Association of fertility traits with markers of oocyte competence in dairy cattle.

Abstract

Fertility traits such as daughter pregnancy rate (DPR), cow conception rate, and heifer conception rate are key predictors of reproductive performance in dairy herds. However, their low heritability, likely due to their multifactorial nature and difficulty in measuring phenotypes, poses challenges for genetic improvement. Oocyte competence, encompassing nuclear and cytoplasmic maturation, is a critical factor influencing fertility. Errors in chromosome segregation during nuclear maturation can result in aneuploidy, which leads to embryonic mortality. During cytoplasmic maturation, mitochondrial replication is vital for energy production to support subsequent embryo development. The objective of this work was to investigate the association between fertility traits and markers of oocyte competence after in vitro maturation in dairy cattle. To do this, 105 Holstein cows with 32 to 52 d of pregnancy and ranging from 1 to 6 lactations were enrolled in the study. Cumulus oocyte complexes (COC) were collected by ovum pickup (OPU). Each cow was subjected to OPU 3 times, and collected COC were classified by grade following International Embryo Transfer Society guidelines. For maturation, COC were split into groups by grade as either grade 1 and 2 or grade 3 and 4. After maturation, COC were used for 2 analyses: aneuploidy screening and mitochondrial copy number. Low-fertility cows, based on DPR, might harbor genetic variants associated with elevated oocyte aneuploidy rates, potentially contributing to their reduced fertility. No association was found between mitochondria copy number and fertility traits. Further research needs to be performed on mitochondria functionality and its relationship with fertility. In conclusion, high-fertility cows based on DPR had fewer oocytes with chromosomal abnormalities, which could explain improved fertility.