Influencia del cuerpo lúteo, lugar de depósito del embrión y tiempo de transferencia de embriones congelados-descongelados sobre la tasa de preñez en receptoras Holstein mestizas

Abstract

The aim of the present study sought to evaluate the total time used in the frozen-thawed embryo transfer (TE), cervix transfer score, morphology and functionality (P4) of the corpus luteum and transfer location of embryo, on pregnancy rates in heifers Holstein mestiza. The research was conducted in livestock farms located in the provinces of Azuay and Cañar, Austral area of Ecuador. A total of 40 heifers were selected as recipients and received the same synchronization protocol from estrus and embryo transfer (TE). At 7 days post-estrus, the recipients that presented a corpus luteum (CL) received an embryo of excellent quality (quality 1). On the day of the transfer, blood samples were taken for subsequent analysis of P4 using the Bovine Progesterone ELISA Kit (Mybiosource Inc.). According to the diameter of the CL animals were grouped into G1 ≤ 17 mm; G2 > 17 and < 20 mm; G3 ≥ 20 mm. Additionally, progesterone levels were organized into G1 ≤ 3.7 ng/ml; G2 > 3.7 and < 4.3 ng/ml; G3 ≥ 4.3 ng/ml. The cervix transfer score was organized in easy ≤ 30 sec; moderate > 30 and < 60 sec; hard ≥ 60 sec. The transfer location of embryo was classified into the cranial, middle, or posterior third of the uterine horn and the total time used for embryo transfer into minimum ≤ 9 min; moderate > 9 and < 13 min; long ≥ 13 min. All these variables were evaluated against the pregnancy rate, by means of the Kruskal-Wallis test and to determine correlation, the Spearman test was used. It was determined that heifers with a larger CL presented a higher pregnancy rate (G1=0%; G2=47.7% and G3=80%), with medium correlation (r= 0.59; p<0.05); in addition, animals with higher levels of P4 showed higher pregnancy rates (G1=9.1%; G2=47.1% and G3=66.7%), with a mean correlation value (r=0.44; p<0.05). It was observed that the more complicated the passage of the cervix was, the less pregnancy is obtained (G1=60%; G2=50% and G3=10%) and this showed a medium correlation (r=0.36; p<0.05). For its part, the transfer location of embryo had pregnancy rates of G1=60%; G2=37.5% and G3=42.1% for the cranial, middle, and posterior third, respectively (r=0.05; p>0.05). Finally, it was established that the longer the total time was necessary for the transfer, the lower the pregnancy rates G1=58.3%; G2=41.2% and G3=27.3% for the minimal, moderate, and long groups, respectively (r=0.24; p>0.05). In conclusion, a larger diameter of CL, higher levels of P4 and a lower cervix transfer score had statistically significant effects on pregnancy rates, therefore these factors must be considered by technicians within TE programs in order to improve conception rates in livestock farms in our region.