0.10). The wide bree d variation pointed to opportunities for selection among breeds, whils t the low heritabilities suggested that within-breed selection will be slow unless early indicator traits can be found.