214 Association between oocyte grade, markers of oocyte competence, and fertility traits in dairy cattle

In dairy cattle, the majority of pregnancy losses occur during the first three weeks of gestation, the time of pre-implantation embryonic development. One of the major determinants of embryo development is oocyte competence. This study aimed to associate phenotypes related to oocyte quality with current fertility traits. To do this, 21 cows at 32–47 days of pregnancy, less than 160 days in milk, and ranging from 1 to 6 lactations were enrolled in the study. Cumulus–oocyte complexes (COCs) were collected by ovum pickup (OPU). Each cow was subjected to OPU three times, 4 days apart. The COCs were classified by grade following IETS guidelines. For maturation, COCs were split into two groups: grade 1–2, and grade 3–4. The COCs were incubated for 22–24 h in HEPES-buffered maturation medium. After maturation, COCs were denuded and used for two analyses: mitochondria copy number (Mtn) and mitochondria super oxidation (mitoSOX). A total of 309 COCs were collected. For statistical analysis, a pairwise comparison of least squares means applying a Tukey test was performed using R v4.2.2. The number of COCs was not different across collections (P > 0.05). The first collection had fewer (P < 0.01) grade 1 COCs (15%) compared with the second and third collections (29% and 32%, respectively). The Mtn was determined by quantitative PCR, using mitochondrial COX1 as template. The COCs grades 1 and 2 had more (P = 0.015) Mtn than grades 3 and 4 (LSMEANS ± s.e.m.: 503 108 ± 19 369 vs 429 883 ± 18 540 copies, respectively). MitoSOX was measured by immunofluorescence to determine mitochondrial activity. Fluorescence intensity was measured using ImageJ v1.44. There was no difference in mitoSOX between COCs grades (P > 0.05). Next, cows were divided into quartiles based on their cow conception rate (CCR) value: Q1 (<−1.2), Q2 (0 to 0.9), Q3 (1.1 to 1.9), and Q4 (>2.3). There were more (P < 0.01) grade 3 and 4 COCs in Q4 (high CCR) than Q1 (low CCR). Interestingly, when evaluating only COCs grades 1 and 2, Mtn was greater (P < 0.05) in Q1 (564 397 ± 30 857) compared with Q4 (424 208 ± 35 420). There was a negative correlation (−0.33; P < 0.01) between CCR value and Mtn of grade 1 and 2 COCs. Cows were also divided into quartiles based on their daughter pregnancy rate (DPR) values: Q1 (<−1.6), Q2 (−1.4 to 0), Q3 (0.1 to 0.2), and Q4 (>0.9). When associating Mtn in COCs grade 1 and 2 with DPR, once more, Q4 (high DPR) had lower (P < 0.01) Mtn compared with Q1 (395 148 ± 33 298 vs 555 008 ± 26 975, respectively). A negative association was also found between Mtn and DPR (−0.59; P < 0.01) of grade 1 and 2 COCs. MitoSOX was not affected (P > 0.05) by CCR or DPR classification. In summary, COCs visually classified as grades 1 and 2 (high quality) had increased Mtn, which is associated with oocyte competence. In addition, oocyte phenotype, defined by mitochondrial measures, is not explained by CCR or DPR. Nevertheless, this is a small sample size and further research is required to elucidate the association between oocyte phenotype and fertility traits.

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