Bovine embryo freezability is closely linked with quality and differs between breeds (Visintin JA et al. 2002 Theriogenology 57, 345–359). Dark embryos have a poor cryotolerance resulting in low pregnancy rates (Hill BR and Kuehner LF 1998 Theriogenology 49, 168), and are characterized by excessive accumulation of lipid droplets which in turn may be influenced by the biochemical composition of the embryonic environment. Earlier work revealed that Holstein Friesian (HF) cows yield significantly darker embryos compared to Belgian Blue beef cows (Leroy JLMR et al. 2004 Reprod. Fert. Dev. 16, 211). In this field examine we aimed to study the differences in embryo quality and color between HF cows (producing milk) and heifers (non-producing) in relation to four serum parameters that have previously been linked with embryo quality and fertility: urea, total protein (TP), total cholesterol (TC), and triglycerides (TG). Blood samples were collected from HF cows (n = 54) and HF heifers (n = 33) prior to embryo flushing on Day 7 after superovulation and subsequent insemination. Serum was stored frozen until assay with commercial photometric assays. Embryos were scored individually by the same operator for quality (excellent to poor: score 1 to 4) and color: light (L), medium (M), or dark (D) using a binocular stereomicroscope (40×). Independent Student's t-test and chi-square test were used when appropriate. Heifers and cows yielded on average (± SEM) 5.1 ± 0.82 and 6.1 ± 0.71 embryos per flushing, respectively. Significantly more cow embryos were classified as dark compared to heifer embryos (L, M, D: 20.4%, 55.5% and 24.1% vs. 70.8%, 27.4%, and 1.8%, for cow and heifer embryos, respectively) (P < 0.05). Significantly more heifer embryos showed an excellent morphological quality (62.5% of heifer embryos compared to 13.1% of cow embryos) (P < 0.05). Serum concentrations (± SEM) of urea (4.5 ± 0.2 vs. 2.8 ± 0.2 mM), TP (7.59 ± 0.07 vs. 6.57 ± 0.09 g/dL) and TC (183 ± 5.3 vs. 105 ± 3.8 mg/dL) were significantly higher and serum concentrations of TG (17.2 ± 0.6 vs. 23.8 ± 0.9 mg/dL) significantly lower in cows compared to heifers (P < 0.05). In this study, embryo color (i.e. lipid content) and quality as well as urea, TP, TC, and TG serum concentrations were clearly influenced by the production parameter (cow = producing vs. heifer = non-producing). These findings imply that producing milk (parity = 0 or >0) may be an important confounder. Therefore, this variable should be taken into account when studying multiple variables influencing embryo quality or color simultaneously. Further analyses are ongoing to reveal which factors at the cow level are affecting the embryo quality and color.

This research was funded by IWT-Flanders (Grant no. 13236).