Magnesium is a critical element for competent development of bovine embryos

The study was designed to determine the impact of magnesium (Mg²⁺) on bovine embryo development. We found that two commercially available sources of bovine serum albumin (BSA) and fetal bovine serum (FBS) contained different amounts of Mg²⁺ residue: 4 ppm in ICPbio BSA, 114 ppm in Sigma BSA, and 44 ppm in FBS. When CR1 was used as basal medium, PVA and ICPbio BSA produced the lowest blastocyst yield (2.2-2.3%), whereas Sigma BSA increased blastocyst yield to 18.9% (P < 0.05). Supplementation of 1.4 mM MgCl₂ into the medium increased the blastocyst rate in the ICPbio BSA group (29.4%) but not in the PVA group (5.4%; P < 0.05) to a level comparable to that of the FBS group (33.7%; P > 0.05). We next found that increasing concentrations of MgCl₂ in the culture medium (ICPbio BSA) elevated blastocyst rate from 2.6% (0 mM), 38.4% (0.35 mM) to 50.2% (1.4 mM; P < 0.05), further maintained at 44.9% (2.1 mM) and 43.4% (2.8 mM) (P > 0.05). However, blastocyst rate was reduced to 31.4% (4.2 mM) and 29.4% (5.6 mM) when MgCl₂ supplement was increased (P < 0.05). Comparable blastocyst development was achieved in both ICPbio BSA (30.0-33.1%) and Sigma BSA (37.4-38.7%) groups when 1.4 mM Mg²⁺ was supplemented regardless of its source (MgCl₂ vs. MgSO₄; P > 0.05). In embryo transfer experiments, higher rates of pregnancy (54.3 vs. 41.5%) and calving (44.3 vs. 32.5%) were achieved in the CR1-Mg²⁺-supplemented BSA group compared with the FBS group with co-culture, respectively (P < 0.05). These results demonstrate that Mg²⁺ is a key ion that promotes competent blastocyst and term development. Therefore, a simple and efficient defined medium (CR1-Mg²⁺-BSA) can successfully replace complex serum and somatic cell co-culture.