Efficacy of timed embryo transfer with fresh and frozen in vitro produced embryos to increase pregnancy rates in heat-stressed dairy cattle

Our objective was to determine whether pregnancy rates in heat-stressed dai ry cattle could be enhanced by timed embryo transfer of fresh (nonfrozen) o r frozen-thawed in vitro-derived embryos compared to timed insemination. Ov ulation in Holstein cows was synchronized by a GnRH injection followed 7 d later by PGF(2 alpha) and a second treatment with GnRH 48 h later. Control cows (n = 129) were inseminated 16 h (d 0) after the second GnRH injection. On d 7, a fresh (n = 133) or frozen-thawed (n = 142) in vitro-derived embr yo was transferred to cows assigned for timed embryo transfer after categor izing the corpus luteum by palpation per rectum as 3 (excellent), 2 (good o r fair), 1 (poor), and 0 (nonpalpable). Response to the synchronization tre atment, determined by plasma progesterone concentration (ng/ml) less than o r equal to 1.5 on d 0 and greater than or equal to 2.0 on d 7, was 76.2%. M ean plasma progesterone concentration on d 7 increased as the quality of co rpus luteum improved from category 0 to 3. Concentrations of progesterone i n plasma were elevated (greater than or equal to 2.0 ng/ml) at 21 d in 64.7 (fresh embryo), 40.3 (frozen embryo), and 41.4 +/- 0.1% (timed inseminatio n) of cows, respectively. Cows that received a fresh embryo had a greater p regnancy rate at 45 to 52 d than did cows that received a frozen-thawed emb ryo or timed insemination (14.3 > 4.8, 4.9 +/- 2.3%). Body condition (d 0) of cows influenced the pregnancy rate and plasma progesterone concentration s. In summary, timed embryo transfer with fresh in vitro-produced embryos i n heat-stressed dairy cattle improved pregnancy rate relative to timed inse mination.