112 Treatment with 325 mg of recombinant bovine somatotropin at the time of embryo transfer increase pregnancy rate of in vitro transfer in Holstein heifer recipients

The utilisation of recombinant bovine somatotropin (rbST) yields a favourable impact on milk production and engenders scientific interest due to its effects on bovine reproductive efficiency. The rbST augments the levels of IGF-1 and insulin, pivotal hormones for reproductive processes. Thus, the present study aimed to evaluate the effect of administering 325 mg of rbST at the time of embryo transfer on the pregnancy rate of in vitro transfer (P/ET) in Holstein heifer recipients. Our hypothesis states that the administration of 325 mg of rbST on the day of embryo transfer should potentially increase the P/ET rate, and this effect is dependent on the type of embryo. For this research, a total of 1612 heifers aged between 13.5 to 15 months, with a body weight of 380 to 410 kg, from first or second service were selected. These animals were housed in freestall stables at Maddox Dairy or in a dry lot at RuAnn Dairy, both located in Riverdale, California, USA. In vitro-produced (IVP) embryos from Holstein donors were then transferred into the uterine horn ipsilateral to the ovary with a corpus luteum, within a timeframe of 6 to 9 days after oestrus detection in the recipients. The embryos were classified into three categories: (1) Fresh high-quality embryos, which were grade 1 embryos that had reached the blastocyst stage on Day 7 of in vitro embryo culture (IVC); (2) Fresh low-quality embryos, including grade 1 embryos at the morula or early blastocyst stage on Day 7 of IVC, and grade 2 embryos; and (3) Frozen embryos, comprising high-quality embryos cryopreserved for direct transfer using ethylene glycol. At the time of transfer, the recipients were divided into two homogeneous treatments: (1) Control group, without treatment at the time of embryo transfer, and (2) rBST group, which received the treatment of 325 mg of rbST (subcutaneous administration) at the time of embryo transfer. Pregnancy diagnosis (P/ET) was determined by analysing blood samples using the PAG test (IDEXX) between 38 to 44 days after oestrus detection. Statistical analyses were performed using GLIMMIX in SAS 9.4. No interaction treatment × embryo category was observed (P = 0.13). Heifers treated with 325 mg of rBST at the time of ET exhibited a higher P/ET (rBST = 47.9% (384/802) vs Control = 43.5% (352/810); P = 0.01). Additionally, fresh high-quality embryos presented higher (P < 0.0001) P/ET (53.3%; 490/919^A) than fresh low-quality embryos (34.3%; 185/538^B) and frozen embryos (39.4%; 61/155^B). In conclusion, the administration of 325 mg of bST at the time of embryo transfer improved the P/ET in recipient Holstein heifers.

This research was supported by Agener União Saúde Animal.