Number of Calves Produced at Specified Age as a Measure of Reproductive Performance in Beef Cattle under Artificially-Inseminated Breeding Scheme

Reproductive abilities in beef cattle herds are receiving increased attention due to recent rises in production costs. To achieve more efficient management, a measure of fertility, namely the number of calves produced at k yr of age ($NCP_k$), was developed and its genetic parameters were estimated from Japanese Black cows by restricted maximum likelihood procedures. The k examined were distributed from 2 to 10 yr of age and $NCP_2$ averaged 1.077 calves over 43,536 cows. The averages increased by approximately 0.9 calf with each additional 1 yr increment in k. Heritabilities of $NCP_k$ were estimated to be low ranging from 0.083 to 0.162, which seemingly suggested a difficulty of genetic improvement. However, large genetic variation and high accuracy were observed in predicted breeding values of $NCP_k$. For example, the breeding values of $NCP_7$ were predicted between -0.303 and +0.213 with average accuracy of 0.607 for cows with observations. Genetic correlations among different k were generally high and positive (0.474 to 0.995). The analyses showed that at least $NCP_4$ was required to maintain the genetic correlations of 0.8 or higher with subsequent $NCP_k$. Also $NCP_5$ maintained the genetic correlations of 0.9 or higher with subsequent $NCP_k$ The results suggested some possibilities for $NCP_k$ to be a selection criterion considering its genetic variation, high accuracy and consistency with subsequent performance.