Evaluation of factors that affect the fertility of the recipient of crossbred Holstein genotype embryos

The study determined the effect of the size of the Corpus Luteum (CL), the concentration of Progesterone (P₄), time needed to pass the cervix, the place of deposit of the embryo and the total time required to complete the embryo transfer process (TE) on the pregnancy rate (PR) of Holstein heifers (n=40). Prior to the transfer, the recipients were ultrasounded and those with CL>15 mm entered the experiment. Fory quality 1 (excellent) frozen-thawed embryos were transferred, obtaining a TR of 45%. The CL size and P₄ concentration of pregnant heifers (20.6 ± 0.31 mm; 4.4 ± 0.12 ng·mL⁻¹) were greater than that of non-pregnant heifers (16.7 ± 0.28 mm; 3.1 ± 0.11 ng·mL⁻¹; P<0.05). The time required to pass the cervix was greater in no-pregnant heifers than in pregnant (60.0 ± 5.32 s; 40.3 ± 4.65 s, respectively); as well as the total time spent in ET was greater in non-pregnant heifers (783.0 ± 31.43 s) compared to pregnant heifers (680.9 ± 29.93 s; P<0.05). A high and positive correlation was established between CL size and P₄ concentration with pregnancy (r=0.84; r=0.77; P<0.001, respectively). However, the correlation observed between the time used to pass the cervix (r=0.35; P<0.05) and the total time spent transferring the embryo with the pregnancy was positive, but low (r=0.45; P<0.05). It concludes that a CL>20 mm at the time of transfer provided a higher concentration of P₄, a factor that increases the probability of establishing a pregnancy. Furthermore, the less time required to pass the cervix, the embryo could be deposited with less manipulation in the cranial third of the uterine horn, decreasing the total time spent transferring the embryo, which increased the conception rate.