Evaluating recipient and embryo factors that affect pregnancy rates of embryo transfer in beef cattle

The objectives of this experiment were to determine the effects of corpus l uteum characteristics, progesterone concentration, donor-recipient synchron y, embryo quality, type, and developmental stage on pregnancy rates after e mbryo transfer. We synchronized 763 potential recipients for estrus using o ne of two synchronization protocols: two doses of PGF2alpha (25 mg im) give n 11 d apart (Location 1); and, a single norgestomet implant for 7 d with o ne dose of PGF2alpha (25 mg im) 24 h before implant removal (Location 2). A t embryo transfer, ovaries were examined by rectal palpation and ultrasonog raphy. Of the 526 recipients presented for embryo transfer, 122 received a fresh embryo and 326 received a frozen embryo. Pregnancy rates were greater (P < 0.05) with fresh embryos (83%) than frozen-thawed embryos (69%). Preg nancy rates were not affected by embryo grade, embryo stage, donor-recipien t synchrony, or the palpated integrity of the CL. Corpus luteum diameter an d luteal tissue volume increased as days post-estrus for the recipients inc reased. However, pregnancy rates did not differ among recipients receiving embryos 6.5 to 8.5 days after estrus (P > 0.1). There was a significant, po sitive simple correlation between CL diameter or luteal tissue volume and p lasma progesterone concentration (r = 0.15, P < 0.01 and r = 0.18, P < 0.01 , respectively). There were no significant: differences in mean CL diameter , luteal volume or plasma progesterone concentration among recipients that did or did not become pregnant after embryo transfer. We conclude that suit ability of a potential embryo transfer recipient is determined by observed estrus and a palpable corpus luteum, regardless of size or quality. (C) 200 1 by Elsevier Science Inc.