16 Genetic contribution of the female to embryo development in dairy cattle

Selection for fertility traits such as daughter pregnancy rate (DPR), heifer conception rate (HCR), and cow conception rate (CCR) has been shown to improve pregnancy success in dairy cattle. This study explored the relationship between fertility traits and in vitro embryo development in Holstein females. Data were collected from 5704 ovum pickup (OPU) and IVF procedures from 778 females across 35 farms during the years 2020 to 2024. The number of OPUs per cow ranged from 1 to 38. The average number of cumulus–oocyte complexes (COCs) collected per OPU and average number of transferable embryos per OPU were estimated using a linear model. The percentage of embryos produced over COCs collected was evaluated using a binomial generalized linear model. Fixed effects included animal age in months and fertility trait value, while cow and block were considered random effects. Blocks were made by combining the year and season when the aspiration was performed with the location and technician who performed the aspiration. A total of 34,171 embryos were classified based on IETS guidelines as grade 1, 2, or 3, with grade 1 being of the highest quality. The percentage of grade 1, 2, and 3 embryos produced per procedure was also analyzed; grade 4 embryos were not included in the analysis. Females included in this study were divided into quartiles by age in months (Q1, 10.9 to 19.2 months; Q4, 31.8 to 60.0 months), and fertility values were based on data distribution: DPR (Q1, −4.7 to −1.8; Q4, −0.2 to 2.9), HCR (Q1, −3.2 to 0.6; Q4, 2.4 to 6), and CCR (Q1, −4.3 to −0.6; Q4, 1.5 to 4.7). Statistical analyses were conducted using R v4.40 including the lmer4 package. Younger animals (Q1) had a higher number of COCs (19.0 ± 0.46) than older cows (Q4; 13.3 ± 0.36; P < 0.0001). In contrast, older females (Q4) produced a higher percentage of embryos (40.7 ± 1.02%; P < 0.0001) and a greater total number of embryos (5.2 ± 0.19; P = 0.007) compared with younger animals (Q1; 26.1 ± 0.76% and 4.7 ± 0.17 embryos, respectively). Furthermore, older cows produced more grade 1 embryos compared with younger animals (71.9 ± 1.01% and 60.1 ± 1.11%, respectively; P < 0.001). In the analysis of fertility traits, females with higher DPR and CCR values produced fewer COCs (14.1 ± 0.58 and 13.8 ± 0.57, respectively) than those with lower DPR (16.1 ± 0.63; P = 0.0146) and CCR (15.9 ± 0.63; P = 0.009). However, fertility traits did not influence embryo production (P > 0.05). Females with lower HCR (Q1) produced fewer (P = 0.004) grade 1 embryos than those in Q4 (64.3 ± 1.22 vs. 68.1 ± 1.17%, respectively) suggesting a relationship between HCR and embryo quality. In conclusion, this study demonstrates a positive effect of cow age on the outcomes of in vitro procedures. Although age affects the number and quality of embryos produced, fertility traits such as DPR, HCR, and CCR appear to have limited influence on these phenotypes. Further research is needed to elucidate the underlying mechanisms linking fertility traits with embryo development, particularly concerning potential influences of HCR on embryo quality.

This work was funded by USDA NIFA Grant 2022-67015-36371.