Genetic Parameters of Post-partum Reproductive Status in Beef Cattle from Northern Australia

Lifetime reproductive performance is a major issue for the Northern Australian beef industry. Delayed cycling of lactating cows after parturition is one of major causes of reproductive inefficiency and impacts on the profitability of beef businesses. In the CRC for Beef Genetic Technologies, genetic markers have been used to develop prediction equations for the improvement of post partum anoestrus interval in tropically adapted beef cattle. An independent cattle population was established to validate these prediction equations. Data were collected at weaning on 4286 cows from 27 herds of 4 breeds in Northern Australia. Using ultrasonic ovarian scans and pregnancy tests, cows were scored for the reproductive status (REP3): 1) being pregnant (P), or 2) having a corpus luteum (CL), or 3) having a follicle (F). REP3 was also rescored into two binary traits: PREG2: pregnant (P) or not pregnant (F or CL) and HEAT2: cycling (P or CL) or not cycling (F). A threshold model was fitted to estimate genetic variance for these three traits. Analyses were implemented using both REML (sire model) and Gibbs Sampler (animal model) for the pooled dataset and two large breeds. The heritability estimates for reproductive status, either in REP3 or binary traits (PREG2 and HEAT2) were low to moderate. Results from REML and Gibbs Sampler were similar for REP3 and PREG2. The practical and important trait is PREG2. For this trait, the estimates of heritability in this study ranged from 0.15 to 0.22. These data may provide a useful resource for validating genomic prediction equations.