Genetic parameters of reproductive traits from artificial insemination records of Japanese Black cows

Decline in reproductive performance directly affects the cost of calf production in the beef industry. The objective of this study was to estimate heritability, repeatability and genetic correlation among female reproductive traits, and to determine the most appropriate trait for improving reproductive performance in Japanese Black cows. The traits studied were; non-return rate (NRR), number of inseminations (IN), days from first to successful insemination (FS), gestation length (GL), days from calving to first insemination (CF), days open (DO), pregnancy rate (PR), calving interval (CI) and age at calving (AC). Data across multiple parities were analyzed by using a multiple-trait model under which the performance of a trait in different parities was treated as a different trait, and by using a repeatability model for treating records of multiple parities as repeated measurements. AC comprises age at first calving (AFC), age at second calving (ASC) and age at third calving (ATC) under the multiple-trait model. Genetic parameters were estimated by restricted maximum likelihood (REML). The estimated heritability of traits showed little variation among parities. Heritability of AC was higher than of the other traits, although it was lower in later parities; 0.158, 0.107 and 0.088 for AFC, ASC and ATC, respectively. The highest repeatability was observed for CF (0.142) and the lowest for FS (0.036). The genetic correlations between the same traits across parities were generally high, ranging from 0.72 to 0.99. The lowest genetic correlation in absolute values was observed between CF and IN (-0.075) and the highest between DO and CI (0.957). AFC was concluded to be the most promising trait for genetic evaluation of female reproductive traits because it demonstrated higher heritability and moderate to high genetic correlations with NRR, GL, CF, DO, PR, CI, IN and FS. Selection of AFC will provide higher genetic improvement in reproductive performance of Japanese Black cows.