21 GENETIC APPROACH TO IMPROVE FERTILITY IN CATTLE

In spite of the economic importance of fertility and increased knowledge of reproductive biology in the cow, efficiency of reproductive performance has continued to decline in dairy cattle. Improvements in fertility through genetic selection may be a possible approach to increase reproductive efficiency in dairy cattle. Although progress may be slow because of low heritabilities, ignoring fertility in genetic improvement programs has contributed to the current fertility problems. The objectives of this study were to (1) estimate genetic parameters and breeding values for fertility traits based on data from milk recording programs on a Holstein population and (2) develop a multi-trait selection index (dollar fertility value; $F) based on estimated breeding values. Data representing 3,282,843 lactations and 1,622,088 animals (cows and bulls from 1936 to 2007) were used for the analysis and obtained from official records maintained by the Argentinean Holstein Association (ACHA). Data were collected from official milk records, and lactations were standardized to 305 DIM for milk, protein, and fat. Gestation period was considered to be 282 days and restrictions were applied to ensure the quality of data (days open <40 and >350 days; calving interval <300 and >600 days; and contemporary groups with <25 lactations were eliminated). Days open (DO), calving interval (CI), age to first calving (AFC), and daughter pregnancy rate (DPR) were traits chosen for genetic parameter estimation. Daughter pregnancy rate was calculated from DO as PR = 0.25 × (233 DO); Kuhn et al. 2004 J. Dairy Sci. 87, 2277-2284. Animal models by lactation for each fertility trait included contemporary group (dairy herd and calving year), calving month, animal effect, milk production as a covariate and error term. Multiple-trait derivative-free restricted maximum likelihood (MTDFREML) was used to estimate covariance components (Boldman et al. 1995 ARS, USDA). Solutions for fixed effects, breeding values, and sampling variances (accuracies) were obtained for each trait. The proposed selection index ($F) included AFC and CI, with economic weights from -0.28 and -4.9 $/year per cow for AFC and CI, respectively (Gonzalez-Recio et al. 2004 J. Dairy Sci. 87, 3053-3061). Heritabilities for DO and DPR ranged from 2 to 7%; CI ranged from 3 to 8%; and AFC was 16%. Predicted transmitting ability values across different lactations ranged from -18.5 to 15.8 days, -23.15 to 16.0 days, -4.0 to 4.6%, and -120.3 to 76.2 days for DO, CI, DPR, and AFC, respectively. Values for $F ranged from -$89.16 to $147.12 in this Holstein population. Results indicate substantial variation in fertility traits, suggesting that genetic selection may be effective in improving declines in fertility. Asociacion Criadores Holando Argentino (ACHA).