138 GENETIC PARAMETER ESTIMATION FOR EMBRYO TRANSFER TRAITS

Abstract

Genetic selection has made tremendous progress on economically important traits in the beef industry. Most of the progress has been from quantitative genetics through use of expected progeny differences (EPD). These values allow prediction of differences in progeny of a sire compared to progeny of other sires. Development of EPD for male and female reproductive traits has largely been ignored because of low heritability of reproductive traits, even though reproduction plays a vital role in the economics of beef operations. Therefore, continued research in the area of genetic selection for fertility is becoming increasingly important. Critical limiting factors for animal breeding programs using MOET nucleus schemes include variability in superovulatory response of donor animals and resulting pregnancy of transferred embryos. Thus, the overall objective of this research was to develop genetic parameters associated with MOET to assist producers in identifying animals with greater genetic merit for these protocols. Records were examined from a large-scale MOET system in beef cattle that contained data only for cows in which at least one transferable embryo was obtained. Data on these animals were extracted and analyzed on 10 425 transferred embryos (2900 collections) from 611 donor animals (Angus, Brangus, and Charolais) utilizing semen from 215 bulls. Phenotypic traits examined included pregnancy status of the recipient following transfer (ET-preg; determined by rectal palpation at 60 days post-transfer and/or confirmed calving date of recipient), number of transferable embryos per collection (ET-trans), and number of unfertilized ova at collection (ET-UFO). Basic statistical analysis and pedigree/trait files were developed using procedures in SAS (SAS Institute, Cary, NC). Genetic parameters were estimated for a single-trait animal model using restricted maximum likelihood (REML) procedures in Wombat (Meyer K 2007 Zhejiang Uni. Science B 8, 815–821). Wombat also computed EPD and standard errors for each trait evaluated. The model included fixed effects of year as well as random animal and residual effects. The EPD for ET-preg ranged from –6.1 to 4.4% (SE = 2.2 to 4.2) for semen sires (sires of the transferred embryos) and –5.3 to 3.8% (SE = 3.2 to 4.2) for donor animals. Additionally, the heritability estimated for ET-preg was 0.03. Heritability estimated for ET-trans was 0.00, indicating minute genetic variation and thus, EPD were not presented. Heritability estimated for ET-UFO was 0.05 with EPD values (deviation of the number of UFO from the mean) ranging from –0.6 to 0.8 (SE = 0.3 to 0.6) for semen sires and –0.4 to 1.1 (SE = 0.5 to 0.6) for donor cows. As previously shown for reproductive traits, heritability of ET-preg, ET-trans, and ET-UFO was low. Genetic improvement in fertility by selection on embryo transfer traits is possible, but progress would be slow. Further studies are underway on a larger dataset to refine these estimates and to examine repeatability.