Transfer of cow blastocysts after sexing.

We obtained 174 Charolais cow blastocysts at day 13 (D(13)) after superovulation and slaughter of 24 donor heifers. Using de Wecker's scissors, we sampled 1 10 to 1 3 of the total blastocyst volume. After colcemid treatment, hypotonic shock, fixation and Giemsa staining, the sex of 149 blastocysts (60 %) was determined. The main difficulty in sexing was the absence of metaphase or the presence of incomplete metaphases on the preparations. During manipulation, the portion of the blastocyst to be transferred was stored at 37 degrees C in a medium of 20 % fetal calf serum containing 80 % B(2) under an atmosphere of 90 % nitrogen, 5 % O(2) and 5 % CO(2) for 6 to 7 hours. After sex determination, the blastocysts were put into an artificial insemination straw and placed with a catheter at the base of the uterine horns. Twenty-eight blastocysts were transferred into both horns of 14 recipient Charolais heifers; 4 heifers (28 %) had an extended cycle and 3 (21 %) were pregnant. When 2, 3 or 4 blastocysts were transferred into the ipsilateral horn with the corpus luteum, 20 to 25 % of the recipients had an extended cycle and 37, 40 and 75 %, respectively, were pregnant. The number of surviving embryos per recipient never exceeded 2.